Development guide

Development guide

• Introduction
• Code layout
• Include files
• Integers
• Common return codes
• Error handling
• Strings
• Overview
• Formatting
• Numeric conversion
• Regular expressions
• Time
• Containers
• Array
• List
• Queue
• Red-Black tree
• Hash
• Memory management
• Heap
• Pool
• Shared memory
• Logging
• Cycle
• Buffer
• Networking
• Connection
• Events
• Event
• I/O events
• Timer events
• Posted events
• Event loop
• Processes
• Threads
• Modules
• Adding new modules
• Core modules
• Configuration directives
• HTTP
• Connection
• Request
• Configuration
• Phases
• Variables
• Complex values
• Request redirection
• Subrequests
• Request finalization
• Request body
• Response
• Response body
• Body filters
• Building filter modules
• Buffer reuse
• Load balancing

Introduction

Code layout

• auto — Build scripts
• src
  • core — Basic types and functions — string, array, log, pool, etc.
  • event — Event core
    • modules — Event notification modules: epoll, kqueue, select etc.
  • http — Core HTTP module and common code
    • modules — Other HTTP modules
    • v2 — HTTP/2
  • mail — Mail modules
  • os — Platform-specific code
    • unix
    • win32
  • stream — Stream modules

Include files

The following two #include statements must appear at the beginning of every nginx file:

#include <ngx_config.h>
#include <ngx_core.h>

In addition to that, HTTP code should include

#include <ngx_http.h>

Mail code should include

#include <ngx_mail.h>

Stream code should include

#include <ngx_stream.h>

Integers

For general purposes, nginx code uses two integer types, ngx_int_t and ngx_uint_t, which are typedefs for intptr_t and uintptr_t respectively.

Common return codes

Most functions in nginx return the following codes:

• NGX_OK — Operation succeeded.
• NGX_ERROR — Operation failed.
• NGX_AGAIN — Operation incomplete; call the function again.
• NGX_DECLINED — Operation rejected, for example, because it is disabled in the configuration. This is never an error.
• NGX_BUSY — Resource is not available.
• NGX_DONE — Operation complete or continued elsewhere. Also used as an alternative success code.
• NGX_ABORT — Function was aborted. Also used as an alternative error code.

Error handling

The ngx_errno macro returns the last system error code. It's mapped to errno on POSIX platforms and to GetLastError() call in Windows. The ngx_socket_errno macro returns the last socket error number. Like the ngx_errno macro, it's mapped to errno on POSIX platforms. It's mapped to the WSAGetLastError() call on Windows. Accessing the values of ngx_errno or ngx_socket_errno more than once in a row can cause performance issues. If the error value might be used multiple times, store it in a local variable of type ngx_err_t. To set errors, use the ngx_set_errno(errno) and ngx_set_socket_errno(errno) macros.

The values of ngx_errno and ngx_socket_errno can be passed to the logging functions ngx_log_error() and ngx_log_debugX(), in which case system error text is added to the log message.

Example using ngx_errno:

void
ngx_my_kill(ngx_pid_t pid, ngx_log_t *log, int signo)
{
    ngx_err_t  err;

    if (kill(pid, signo) == -1) {
        err = ngx_errno;

        ngx_log_error(NGX_LOG_ALERT, log, err, "kill(%P, %d) failed", pid, signo);

        if (err == NGX_ESRCH) {
            return 2;
        }

        return 1;
    }

    return 0;
}

Strings

Overview

For C strings, nginx uses the unsigned character type pointer u_char *.

The nginx string type ngx_str_t is defined as follows:

typedef struct {
    size_t      len;
    u_char     *data;
} ngx_str_t;

The len field holds the string length and data holds the string data. The string, held in ngx_str_t, may or may not be null-terminated after the len bytes. In most cases it’s not. However, in certain parts of the code (for example, when parsing configuration), ngx_str_t objects are known to be null-terminated, which simplifies string comparison and makes it easier to pass the strings to syscalls.

The string operations in nginx are declared in src/core/ngx_string.h Some of them are wrappers around standard C functions:

• ngx_strcmp()
• ngx_strncmp()
• ngx_strstr()
• ngx_strlen()
• ngx_strchr()
• ngx_memcmp()
• ngx_memset()
• ngx_memcpy()
• ngx_memmove()

Other string functions are nginx-specific

• ngx_memzero() — Fills memory with zeroes.
• ngx_cpymem() — Does the same as ngx_memcpy(), but returns the final destination address This one is handy for appending multiple strings in a row.
• ngx_movemem() — Does the same as ngx_memmove(), but returns the final destination address.
• ngx_strlchr() — Searches for a character in a string, delimited by two pointers.

The following functions perform case conversion and comparison:

• ngx_tolower()
• ngx_toupper()
• ngx_strlow()
• ngx_strcasecmp()
• ngx_strncasecmp()

The following macros simplify string initialization:

• ngx_string(text) — static initializer for the ngx_str_t type from the C string literal text
• ngx_null_string — static empty string initializer for the ngx_str_t type
• ngx_str_set(str, text) — initializes string str of ngx_str_t * type with the C string literal text
• ngx_str_null(str) — initializes string str of ngx_str_t * type with the empty string

Formatting

The following formatting functions support nginx-specific types:

• ngx_sprintf(buf, fmt, ...)
• ngx_snprintf(buf, max, fmt, ...)
• ngx_slprintf(buf, last, fmt, ...)
• ngx_vslprint(buf, last, fmt, args)
• ngx_vsnprint(buf, max, fmt, args)

The full list of formatting options, supported by these functions is in src/core/ngx_string.c. Some of them are:

• %O — off_t
• %T — time_t
• %z — ssize_t
• %i — ngx_int_t
• %p — void *
• %V — ngx_str_t *
• %s — u_char * (null-terminated)
• %*s — size_t + u_char *

You can prepend u on most types to make them unsigned. To convert output to hex, use X or x.

For example:

u_char      buf[NGX_INT_T_LEN];
size_t      len;
ngx_uint_t  n;

/* set n here */

len = ngx_sprintf(buf, "%ui", n) — buf;

Numeric conversion

Several functions for numeric conversion are implemented in nginx. The first four each convert a string of given length to a positive integer of the indicated type. They return NGX_ERROR on error.

• ngx_atoi(line, n) — ngx_int_t
• ngx_atosz(line, n) — ssize_t
• ngx_atoof(line, n) — off_t
• ngx_atotm(line, n) — time_t

There are two additional numeric conversion functions. Like the first four, they return NGX_ERROR on error.

• ngx_atofp(line, n, point) — Converts a fixed point floating number of given length to a positive integer of type ngx_int_t. The result is shifted left by point decimal positions. The string representation of the number is expected to have no more than points fractional digits. For example, ngx_atofp("10.5", 4, 2) returns 1050.
• ngx_hextoi(line, n) — Converts a hexadecimal representation of a positive integer to ngx_int_t.

Regular expressions

The regular expressions interface in nginx is a wrapper around the PCRE library. The corresponding header file is src/core/ngx_regex.h.

To use a regular expression for string matching, it first needs to be compiled, which is usually done at the configuration phase. Note that since PCRE support is optional, all code using the interface must be protected by the surrounding NGX_PCRE macro:

#if (NGX_PCRE)
ngx_regex_t          *re;
ngx_regex_compile_t   rc;

u_char                errstr[NGX_MAX_CONF_ERRSTR];

ngx_str_t  value = ngx_string("message (\\d\\d\\d).*Codeword is '(?<cw>\\w+)'");

ngx_memzero(&rc, sizeof(ngx_regex_compile_t));

rc.pattern = value;
rc.pool = cf->pool;
rc.err.len = NGX_MAX_CONF_ERRSTR;
rc.err.data = errstr;
/* rc.options are passed as is to pcre_compile() */

if (ngx_regex_compile(&rc) != NGX_OK) {
    ngx_conf_log_error(NGX_LOG_EMERG, cf, 0, "%V", &rc.err);
    return NGX_CONF_ERROR;
}

re = rc.regex;
#endif

After successful compilation, the captures and named_captures fields in the ngx_regex_compile_t structure contain the count of all captures and named captures, respectively, found in the regular expression.

The compiled regular expression can then be used for matching against strings:

ngx_int_t  n;
int        captures[(1 + rc.captures) * 3];

ngx_str_t input = ngx_string("This is message 123. Codeword is 'foobar'.");

n = ngx_regex_exec(re, &input, captures, (1 + rc.captures) * 3);
if (n >= 0) {
    /* string matches expression */

} else if (n == NGX_REGEX_NO_MATCHED) {
    /* no match was found */

} else {
    /* some error */
    ngx_log_error(NGX_LOG_ALERT, log, 0, ngx_regex_exec_n " failed: %i", n);
}

The arguments to ngx_regex_exec() are the compiled regular expression re, the string to match s, an optional array of integers to hold any captures that are found, and the array's size. The size of the captures array must be a multiple of three, as required by the PCRE API. In the example, the size is calculated from the total number of captures plus 1one for the matched string itself.

If there are matches, captures can be accessed as follows:

u_char     *p;
size_t      size;
ngx_str_t   name, value;

/* all captures */
for (i = 0; i < n * 2; i += 2) {
    value.data = input.data + captures[i];
    value.len = captures[i + 1] — captures[i];
}

/* accessing named captures */

size = rc.name_size;
p = rc.names;

for (i = 0; i < rc.named_captures; i++, p += size) {

    /* capture name */
    name.data = &p[2];
    name.len = ngx_strlen(name.data);

    n = 2 * ((p[0] << 8) + p[1]);

    /* captured value */
    value.data = &input.data[captures[n]];
    value.len = captures[n + 1] — captures[n];
}

The ngx_regex_exec_array() function accepts the array of ngx_regex_elt_t elements (which are just compiled regular expressions with associated names), a string to match, and a log. The function applies expressions from the array to the string until either a match is found or no more expressions are left. The return value is NGX_OK when there is a match and NGX_DECLINED otherwise, or NGX_ERROR in case of error.

Time

The ngx_time_t structure represents time with three separate types for seconds, milliseconds, and the GMT offset:

typedef struct {
    time_t      sec;
    ngx_uint_t  msec;
    ngx_int_t   gmtoff;
} ngx_time_t;

The ngx_tm_t structure is an alias for struct tm on UNIX platforms and SYSTEMTIME on Windows.

To obtain the current time, it is usually sufficient to access one of the available global variables, representing the cached time value in the desired format. For example, the ngx_current_msec variable holds the number of milliseconds elapsed since Epoch and truncated to ngx_msec_t.

The available string representations are:

• ngx_cached_err_log_time — Used in error log entries: "1970/09/28 12:00:00"
• ngx_cached_http_log_time — Used in HTTP access log entries: "28/Sep/1970:12:00:00 +0600"
• ngx_cached_syslog_time — Used in syslog entries: "Sep 28 12:00:00"
• ngx_cached_http_time — Used in HTTP headers: "Mon, 28 Sep 1970 06:00:00 GMT"
• ngx_cached_http_log_iso8601 — The ISO 8601 standard format: "1970-09-28T12:00:00+06:00"

The ngx_time() and ngx_timeofday() macros return the current time value in seconds and are the preferred way to access the cached time value.

To obtain the time explicitly, use ngx_gettimeofday(), which updates its argument (pointer to struct timeval). The time is always updated when nginx returns to the event loop from system calls. To update the time immediately, call ngx_time_update(), or ngx_time_sigsafe_update() if updating the time in the signal handler context.

The following functions convert time_t into the indicated broken-down time representation. The first function in each pair converts time_t to ngx_tm_t and the second (with the _libc_ infix) to struct tm:

• ngx_gmtime(), ngx_libc_gmtime() — Time expressed as UTC
• ngx_localtime(), ngx_libc_localtime() — Time expressed relative to the local time zone

The ngx_http_time(buf, time) function returns a string representation suitable for use in HTTP headers (for example, "Mon, 28 Sep 1970 06:00:00 GMT"). The ngx_http_cookie_time(buf, time) returns a string representation function returns a string representation suitable for HTTP cookies ("Thu, 31-Dec-37 23:55:55 GMT").

Containers

Array

The nginx array type ngx_array_t is defined as follows

typedef struct {
    void        *elts;
    ngx_uint_t   nelts;
    size_t       size;
    ngx_uint_t   nalloc;
    ngx_pool_t  *pool;
} ngx_array_t;

The elements of the array are available in the elts field. The nelts field holds the number of elements. The size field holds the size of a single element and is set when the array is initialized.

Use the ngx_array_create(pool, n, size) call to create an array in a pool, and the ngx_array_init(array, pool, n, size) call to initialize an array object that has already been allocated.

ngx_array_t  *a, b;

/* create an array of strings with preallocated memory for 10 elements */
a = ngx_array_create(pool, 10, sizeof(ngx_str_t));

/* initialize string array for 10 elements */
ngx_array_init(&b, pool, 10, sizeof(ngx_str_t));

Use the following functions to add elements to an array:

• ngx_array_push(a) adds one tail element and returns pointer to it
• ngx_array_push_n(a, n) adds n tail elements and returns pointer to the first one

If the currently allocated amount of memory is not large enough to accommodate the new elements, a new block of memory is allocated and the existing elements are copied to it. The new memory block is normally twice as large as the existing one.

s = ngx_array_push(a);
ss = ngx_array_push_n(&b, 3);

List

In nginx a list is a sequence of arrays, optimized for inserting a potentially large number of items. The ngx_list_t list type is defined as follows:

typedef struct {
    ngx_list_part_t  *last;
    ngx_list_part_t   part;
    size_t            size;
    ngx_uint_t        nalloc;
    ngx_pool_t       *pool;
} ngx_list_t;

The actual items are stored in list parts, which are defined as follows:

typedef struct ngx_list_part_s  ngx_list_part_t;

struct ngx_list_part_s {
    void             *elts;
    ngx_uint_t        nelts;
    ngx_list_part_t  *next;
};

Before use, a list must be initialized by calling ngx_list_init(list, pool, n, size) or created by calling ngx_list_create(pool, n, size). Both functions take as arguments the size of a single item and a number of items per list part. To add an item to a list, use the ngx_list_push(list) function. To iterate over the items, directly access the list fields as shown in the example:

ngx_str_t        *v;
ngx_uint_t        i;
ngx_list_t       *list;
ngx_list_part_t  *part;

list = ngx_list_create(pool, 100, sizeof(ngx_str_t));
if (list == NULL) { /* error */ }

/* add items to the list */

v = ngx_list_push(list);
if (v == NULL) { /* error */ }
ngx_str_set(v, "foo");

v = ngx_list_push(list);
if (v == NULL) { /* error */ }
ngx_str_set(v, "bar");

/* iterate over the list */

part = &list->part;
v = part->elts;

for (i = 0; /* void */; i++) {

    if (i >= part->nelts) {
        if (part->next == NULL) {
            break;
        }

        part = part->next;
        v = part->elts;
        i = 0;
    }

    ngx_do_smth(&v[i]);
}

Lists are primarily used for HTTP input and output headers.

Lists do not support item removal. However, when needed, items can internally be marked as missing without actually being removed from the list. For example, to mark HTTP output headers (which are stored as ngx_table_elt_t objects) as missing, set the hash field in ngx_table_elt_t to zero. Items marked in this way are explicitly skipped when the headers are iterated over.

Queue

In nginx a queue is an intrusive doubly linked list, with each node defined as follows:

typedef struct ngx_queue_s  ngx_queue_t;

struct ngx_queue_s {
    ngx_queue_t  *prev;
    ngx_queue_t  *next;
};

The head queue node is not linked with any data. Use the ngx_queue_init(q) call to initialize the list head before use. Queues support the following operations:

• ngx_queue_insert_head(h, x), ngx_queue_insert_tail(h, x) — Insert a new node
• ngx_queue_remove(x) — Remove a queue node
• ngx_queue_split(h, q, n) — Split a queue at a node, returning the queue tail in a separate queue
• ngx_queue_add(h, n) — Add a second queue to the first queue
• ngx_queue_head(h), ngx_queue_last(h) — Get first or last queue node
• ngx_queue_sentinel(h) - Get a queue sentinel object to end iteration at
• ngx_queue_data(q, type, link) — Get a reference to the beginning of a queue node data structure, considering the queue field offset in it

An example:

typedef struct {
    ngx_str_t    value;
    ngx_queue_t  queue;
} ngx_foo_t;

ngx_foo_t    *f;
ngx_queue_t   values;

ngx_queue_init(&values);

f = ngx_palloc(pool, sizeof(ngx_foo_t));
if (f == NULL) { /* error */ }
ngx_str_set(&f->value, "foo");

ngx_queue_insert_tail(&values, f);

/* insert more nodes here */

for (q = ngx_queue_head(&values);
     q != ngx_queue_sentinel(&values);
     q = ngx_queue_next(q))
{
    f = ngx_queue_data(q, ngx_foo_t, queue);

    ngx_do_smth(&f->value);
}

Red-Black tree

The src/core/ngx_rbtree.h header file provides access to the effective implementation of red-black trees.

typedef struct {
    ngx_rbtree_t       rbtree;
    ngx_rbtree_node_t  sentinel;

    /* custom per-tree data here */
} my_tree_t;

typedef struct {
    ngx_rbtree_node_t  rbnode;

    /* custom per-node data */
    foo_t              val;
} my_node_t;

To deal with a tree as a whole, you need two nodes: root and sentinel. Typically, they are added to a custom structure, allowing you to organize your data into a tree in which the leaves contain a link to or embed your data.

To initialize a tree:

my_tree_t  root;

ngx_rbtree_init(&root.rbtree, &root.sentinel, insert_value_function);

To traverse a tree and insert new values, use the "insert_value" functions. For example, the ngx_str_rbtree_insert_value function deals with the ngx_str_t type. Its arguments are pointers to a root node of an insertion, the newly created node to be added, and a tree sentinel.

void ngx_str_rbtree_insert_value(ngx_rbtree_node_t *temp,
                                 ngx_rbtree_node_t *node,
                                 ngx_rbtree_node_t *sentinel)

The traversal is pretty straightforward and can be demonstrated with the following lookup function pattern:

my_node_t *
my_rbtree_lookup(ngx_rbtree_t *rbtree, foo_t *val, uint32_t hash)
{
    ngx_int_t           rc;
    my_node_t          *n;
    ngx_rbtree_node_t  *node, *sentinel;

    node = rbtree->root;
    sentinel = rbtree->sentinel;

    while (node != sentinel) {

        n = (my_node_t *) node;

        if (hash != node->key) {
            node = (hash < node->key) ? node->left : node->right;
            continue;
        }

        rc = compare(val, node->val);

        if (rc < 0) {
            node = node->left;
            continue;
        }

        if (rc > 0) {
            node = node->right;
            continue;
        }

        return n;
    }

    return NULL;
}

The compare() function is a classic comparator function that returns a value less than, equal to, or greater than zero. To speed up lookups and avoid comparing user objects that can be big, an integer hash field is used.

To add a node to a tree, allocate a new node, initialize it and call ngx_rbtree_insert():

    my_node_t          *my_node;
    ngx_rbtree_node_t  *node;

    my_node = ngx_palloc(...);
    init_custom_data(&my_node->val);

    node = &my_node->rbnode;
    node->key = create_key(my_node->val);

    ngx_rbtree_insert(&root->rbtree, node);

To remove a node, call the ngx_rbtree_delete() function:

ngx_rbtree_delete(&root->rbtree, node);

Hash

Hash table functions are declared in src/core/ngx_hash.h. Both exact and wildcard matching are supported. The latter requires extra setup and is described in a separate section below.

Before initializing a hash, you need to know the number of elements it will hold so that nginx can build it optimally. Two parameters that need to be configured are max_size and bucket_size, as detailed in a separate document. They are usually configurable by the user. Hash initialization settings are stored with the ngx_hash_init_t type, and the hash itself is ngx_hash_t:

ngx_hash_t       foo_hash;
ngx_hash_init_t  hash;

hash.hash = &foo_hash;
hash.key = ngx_hash_key;
hash.max_size = 512;
hash.bucket_size = ngx_align(64, ngx_cacheline_size);
hash.name = "foo_hash";
hash.pool = cf->pool;
hash.temp_pool = cf->temp_pool;

The key is a pointer to a function that creates the hash integer key from a string. There are two generic key-creation functions: ngx_hash_key(data, len) and ngx_hash_key_lc(data, len). The latter converts a string to all lowercase characters, so the passed string must be writeable. If that is not true, pass the NGX_HASH_READONLY_KEY flag to the function, initializing the key array (see below).

The hash keys are stored in ngx_hash_keys_arrays_t and are initialized with ngx_hash_keys_array_init(arr, type): The second parameter (type) controls the amount of resources preallocated for the hash and can be either NGX_HASH_SMALL or NGX_HASH_LARGE. The latter is appropriate if you expect the hash to contain thousands of elements.

ngx_hash_keys_arrays_t  foo_keys;

foo_keys.pool = cf->pool;
foo_keys.temp_pool = cf->temp_pool;

ngx_hash_keys_array_init(&foo_keys, NGX_HASH_SMALL);

To insert keys into a hash keys array, use the ngx_hash_add_key(keys_array, key, value, flags) function:

ngx_str_t k1 = ngx_string("key1");
ngx_str_t k2 = ngx_string("key2");

ngx_hash_add_key(&foo_keys, &k1, &my_data_ptr_1, NGX_HASH_READONLY_KEY);
ngx_hash_add_key(&foo_keys, &k2, &my_data_ptr_2, NGX_HASH_READONLY_KEY);

To build the hash table, call the ngx_hash_init(hinit, key_names, nelts) function:

ngx_hash_init(&hash, foo_keys.keys.elts, foo_keys.keys.nelts);

The function fails if max_size or bucket_size parameters are not big enough.

When the hash is built, use the ngx_hash_find(hash, key, name, len) function to look up elements:

my_data_t   *data;
ngx_uint_t   key;

key = ngx_hash_key(k1.data, k1.len);

data = ngx_hash_find(&foo_hash, key, k1.data, k1.len);
if (data == NULL) {
    /* key not found */
}

Wildcard matching

To create a hash that works with wildcards, use the ngx_hash_combined_t type. It includes the hash type described above and has two additional keys arrays: dns_wc_head and dns_wc_tail. The initialization of basic properties is similar to a regular hash:

ngx_hash_init_t      hash
ngx_hash_combined_t  foo_hash;

hash.hash = &foo_hash.hash;
hash.key = ...;

It is possible to add wildcard keys using the NGX_HASH_WILDCARD_KEY flag:

/* k1 = ".example.org"; */
/* k2 = "foo.*";        */
ngx_hash_add_key(&foo_keys, &k1, &data1, NGX_HASH_WILDCARD_KEY);
ngx_hash_add_key(&foo_keys, &k2, &data2, NGX_HASH_WILDCARD_KEY);

The function recognizes wildcards and adds keys into the corresponding arrays. Please refer to the map module documentation for the description of the wildcard syntax and the matching algorithm.

Depending on the contents of added keys, you may need to initialize up to three key arrays: one for exact matching (described above), and two more to enable matching starting from the head or tail of a string:

if (foo_keys.dns_wc_head.nelts) {

    ngx_qsort(foo_keys.dns_wc_head.elts,
              (size_t) foo_keys.dns_wc_head.nelts,
              sizeof(ngx_hash_key_t),
              cmp_dns_wildcards);

    hash.hash = NULL;
    hash.temp_pool = pool;

    if (ngx_hash_wildcard_init(&hash, foo_keys.dns_wc_head.elts,
                               foo_keys.dns_wc_head.nelts)
        != NGX_OK)
    {
        return NGX_ERROR;
    }

    foo_hash.wc_head = (ngx_hash_wildcard_t *) hash.hash;
}

The keys array needs to be sorted, and initialization results must be added to the combined hash. The initialization of dns_wc_tail array is done similarly.

The lookup in a combined hash is handled by the ngx_hash_find_combined(chash, key, name, len):

/* key = "bar.example.org"; — will match ".example.org" */
/* key = "foo.example.com"; — will match "foo.*"        */

hkey = ngx_hash_key(key.data, key.len);
res = ngx_hash_find_combined(&foo_hash, hkey, key.data, key.len);

Memory management

Heap

To allocate memory from system heap, use the following functions:

• ngx_alloc(size, log) — Allocate memory from system heap. This is a wrapper around malloc() with logging support. Allocation error and debugging information is logged to log.
• ngx_calloc(size, log) — Allocate memory from system heap like ngx_alloc(), but fill memory with zeros after allocation.
• ngx_memalign(alignment, size, log) — Allocate aligned memory from system heap. This is a wrapper around posix_memalign() on those platforms that provide that function. Otherwise implementation falls back to ngx_alloc() which provides maximum alignment.
• ngx_free(p) — Free allocated memory. This is a wrapper around free()

Pool

Most nginx allocations are done in pools. Memory allocated in an nginx pool is freed automatically when the pool is destroyed. This provides good allocation performance and makes memory control easy.

A pool internally allocates objects in continuous blocks of memory. Once a block is full, a new one is allocated and added to the pool memory block list. When the requested allocation is too large to fit into a block, the request is forwarded to the system allocator and the returned pointer is stored in the pool for further deallocation.

The type for nginx pools is ngx_pool_t. The following operations are supported:

• ngx_create_pool(size, log) — Create a pool with specified block size. The pool object returned is allocated in the pool as well.
• ngx_destroy_pool(pool) — Free all pool memory, including the pool object itself.
• ngx_palloc(pool, size) — Allocate aligned memory from the specified pool.
• ngx_pcalloc(pool, size) — Allocate aligned memory from the specified pool and fill it with zeroes.
• ngx_pnalloc(pool, size) — Allocate unaligned memory from the specified pool. Mostly used for allocating strings.
• ngx_pfree(pool, p) — Free memory that was previously allocated in the specified pool. Only allocations that result from requests forwarded to the system allocator can be freed.

u_char      *p;
ngx_str_t   *s;
ngx_pool_t  *pool;

pool = ngx_create_pool(1024, log);
if (pool == NULL) { /* error */ }

s = ngx_palloc(pool, sizeof(ngx_str_t));
if (s == NULL) { /* error */ }
ngx_str_set(s, "foo");

p = ngx_pnalloc(pool, 3);
if (p == NULL) { /* error */ }
ngx_memcpy(p, "foo", 3);

Chain links (ngx_chain_t) are actively used in nginx, so the nginx pool implementation provides a way to reuse them. The chain field of ngx_pool_t keeps a list of previously allocated links ready for reuse. For efficient allocation of a chain link in a pool, use the ngx_alloc_chain_link(pool) function. This function looks up a free chain link in the pool list and allocates a new chain link if the pool list is empty. To free a link, call the ngx_free_chain(pool, cl) function.

Cleanup handlers can be registered in a pool. A cleanup handler is a callback with an argument which is called when pool is destroyed. A pool is usually tied to a specific nginx object (like an HTTP request) and is destroyed when the object reaches the end of its lifetime. Registering a pool cleanup is a convenient way to release resources, close file descriptors or make final adjustments to the shared data associated with the main object.

To register a pool cleanup, call ngx_pool_cleanup_add(pool, size), which returns a ngx_pool_cleanup_t pointer to be filled in by the caller. Use the size argument to allocate context for the cleanup handler.

ngx_pool_cleanup_t  *cln;

cln = ngx_pool_cleanup_add(pool, 0);
if (cln == NULL) { /* error */ }

cln->handler = ngx_my_cleanup;
cln->data = "foo";

...

static void
ngx_my_cleanup(void *data)
{
    u_char  *msg = data;

    ngx_do_smth(msg);
}

Shared memory

Shared memory is used by nginx to share common data between processes. The ngx_shared_memory_add(cf, name, size, tag) function adds a new shared memory entry ngx_shm_zone_t to a cycle. The function receives the name and size of the zone. Each shared zone must have a unique name. If a shared zone entry with the provided name and tag already exists, the existing zone entry is reused. The function fails with an error if an existing entry with the same name has a different tag. Usually, the address of the module structure is passed as tag, making it possible to reuse shared zones by name within one nginx module.

The shared memory entry structure ngx_shm_zone_t has the following fields:

• init — Initialization callback, called after the shared zone is mapped to actual memory
• data — Data context, used to pass arbitrary data to the init callback
• noreuse — Flag that disables reuse of a shared zone from the old cycle
• tag — Shared zone tag
• shm — Platform-specific object of type ngx_shm_t, having at least the following fields:
  • addr — Mapped shared memory address, initially NULL
  • size — Shared memory size
  • name — Shared memory name
  • log — Shared memory log
  • exists — Flag that indicates shared memory was inherited from the master process (Windows-specific)

Shared zone entries are mapped to actual memory in ngx_init_cycle() after the configuration is parsed. On POSIX systems, the mmap() syscall is used to create the shared anonymous mapping. On Windows, the CreateFileMapping()/ MapViewOfFileEx() pair is used.

For allocating in shared memory, nginx provides the slab pool ngx_slab_pool_t type. A slab pool for allocating memory is automatically created in each nginx shared zone. The pool is located in the beginning of the shared zone and can be accessed by the expression (ngx_slab_pool_t *) shm_zone->shm.addr. To allocate memory in a shared zone, call either ngx_slab_alloc(pool, size) or ngx_slab_calloc(pool, size). To free memory, call ngx_slab_free(pool, p).

Slab pool divides all shared zone into pages. Each page is used for allocating objects of the same size. The specified size must be a power of 2, and greater than the minimum size of 8 bytes. Nonconforming values are rounded up. A bitmask for each page tracks which blocks are in use and which are free for allocation. For sizes greater than a half page (which is usually 2048 bytes), allocation is done an entire page at a time

To protect data in shared memory from concurrent access, use the mutex available in the mutex field of ngx_slab_pool_t. A mutex is most commonly used by the slab pool while allocating and freeing memory, but it can be used to protect any other user data structures allocated in the shared zone. To lock or unlock a mutex, call ngx_shmtx_lock(&shpool->mutex) or ngx_shmtx_unlock(&shpool->mutex) respectively.

ngx_str_t        name;
ngx_foo_ctx_t   *ctx;
ngx_shm_zone_t  *shm_zone;

ngx_str_set(&name, "foo");

/* allocate shared zone context */
ctx = ngx_pcalloc(cf->pool, sizeof(ngx_foo_ctx_t));
if (ctx == NULL) {
    /* error */
}

/* add an entry for 64k shared zone */
shm_zone = ngx_shared_memory_add(cf, &name, 65536, &ngx_foo_module);
if (shm_zone == NULL) {
    /* error */
}

/* register init callback and context */
shm_zone->init = ngx_foo_init_zone;
shm_zone->data = ctx;


...


static ngx_int_t
ngx_foo_init_zone(ngx_shm_zone_t *shm_zone, void *data)
{
    ngx_foo_ctx_t  *octx = data;

    size_t            len;
    ngx_foo_ctx_t    *ctx;
    ngx_slab_pool_t  *shpool;

    value = shm_zone->data;

    if (octx) {
        /* reusing a shared zone from old cycle */
        ctx->value = octx->value;
        return NGX_OK;
    }

    shpool = (ngx_slab_pool_t *) shm_zone->shm.addr;

    if (shm_zone->shm.exists) {
        /* initialize shared zone context in Windows nginx worker */
        ctx->value = shpool->data;
        return NGX_OK;
    }

    /* initialize shared zone */

    ctx->value = ngx_slab_alloc(shpool, sizeof(ngx_uint_t));
    if (ctx->value == NULL) {
        return NGX_ERROR;
    }

    shpool->data = ctx->value;

    return NGX_OK;
}

Logging

For logging nginx uses ngx_log_t objects. The nginx logger supports several types of output:

• stderr — Logging to standard error (stderr)
• file — Logging to a file
• syslog — Logging to syslog
• memory — Logging to internal memory storage for development purposes; the memory can be accessed later with a debugger

A logger instance can be a chain of loggers, linked to each other with the next field. In this case, each message is written to all loggers in the chain.

For each logger, a severity level controls which messages are written to the log (only events assigned that level or higher are logged). The following severity levels are supported:

• NGX_LOG_EMERG
• NGX_LOG_ALERT
• NGX_LOG_CRIT
• NGX_LOG_ERR
• NGX_LOG_WARN
• NGX_LOG_NOTICE
• NGX_LOG_INFO
• NGX_LOG_DEBUG

For debug logging, the debug mask is checked as well. The debug masks are:

• NGX_LOG_DEBUG_CORE
• NGX_LOG_DEBUG_ALLOC
• NGX_LOG_DEBUG_MUTEX
• NGX_LOG_DEBUG_EVENT
• NGX_LOG_DEBUG_HTTP
• NGX_LOG_DEBUG_MAIL
• NGX_LOG_DEBUG_STREAM

Normally, loggers are created by existing nginx code from error_log directives and are available at nearly every stage of processing in cycle, configuration, client connection and other objects.

Nginx provides the following logging macros:

• ngx_log_error(level, log, err, fmt, ...) — Error logging
• ngx_log_debug0(level, log, err, fmt), ngx_log_debug1(level, log, err, fmt, arg1) etc — Debug logging with up to eight supported formatting arguments

A log message is formatted in a buffer of size NGX_MAX_ERROR_STR (currently, 2048 bytes) on stack. The message is prepended with the severity level, process ID (PID), connection ID (stored in log->connection), and the system error text. For non-debug messages log->handler is called as well to prepend more specific information to the log message. HTTP module sets ngx_http_log_error() function as log handler to log client and server addresses, current action (stored in log->action), client request line, server name etc.

/* specify what is currently done */
log->action = "sending mp4 to client”;

/* error and debug log */
ngx_log_error(NGX_LOG_INFO, c->log, 0, "client prematurely
              closed connection”);

ngx_log_debug2(NGX_LOG_DEBUG_HTTP, mp4->file.log, 0,
               "mp4 start:%ui, length:%ui”, mp4->start, mp4->length);

The example above results in log entries like these:

2016/09/16 22:08:52 [info] 17445#0: *1 client prematurely closed connection while
sending mp4 to client, client: 127.0.0.1, server: , request: "GET /file.mp4 HTTP/1.1”
2016/09/16 23:28:33 [debug] 22140#0: *1 mp4 start:0, length:10000

Cycle

A cycle object stores the nginx runtime context created from a specific configuration. Its type is ngx_cycle_t. The current cycle is referenced by the ngx_cycle global variable and inherited by nginx workers as they start. Each time the nginx configuration is reloaded, a new cycle is created from the new nginx configuration; the old cycle is usually deleted after the new one is successfully created.

A cycle is created by the ngx_init_cycle() function, which takes the previous cycle as its argument. The function locates the previous cycle's configuration file and inherits as many resources as possible from the previous cycle. A placeholder cycle called "init cycle" is created as nginx start, then is replaced by an actual cycle built from configuration.

Members of the cycle include:

• pool — Cycle pool. Created for each new cycle.
• log — Cycle log. Initially inherited from the old cycle, it is set to point to new_log after the configuration is read.
• new_log — Cycle log, created by the configuration. It's affected by the root-scope error_log directive.
• connections, connection_n — Array of connections of type ngx_connection_t, created by the event module while initializing each nginx worker. The worker_connections directive in the nginx configuration sets the number of connections connection_n.
• free_connections, free_connection_n — List and number of currently available connections. If no connections are available, an nginx worker refuses to accept new clients or connect to upstream servers.
• files, files_n — Array for mapping file descriptors to nginx connections. This mapping is used by the event modules, having the NGX_USE_FD_EVENT flag (currently, it's poll and devpoll).
• conf_ctx — Array of core module configurations. The configurations are created and filled during reading of nginx configuration files.
• modules, modules_n — Array of modules of type ngx_module_t, both static and dynamic, loaded by the current configuration.
• listening — Array of listening objects of type ngx_listening_t. Listening objects are normally added by the listen directive of different modules which call the ngx_create_listening() function. Listen sockets are created based on the listening objects.
• paths — Array of paths of type ngx_path_t. Paths are added by calling the function ngx_add_path() from modules which are going to operate on certain directories. These directories are created by nginx after reading configuration, if missing. Moreover, two handlers can be added for each path:
  • path loader — Executes only once in 60 seconds after starting or reloading nginx. Normally, the loader reads the directory and stores data in nginx shared memory. The handler is called from the dedicated nginx process “nginx cache loader”.
  • path manager — Executes periodically. Normally, the manager removes old files from the directory and updates nginx memory to reflect the changes. The handler is called from the dedicated “nginx cache manager” process.
• open_files — List of open file objects of type ngx_open_file_t, which are created by calling the function ngx_conf_open_file(). Currently, nginx uses this kind of open files for logging. After reading the configuration, nginx opens all files in the open_files list and stores each file descriptor in the object's fd field. The files are opened in append mode and are created if missing. The files in the list are reopened by nginx workers upon receiving the reopen signal (most often USR1). In this case the descriptor in the fd field is changed to a new value.
• shared_memory — List of shared memory zones, each added by calling the ngx_shared_memory_add() function. Shared zones are mapped to the same address range in all nginx processes and are used to share common data, for example the HTTP cache in-memory tree.

Buffer

For input/output operations, nginx provides the buffer type ngx_buf_t. Normally, it's used to hold data to be written to a destination or read from a source. A buffer can reference data in memory or in a file and it's technically possible for a buffer to reference both at the same time. Memory for the buffer is allocated separately and is not related to the buffer structure ngx_buf_t.

The ngx_buf_t structure has the following fields:

• start, end — The boundaries of the memory block allocated for the buffer.
• pos, last — The boundaries of the memory buffer; normally a subrange of start .. end.
• file_pos, file_last — The boundaries of a file buffer, expressed as offsets from the beginning of the file.
• tag — Unique value used to distinguish buffers; created by different nginx modules, usually for the purpose of buffer reuse.
• file — File object.
• temporary — Flag indicating that the buffer references writable memory.
• memory — Flag indicating that the buffer references read-only memory.
• in_file — Flag indicating that the buffer references data in a file.
• flush — Flag indicating that all data prior to the buffer need to be flushed.
• recycled — Flag indicating that the buffer can be reused and needs to be consumed as soon as possible.
• sync — Flag indicating that the buffer carries no data or special signal like flush or last_buf. By default nginx considers such buffers an error condition, but this flag tells nginx to skip the error check.
• last_buf — Flag indicating that the buffer is the last in output.
• last_in_chain — Flag indicating that there are no more data buffers in a request or subrequest.
• shadow — Reference to another ("shadow") buffer related to the current buffer, usually in the sense that the buffer uses data from the shadow. When the buffer is consumed, the shadow buffer is normally also marked as consumed.
• last_shadow — Flag indicating that the buffer is the last one that references a particular shadow buffer.
• temp_file — Flag indicating that the buffer is in a temporary file.

For input and output operations buffers are linked in chains. A chain is a sequence of chain links of type ngx_chain_t, defined as follows:

typedef struct ngx_chain_s  ngx_chain_t;

struct ngx_chain_s {
    ngx_buf_t    *buf;
    ngx_chain_t  *next;
};

Each chain link keeps a reference to its buffer and a reference to the next chain link.

An example of using buffers and chains:

ngx_chain_t *
ngx_get_my_chain(ngx_pool_t *pool)
{
    ngx_buf_t    *b;
    ngx_chain_t  *out, *cl, **ll;

    /* first buf */
    cl = ngx_alloc_chain_link(pool);
    if (cl == NULL) { /* error */ }

    b = ngx_calloc_buf(pool);
    if (b == NULL) { /* error */ }

    b->start = (u_char *) "foo";
    b->pos = b->start;
    b->end = b->start + 3;
    b->last = b->end;
    b->memory = 1; /* read-only memory */

    cl->buf = b;
    out = cl;
    ll = &cl->next;

    /* second buf */
    cl = ngx_alloc_chain_link(pool);
    if (cl == NULL) { /* error */ }

    b = ngx_create_temp_buf(pool, 3);
    if (b == NULL) { /* error */ }

    b->last = ngx_cpymem(b->last, "foo", 3);

    cl->buf = b;
    cl->next = NULL;
    *ll = cl;

    return out;
}

Networking

Connection

The connection type ngx_connection_t is a wrapper around a socket descriptor. It includes the following fields:

• fd — Socket descriptor
• data — Arbitrary connection context. Normally, it is a pointer to a higher-level object built on top of the connection, such as an HTTP request or a Stream session.
• read, write — Read and write events for the connection.
• recv, send, recv_chain, send_chain — I/O operations for the connection.
• pool — Connection pool.
• log — Connection log.
• sockaddr, socklen, addr_text — Remote socket address in binary and text forms.
• local_sockaddr, local_socklen — Local socket address in binary form. Initially, these fields are empty. Use the ngx_connection_local_sockaddr() function to get the local socket address.
• proxy_protocol_addr, proxy_protocol_port - PROXY protocol client address and port, if the PROXY protocol is enabled for the connection.
• ssl — SSL context for the connection.
• reusable — Flag indicating the connection is in a state that makes it eligible for reuse.
• close — Flag indicating that the connection is being reused and needs to be closed.

An nginx connection can transparently encapsulate the SSL layer. In this case the connection's ssl field holds a pointer to an ngx_ssl_connection_t structure, keeping all SSL-related data for the connection, including SSL_CTX and SSL. The recv, send, recv_chain, and send_chain handlers are set to SSL-enabled functions as well.

The worker_connections directive in the nginx configuration limits the number of connections per nginx worker. All connection structures are precreated when a worker starts and stored in the connections field of the cycle object. To retrieve a connection structure, use the ngx_get_connection(s, log) function. It takes as its s argument a socket descriptor, which needs to be wrapped in a connection structure.

Because the number of connections per worker is limited, nginx provides a way to grab connections that are currently in use. To enable or disable reuse of a connection, call the ngx_reusable_connection(c, reusable) function. Calling ngx_reusable_connection(c, 1) sets the reuse flag in the connection structure and inserts the connection into the reusable_connections_queue of the cycle. Whenever ngx_get_connection() finds out there are no available connections in the cycle's free_connections list, it calls ngx_drain_connections() to release a specific number of reusable connections. For each such connection, the close flag is set and its read handler is called which is supposed to free the connection by calling ngx_close_connection(c) and make it available for reuse. To exit the state when a connection can be reused ngx_reusable_connection(c, 0) is called. HTTP client connections are an example of reusable connections in nginx; they are marked as reusable until the first request byte is received from the client.

Events

Event

Event object ngx_event_t in nginx provides a mechanism for notification that a specific event has occurred.

Fields in ngx_event_t include:

• data — Arbitrary event context used in event handlers, usually as pointer to a connection related to the event.
• handler — Callback function to be invoked when the event happens.
• write — Flag indicating a write event. Absence of the flag indicates a read event.
• active — Flag indicating that the event is registered for receiving I/O notifications, normally from notification mechanisms like epoll, kqueue, poll.
• ready — Flag indicating that the event has received an I/O notification.
• delayed — Flag indicating that I/O is delayed due to rate limiting.
• timer — Red-black tree node for inserting the event into the timer tree.
• timer_set — Flag indicating that the event timer is set and not yet expired.
• timedout — Flag indicating that the event timer has expired.
• eof — Flag indicating that EOF occurred while reading data.
• pending_eof — Flag indicating that EOF is pending on the socket, even though there may be some data available before it. The flag is delivered via the EPOLLRDHUP epoll event or EV_EOF kqueue flag.
• error — Flag indicating that an error occurred during reading (for a read event) or writing (for a write event).
• cancelable — Timer event flag indicating that the event should be ignored while shutting down the worker. Graceful worker shutdown is delayed until there are no non-cancelable timer events scheduled.
• posted — Flag indicating that the event is posted to a queue.
• queue — Queue node for posting the event to a queue.

I/O events

Each connection obtained by calling the ngx_get_connection() function has two attached events, c->read and c->write, which are used for receiving notification that the socket is ready for reading or writing. All such events operate in Edge-Triggered mode, meaning that they only trigger notifications when the state of the socket changes. For example, doing a partial read on a socket does not make nginx deliver a repeated read notification until more data arrives on the socket. Even when the underlying I/O notification mechanism is essentially Level-Triggered (poll, select etc), nginx converts the notifications to Edge-Triggered. To make nginx event notifications consistent across all notifications systems on different platforms, the functions ngx_handle_read_event(rev, flags) and ngx_handle_write_event(wev, lowat) must be called after handling an I/O socket notification or calling any I/O functions on that socket. Normally, the functions are called once at the end of each read or write event handler.

Timer events

An event can be set to send a notification when a timeout expires. The function ngx_add_timer(ev, timer) sets a timeout for an event, ngx_del_timer(ev) deletes a previously set timeout. The global timeout red-black tree ngx_event_timer_rbtree stores all timeouts currently set. The key in the tree is of type ngx_msec_t and is the time when the event expires, expressed in milliseconds since the midnight on of January 1, 1970 modulus ngx_msec_t max value. The tree structure enables fast insertion and deletion operations, as well as access to the nearest timeouts, which nginx uses to find out how long to wait for I/O events and for expiring timeout events.

Posted events

An event can be posted which means that its handler will be called at some point later within the current event loop iteration. Posting events is a good practice for simplifying code and escaping stack overflows. Posted events are held in a post queue. The ngx_post_event(ev, q) mscro posts the event ev to the post queue q. The ngx_delete_posted_event(ev) macro deletes the event ev from the queue it's currently posted in. Normally, events are posted to the ngx_posted_events queue, which is processed late in the event loop — after all I/O and timer events are already handled. The function ngx_event_process_posted() is called to process an event queue. It calls event handlers until the queue is not empty. This means that a posted event handler can post more events to be processed within the current event loop iteration.

An example:

void
ngx_my_connection_read(ngx_connection_t *c)
{
    ngx_event_t  *rev;

    rev = c->read;

    ngx_add_timer(rev, 1000);

    rev->handler = ngx_my_read_handler;

    ngx_my_read(rev);
}


void
ngx_my_read_handler(ngx_event_t *rev)
{
    ssize_t            n;
    ngx_connection_t  *c;
    u_char             buf[256];

    if (rev->timedout) { /* timeout expired */ }

    c = rev->data;

    while (rev->ready) {
        n = c->recv(c, buf, sizeof(buf));

        if (n == NGX_AGAIN) {
            break;
        }

        if (n == NGX_ERROR) { /* error */ }

        /* process buf */
    }

    if (ngx_handle_read_event(rev, 0) != NGX_OK) { /* error */ }
}

Event loop

Except for the nginx master process, all nginx processes do I/O and so have an event loop. (The nginx master process instead spends most of its time in the sigsuspend() call waiting for signals to arrive.) The nginx event loop is implemented in the ngx_process_events_and_timers() function, which is called repeatedly until the process exits.

The event loop has the following stages:

• Find the timeout that is closest to expiring, by calling ngx_event_find_timer(). This function finds the leftmost node in the timer tree and returns the number of milliseconds until the node expires.
• Process I/O events by calling a handler, specific to the event notification mechanism, chosen by nginx configuration. This handler waits for at least one I/O event to happen, but only until the next timeout expires. When a read or write event occurs, the ready flag is set and the event's handler is called. For Linux, the ngx_epoll_process_events() handler is normally used, which calls epoll_wait() to wait for I/O events.
• Expire timers by calling ngx_event_expire_timers(). The timer tree is iterated from the leftmost element to the right until an unexpired timeout is found. For each expired node the timedout event flag is set, the timer_set flag is reset, and the event handler is called
• Process posted events by calling ngx_event_process_posted(). The function repeatedly removes the first element from the posted events queue and calls the element's handler, until the queue is empty.

All nginx processes handle signals as well. Signal handlers only set global variables which are checked after the ngx_process_events_and_timers() call.

Processes

There are several types of processes in nginx. The type of current process is kept in the ngx_process global variable:

• NGX_PROCESS_MASTER — the master process runs the ngx_master_process_cycle() function. Master process does not have any I/O and responds only to signals. It reads configuration, creates cycles, starts and controls child processes

• NGX_PROCESS_WORKER — the worker process runs the ngx_worker_process_cycle() function. Worker processes are started by master and handle client connections. They also respond to signals and channel commands, sent from master

• NGX_PROCESS_SINGLE — single process is the only type of processes which exist in the master_process off mode. The cycle function for this process is ngx_single_process_cycle(). This process creates cycles and handles client connections

• NGX_PROCESS_HELPER — currently, there are two types of helper processes: cache manager and cache loader. Both of them share the same cycle function ngx_cache_manager_process_cycle().

All nginx processes handle the following signals:

• NGX_SHUTDOWN_SIGNAL (SIGQUIT) — graceful shutdown. Upon receiving this signal master process sends shutdown signal to all child processes. When no child processes are left, master destroys cycle pool and exits. A worker process which received this signal, closes all listening sockets and waits until timeout tree becomes empty, then destroys cycle pool and exits. A cache manager process exits right after receiving this signal. The variable ngx_quit is set to one after receiving this signal and immediately reset after being processed. The variable ngx_exiting is set to one when worker process is in shutdown state

• NGX_TERMINATE_SIGNAL (SIGTERM) - terminate. Upon receiving this signal master process sends terminate signal to all child processes. If child processes do not exit in 1 second, they are killed with the SIGKILL signal. When no child processes are left, master process destroys cycle pool and exits. A worker or cache manager process which received this signal destroys cycle pool and exits. The variable ngx_terminate is set to one after receiving this signal

• NGX_NOACCEPT_SIGNAL (SIGWINCH) - gracefully shut down worker processes

• NGX_RECONFIGURE_SIGNAL (SIGHUP) - reconfigure. Upon receiving this signal master process creates a new cycle from configuration file. If the new cycle was created successfully, the old cycle is deleted and new child processes are started. Meanwhile, the old processes receive the shutdown signal. In single-process mode, nginx creates a new cycle as well, but keeps the old one until all clients, tied to the old cycle, are gone. Worker and helper processes ignore this signal

• NGX_REOPEN_SIGNAL (SIGUSR1) — reopen files. Master process passes this signal to workers. Worker processes reopen all open_files from the cycle

• NGX_CHANGEBIN_SIGNAL (SIGUSR2) — change nginx binary. Master process starts a new nginx binary and passes there a list of all listen sockets. The list is passed in the environment variable “NGINX” in text format, where descriptor numbers separated with semicolons. A new nginx instance reads that variable and adds the sockets to its init cycle. Other processes ignore this signal

While all nginx worker processes are able to receive and properly handle POSIX signals, master process normally does not pass any signals to workers and helpers with the standard kill() syscall. Instead, nginx uses inter-process channels which allow sending messages between all nginx processes. Currently, however, messages are only sent from master to its children. Those messages carry the same signals. The channels are socketpairs with their ends in different processes.

When running nginx binary, several values can be specified next to -s parameter. Those values are stop, quit, reopen, reload. They are converted to signals NGX_TERMINATE_SIGNAL, NGX_SHUTDOWN_SIGNAL, NGX_REOPEN_SIGNAL and NGX_RECONFIGURE_SIGNAL and sent to the nginx master process, whose pid is read from nginx pid file.

Threads

It is possible to offload tasks that would otherwise block nginx worker process into a separate thread. For example, nginx may be configured to use threads to perform file I/O. Another example is using a library that doesn't have asynchronous interface and thus cannot be normally used with nginx. Keep in mind that threads interface is a helper for existing asynchronous approach in processing client connections, and by no means a replacement.

To deal with synchronization the following wrappers over pthreads primitives are available:

typedef pthread_mutex_t  ngx_thread_mutex_t;

ngx_int_t ngx_thread_mutex_create(ngx_thread_mutex_t *mtx, ngx_log_t *log);
ngx_int_t ngx_thread_mutex_destroy(ngx_thread_mutex_t *mtx, ngx_log_t *log);
ngx_int_t ngx_thread_mutex_lock(ngx_thread_mutex_t *mtx, ngx_log_t *log);
ngx_int_t ngx_thread_mutex_unlock(ngx_thread_mutex_t *mtx, ngx_log_t *log);

typedef pthread_cond_t  ngx_thread_cond_t;

ngx_int_t ngx_thread_cond_create(ngx_thread_cond_t *cond, ngx_log_t *log);
ngx_int_t ngx_thread_cond_destroy(ngx_thread_cond_t *cond, ngx_log_t *log);
ngx_int_t ngx_thread_cond_signal(ngx_thread_cond_t *cond, ngx_log_t *log);
ngx_int_t ngx_thread_cond_wait(ngx_thread_cond_t *cond, ngx_thread_mutex_t *mtx,
    ngx_log_t *log);

Instead of creating a new thread for each task, nginx implements a thread_pool strategy. Multiple thread pools may be configured intended for different purposes (for example, performing I/O on different sets of disks). Each thread pool is created on start and contains a limited number of threads that process a queue of tasks. When a task is completed, a predefined completion handler is called.

The src/core/ngx_thread_pool.h header file contains corresponding definitions:

struct ngx_thread_task_s {
    ngx_thread_task_t   *next;
    ngx_uint_t           id;
    void                *ctx;
    void               (*handler)(void *data, ngx_log_t *log);
    ngx_event_t          event;
};

typedef struct ngx_thread_pool_s  ngx_thread_pool_t;

ngx_thread_pool_t *ngx_thread_pool_add(ngx_conf_t *cf, ngx_str_t *name);
ngx_thread_pool_t *ngx_thread_pool_get(ngx_cycle_t *cycle, ngx_str_t *name);

ngx_thread_task_t *ngx_thread_task_alloc(ngx_pool_t *pool, size_t size);
ngx_int_t ngx_thread_task_post(ngx_thread_pool_t *tp, ngx_thread_task_t *task);

At configuration time, a module willing to use threads has to obtain a reference to thread pool by calling ngx_thread_pool_add(cf, name) which will either create a new thread pool with given name or return a reference to an existing one if a pool with such name already exists.

At runtime, the ngx_thread_task_post(tp, task) function is used to add a task into a queue of a thread pool tp. The ngx_thread_task_t structure contains all necessary to execute user function in thread, pass parameters and setup completion handler:

typedef struct {
    int    foo;
} my_thread_ctx_t;


static void
my_thread_func(void *data, ngx_log_t *log)
{
    my_thread_ctx_t *ctx = data;

    /* this function is executed in a separate thread */
}


static void
my_thread_completion(ngx_event_t *ev)
{
    my_thread_ctx_t *ctx = ev->data;

    /* executed in nginx event loop */
}


ngx_int_t
my_task_offload(my_conf_t *conf)
{
    my_thread_ctx_t    *ctx;
    ngx_thread_task_t  *task;

    task = ngx_thread_task_alloc(conf->pool, sizeof(my_thread_ctx_t));
    if (task == NULL) {
        return NGX_ERROR;
    }

    ctx = task->ctx;

    ctx->foo = 42;

    task->handler = my_thread_func;
    task->event.handler = my_thread_completion;
    task->event.data = ctx;

    if (ngx_thread_task_post(conf->thread_pool, task) != NGX_OK) {
        return NGX_ERROR;
    }

    return NGX_OK;
}

Modules

Adding new modules

The standalone nginx module resides in a separate directory that contains at least two files: config and a file with the module source. The first file contains all information needed for nginx to integrate the module, for example:

ngx_module_type=CORE
ngx_module_name=ngx_foo_module
ngx_module_srcs="$ngx_addon_dir/ngx_foo_module.c"

. auto/module

ngx_addon_name=$ngx_module_name

The file is a POSIX shell script and it can set (or access) the following variables:

• ngx_module_type — the type of module to build. Possible options are CORE, HTTP, HTTP_FILTER, HTTP_INIT_FILTER, HTTP_AUX_FILTER, MAIL, STREAM, or MISC
• ngx_module_name — the name of the module. A whitespace separated values list is accepted and may be used to build multiple modules from a single set of source files. The first name indicates the name of the output binary for a dynamic module. The names in this list should match the names used in the module.
• ngx_addon_name — supplies the name of the module in the console output text of the configure script.
• ngx_module_srcs — a whitespace separated list of source files used to compile the module. The $ngx_addon_dir variable can be used as a placeholder for the path of the module source.
• ngx_module_incs — include paths required to build the module
• ngx_module_deps — a list of module's header files.
• ngx_module_libs — a list of libraries to link with the module. For example, libpthread would be linked using ngx_module_libs=-lpthread. The following macros can be used to link against the same libraries as nginx: LIBXSLT, LIBGD, GEOIP, PCRE, OPENSSL, MD5, SHA1, ZLIB, and PERL
• ngx_module_link — set by the build system to DYNAMIC for a dynamic module or ADDON for a static module and used to perform different actions depending on linking type.
• ngx_module_order — sets the load order for the module which is useful for HTTP_FILTER and HTTP_AUX_FILTER module types. The order is stored in a reverse list.

  The ngx_http_copy_filter_module is near the bottom of the list so is one of the first to be executed. This reads the data for other filters. Near the top of the list is ngx_http_write_filter_module which writes the data out and is one of the last to be executed.

  The format for this option is typically the current module’s name followed by a whitespace separated list of modules to insert before, and therefore execute after. The module will be inserted before the last module in the list that is found to be currently loaded.

  By default for filter modules this is set to “ngx_http_copy_filter” which will insert the module before the copy filter in the list and therefore will execute after the copy filter. For other module types the default is empty.

A module can be added to nginx by means of the configure script using --add-module=/path/to/module for static compilation and --add-dynamic-module=/path/to/module for dynamic compilation.

Core modules

Modules are building blocks of nginx, and most of its functionality is implemented as modules. The module source file must contain a global variable of ngx_module_t type which is defined as follows:

struct ngx_module_s {

    /* private part is omitted */

    void                 *ctx;
    ngx_command_t        *commands;
    ngx_uint_t            type;

    ngx_int_t           (*init_master)(ngx_log_t *log);

    ngx_int_t           (*init_module)(ngx_cycle_t *cycle);

    ngx_int_t           (*init_process)(ngx_cycle_t *cycle);
    ngx_int_t           (*init_thread)(ngx_cycle_t *cycle);
    void                (*exit_thread)(ngx_cycle_t *cycle);
    void                (*exit_process)(ngx_cycle_t *cycle);

    void                (*exit_master)(ngx_cycle_t *cycle);

    /* stubs for future extensions are omitted */
};

The omitted private part includes module version, signature and is filled using the predefined macro NGX_MODULE_V1.

Each module keeps its private data in the ctx field, recognizes specific configuration directives, specified in the commands array, and may be invoked at certain stages of nginx lifecycle. The module lifecycle consists of the following events:

• Configuration directive handlers are called as they appear in configuration files in the context of the master process
• The init_module handler is called in the context of the master process after the configuration is parsed successfully
• The master process creates worker process(es) and init_process handler is called in each of them
• When a worker process receives the shutdown command from master, it invokes the exit_process handler
• The master process calls the exit_master handler before exiting.

init_module handler may be called multiple times in the master process if the configuration reload is requested.

The init_master, init_thread and exit_thread handlers are not implemented at the moment; Threads in nginx are only used as supplementary I/O facility with its own API and init_master handler looks unnecessary.

The module type defines what exactly is stored in the ctx field. There are several types of modules:

• NGX_CORE_MODULE
• NGX_EVENT_MODULE
• NGX_HTTP_MODULE
• NGX_MAIL_MODULE
• NGX_STREAM_MODULE

The NGX_CORE_MODULE is the most basic and thus the most generic and most low-level type of module. Other module types are implemented on top of it and provide more convenient way to deal with corresponding problem domains, like handling events or http requests.

The examples of core modules are ngx_core_module, ngx_errlog_module, ngx_regex_module, ngx_thread_pool_module, ngx_openssl_module modules and, of course, http, stream, mail and event modules itself. The context of a core module is defined as:

typedef struct {
    ngx_str_t             name;
    void               *(*create_conf)(ngx_cycle_t *cycle);
    char               *(*init_conf)(ngx_cycle_t *cycle, void *conf);
} ngx_core_module_t;

where the name is a string with a module name for convenience, create_conf and init_conf are pointers to functions that create and initialize module configuration correspondingly. For core modules, nginx will call create_conf before parsing a new configuration and init_conf after all configuration was parsed successfully. The typical create_conf function allocates memory for the configuration and sets default values. The init_conf deals with known configuration and thus may perform sanity checks and complete initialization.

For example, the simplistic ngx_foo_module can look like this:

/*
 * Copyright (C) Author.
 */


#include <ngx_config.h>
#include <ngx_core.h>


typedef struct {
    ngx_flag_t  enable;
} ngx_foo_conf_t;


static void *ngx_foo_create_conf(ngx_cycle_t *cycle);
static char *ngx_foo_init_conf(ngx_cycle_t *cycle, void *conf);

static char *ngx_foo_enable(ngx_conf_t *cf, void *post, void *data);
static ngx_conf_post_t  ngx_foo_enable_post = { ngx_foo_enable };


static ngx_command_t  ngx_foo_commands[] = {

    { ngx_string("foo_enabled"),
      NGX_MAIN_CONF|NGX_DIRECT_CONF|NGX_CONF_FLAG,
      ngx_conf_set_flag_slot,
      0,
      offsetof(ngx_foo_conf_t, enable),
      &ngx_foo_enable_post },

      ngx_null_command
};


static ngx_core_module_t  ngx_foo_module_ctx = {
    ngx_string("foo"),
    ngx_foo_create_conf,
    ngx_foo_init_conf
};


ngx_module_t  ngx_foo_module = {
    NGX_MODULE_V1,
    &ngx_foo_module_ctx,                   /* module context */
    ngx_foo_commands,                      /* module directives */
    NGX_CORE_MODULE,                       /* module type */
    NULL,                                  /* init master */
    NULL,                                  /* init module */
    NULL,                                  /* init process */
    NULL,                                  /* init thread */
    NULL,                                  /* exit thread */
    NULL,                                  /* exit process */
    NULL,                                  /* exit master */
    NGX_MODULE_V1_PADDING
};


static void *
ngx_foo_create_conf(ngx_cycle_t *cycle)
{
    ngx_foo_conf_t  *fcf;

    fcf = ngx_pcalloc(cycle->pool, sizeof(ngx_foo_conf_t));
    if (fcf == NULL) {
        return NULL;
    }

    fcf->enable = NGX_CONF_UNSET;

    return fcf;
}


static char *
ngx_foo_init_conf(ngx_cycle_t *cycle, void *conf)
{
    ngx_foo_conf_t *fcf = conf;

    ngx_conf_init_value(fcf->enable, 0);

    return NGX_CONF_OK;
}


static char *
ngx_foo_enable(ngx_conf_t *cf, void *post, void *data)
{
    ngx_flag_t  *fp = data;

    if (*fp == 0) {
        return NGX_CONF_OK;
    }

    ngx_log_error(NGX_LOG_NOTICE, cf->log, 0, "Foo Module is enabled");

    return NGX_CONF_OK;
}

Configuration directives

The ngx_command_t describes single configuration directive. Each module, supporting configuration, provides an array of such specifications that describe how to process arguments and what handlers to call:

struct ngx_command_s {
    ngx_str_t             name;
    ngx_uint_t            type;
    char               *(*set)(ngx_conf_t *cf, ngx_command_t *cmd, void *conf);
    ngx_uint_t            conf;
    ngx_uint_t            offset;
    void                 *post;
};

The array should be terminated by a special value “ngx_null_command”. The name is the literal name of a directive, as it appears in configuration file, for example “worker_processes” or “listen”. The type is a bitfield that controls number of arguments, command type and other properties using corresponding flags. Arguments flags:

• NGX_CONF_NOARGS — directive without arguments
• NGX_CONF_1MORE — one required argument
• NGX_CONF_2MORE — two required arguments
• NGX_CONF_TAKE1..7 — exactly 1..7 arguments
• NGX_CONF_TAKE12, 13, 23, 123, 1234 — one or two arguments, or other combinations

Directive types:

• NGX_CONF_BLOCK — the directive is a block, i.e. it may contain other directives in curly braces, or even implement its own parser to handle contents inside.
• NGX_CONF_FLAG — the directive value is a flag, a boolean value represented by “on” or “off” strings.

Context of a directive defines where in the configuration it may appear and how to access module context to store corresponding values:

• NGX_MAIN_CONF — top level configuration
• NGX_HTTP_MAIN_CONF — in the http block
• NGX_HTTP_SRV_CONF — in the http server block
• NGX_HTTP_LOC_CONF — in the http location
• NGX_HTTP_UPS_CONF — in the http upstream block
• NGX_HTTP_SIF_CONF — in the http server “if”
• NGX_HTTP_LIF_CONF — in the http location “if”
• NGX_HTTP_LMT_CONF — in the http “limit_except”
• NGX_STREAM_MAIN_CONF — in the stream block
• NGX_STREAM_SRV_CONF — in the stream server block
• NGX_STREAM_UPS_CONF — in the stream upstream block
• NGX_MAIL_MAIN_CONF — in the the mail block
• NGX_MAIL_SRV_CONF — in the mail server block
• NGX_EVENT_CONF — in the event block
• NGX_DIRECT_CONF — used by modules that don't create a hierarchy of contexts and store module configuration directly in ctx

The configuration parser uses this flags to throw an error in case of a misplaced directive and calls directive handlers supplied with a proper configuration pointer, so that same directives in different locations could store their values in distinct places.

The set field defines a handler that processes a directive and stores parsed values into corresponding configuration. Nginx offers a convenient set of functions that perform common conversions:

• ngx_conf_set_flag_slot — converts literal “on” or “off” strings into ngx_flag_t type with values 1 or 0
• ngx_conf_set_str_slot — stores string as a value of the ngx_str_t type
• ngx_conf_set_str_array_slot — appends ngx_array_t of ngx_str_t with a new value. The array is created if not yet exists
• ngx_conf_set_keyval_slot — appends ngx_array_t of ngx_keyval_t with a new value, where key is the first string and value is second. The array is created if not yet exists
• ngx_conf_set_num_slot — converts directive argument to a ngx_int_t value
• ngx_conf_set_size_slot — converts size to size_t value in bytes
• ngx_conf_set_off_slot — converts offset to off_t value in bytes
• ngx_conf_set_msec_slot — converts time to ngx_msec_t value in milliseconds
• ngx_conf_set_sec_slot — converts time to time_t value in seconds
• ngx_conf_set_bufs_slot — converts two arguments into ngx_bufs_t that holds ngx_int_t number and size of buffers
• ngx_conf_set_enum_slot — converts argument into ngx_uint_t value. The null-terminated array of ngx_conf_enum_t passed in the post field defines acceptable strings and corresponding integer values
• ngx_conf_set_bitmask_slot — arguments are converted to ngx_uint_t value and OR'ed with the resulting value, forming a bitmask. The null-terminated array of ngx_conf_bitmask_t passed in the post field defines acceptable strings and corresponding mask values
• set_path_slot — converts arguments to ngx_path_t type and performs all required initializations. See the proxy_temp_path directive description for details
• set_access_slot — converts arguments to file permissions mask. See the proxy_store_access directive description for details

The conf field defines which context is used to store the value of the directive, or zero if contexts are not used. Only simple core modules use configuration without context and set NGX_DIRECT_CONF flag. In real life, such modules like http or stream require more sophisticated configuration that can be applied per-server or per-location, or even more precisely, in the context of the “if” directive or some limit. In this modules, configuration structure is more complex. Please refer to corresponding modules description to understand how they manage their configuration.

• NGX_HTTP_MAIN_CONF_OFFSET — http block configuration
• NGX_HTTP_SRV_CONF_OFFSET — http server configuration
• NGX_HTTP_LOC_CONF_OFFSET — http location configuration
• NGX_STREAM_MAIN_CONF_OFFSET — stream block configuration
• NGX_STREAM_SRV_CONF_OFFSET — stream server configuration
• NGX_MAIL_MAIN_CONF_OFFSET — mail block configuration
• NGX_MAIL_SRV_CONF_OFFSET — mail server configuration

The offset defines an offset of a field in a module configuration structure that holds values of this particular directive. The typical use is to employ offsetof() macro.

The post is a twofold field: it may be used to define a handler to be called after main handler completed or to pass additional data to the main handler. In the first case, ngx_conf_post_t structure needs to be initialized with a pointer to handler, for example:

static char *ngx_do_foo(ngx_conf_t *cf, void *post, void *data);
static ngx_conf_post_t  ngx_foo_post = { ngx_do_foo };

The post argument is the ngx_conf_post_t object itself, and the data is a pointer to value, converted from arguments by the main handler with the appropriate type.

HTTP

Connection

Each client HTTP connection runs through the following stages:

• ngx_event_accept() accepts a client TCP connection. This handler is called in response to a read notification on a listen socket. A new ngx_connecton_t object is created at this stage. The object wraps the newly accepted client socket. Each nginx listener provides a handler to pass the new connection object to. For HTTP connections it's ngx_http_init_connection(c)
• ngx_http_init_connection() performs early initialization of an HTTP connection. At this stage an ngx_http_connection_t object is created for the connection and its reference is stored in connection's data field. Later it will be substituted with an HTTP request object. PROXY protocol parser and SSL handshake are started at this stage as well
• ngx_http_wait_request_handler() is a read event handler, that is called when data is available in the client socket. At this stage an HTTP request object ngx_http_request_t is created and set to connection's data field
• ngx_http_process_request_line() is a read event handler, which reads client request line. The handler is set by ngx_http_wait_request_handler(). Reading is done into connection's buffer. The size of the buffer is initially set by the directive client_header_buffer_size. The entire client header is supposed to fit the buffer. If the initial size is not enough, a bigger buffer is allocated, whose size is set by the large_client_header_buffers directive
• ngx_http_process_request_headers() is a read event handler, which is set after ngx_http_process_request_line() to read client request header
• ngx_http_core_run_phases() is called when the request header is completely read and parsed. This function runs request phases from NGX_HTTP_POST_READ_PHASE to NGX_HTTP_CONTENT_PHASE. The last phase is supposed to generate response and pass it along the filter chain. The response is not necessarily sent to the client at this phase. It may remain buffered and will be sent at the finalization stage
• ngx_http_finalize_request() is usually called when the request has generated all the output or produced an error. In the latter case an appropriate error page is looked up and used as the response. If the response is not completely sent to the client by this point, an HTTP writer ngx_http_writer() is activated to finish sending outstanding data
• ngx_http_finalize_connection() is called when the response is completely sent to the client and the request can be destroyed. If client connection keepalive feature is enabled, ngx_http_set_keepalive() is called, which destroys current request and waits for the next request on the connection. Otherwise, ngx_http_close_request() destroys both the request and the connection

Request

For each client HTTP request the ngx_http_request_t object is created. Some of the fields of this object:

• connection — pointer to a ngx_connection_t client connection object. Several requests may reference the same connection object at the same time - one main request and its subrequests. After a request is deleted, a new request may be created on the same connection.

  Note that for HTTP connections ngx_connection_t's data field points back to the request. Such request is called active, as opposed to the other requests tied with the connection. Active request is used to handle client connection events and is allowed to output its response to the client. Normally, each request becomes active at some point to be able to send its output

• ctx — array of HTTP module contexts. Each module of type NGX_HTTP_MODULE can store any value (normally, a pointer to a structure) in the request. The value is stored in the ctx array at the module's ctx_index position. The following macros provide a convenient way to get and set request contexts:

  • ngx_http_get_module_ctx(r, module) — returns module's context
  • ngx_http_set_ctx(r, c, module) — sets c as module's context
• main_conf, srv_conf, loc_conf — arrays of current request configurations. Configurations are stored at module's ctx_index positions
• read_event_handler, write_event_handler - read and write event handlers for the request. Normally, an HTTP connection has ngx_http_request_handler() set as both read and write event handlers. This function calls read_event_handler and write_event_handler handlers of the currently active request
• cache — request cache object for caching upstream response
• upstream — request upstream object for proxying
• pool — request pool. This pool is destroyed when the request is deleted. The request object itself is allocated in this pool. For allocations which should be available throughout the client connection's lifetime, ngx_connection_t's pool should be used instead
• header_in — buffer where client HTTP request header in read
• headers_in, headers_out — input and output HTTP headers objects. Both objects contain the headers field of type ngx_list_t keeping the raw list of headers. In addition to that, specific headers are available for getting and setting as separate fields, for example content_length_n, status etc
• request_body — client request body object
• start_sec, start_msec — time point when the request was created. Used for tracking request duration
• method, method_name — numeric and textual representation of client HTTP request method. Numeric values for methods are defined in src/http/ngx_http_request.h with macros NGX_HTTP_GET, NGX_HTTP_HEAD, NGX_HTTP_POST etc
• http_protocol, http_version, http_major, http_minor - client HTTP protocol version in its original textual form (“HTTP/1.0”, “HTTP/1.1” etc), numeric form (NGX_HTTP_VERSION_10, NGX_HTTP_VERSION_11 etc) and separate major and minor versions
• request_line, unparsed_uri — client original request line and URI
• uri, args, exten — current request URI, arguments and file extention. The URI value here might differ from the original URI sent by the client due to normalization. Throughout request processing, these value can change while performing internal redirects
• main — pointer to a main request object. This object is created to process client HTTP request, as opposed to subrequests, created to perform a specific sub-task within the main request
• parent — pointer to a parent request of a subrequest
• postponed — list of output buffers and subrequests in the order they are sent and created. The list is used by the postpone filter to provide consistent request output, when parts of it are created by subrequests
• post_subrequest — pointer to a handler with context to be called when a subrequest gets finalized. Unused for main requests

• posted_requests — list of requests to be started or resumed. Starting or resuming is done by calling the request's write_event_handler. Normally, this handler holds the request main function, which at first runs request phases and then produces the output.

  A request is usually posted by the ngx_http_post_request(r, NULL) call. It is always posted to the main request posted_requests list. The function ngx_http_run_posted_requests(c) runs all requests, posted in the main request of the passed connection's active request. This function should be called in all event handlers, which can lead to new posted requests. Normally, it's called always after invoking a request's read or write handler

• phase_handler — index of current request phase
• ncaptures, captures, captures_data — regex captures produced by the last regex match of the request. While processing a request, there's a number of places where a regex match can happen: map lookup, server lookup by SNI or HTTP Host, rewrite, proxy_redirect etc. Captures produced by a lookup are stored in the above mentioned fields. The field ncaptures holds the number of captures, captures holds captures boundaries, captures_data holds a string, against which the regex was matched and which should be used to extract captures. After each new regex match request captures are reset to hold new values
• count — request reference counter. The field only makes sense for the main request. Increasing the counter is done by simple r->main->count++. To decrease the counter ngx_http_finalize_request(r, rc) should be called. Creation of a subrequest or running request body read process increase the counter
• subrequests — current subrequest nesting level. Each subrequest gets the nesting level of its parent decreased by one. Once the value reaches zero an error is generated. The value for the main request is defined by the NGX_HTTP_MAX_SUBREQUESTS constant
• uri_changes — number of URI changes left for the request. The total number of times a request can change its URI is limited by the NGX_HTTP_MAX_URI_CHANGES constant. With each change the value is decreased until it reaches zero. In the latter case an error is generated. The actions considered as URI changes are rewrites and internal redirects to normal or named locations
• blocked — counter of blocks held on the request. While this value is non-zero, request cannot be terminated. Currently, this value is increased by pending AIO operations (POSIX AIO and thread operations) and active cache lock
• buffered — bitmask showing which modules have buffered the output produced by the request. A number of filters can buffer output, for example sub_filter can buffer data due to a partial string match, copy filter can buffer data because of the lack of free output_buffers etc. As long as this value is non-zero, request is not finalized, expecting the flush
• header_only — flag showing that output does not require body. For example, this flag is used by HTTP HEAD requests

• keepalive — flag showing if client connection keepalive is supported. The value is inferred from HTTP version and “Connection” header value

• header_sent — flag showing that output header has already been sent by the request
• internal — flag showing that current request is internal. To enter the internal state, a request should pass through an internal redirect or be a subrequest. Internal requests are allowed to enter internal locations
• allow_ranges — flag showing that partial response can be sent to client, if requested by the HTTP Range header
• subrequest_ranges — flag showing that a partial response is allowed to be sent while processing a subrequest
• single_range — flag showing that only a single continuous range of output data can be sent to the client. This flag is usually set when sending a stream of data, for example from a proxied server, and the entire response is not available at once
• main_filter_need_in_memory, filter_need_in_memory — flags showing that the output should be produced in memory buffers but not in files. This is a signal to the copy filter to read data from file buffers even if sendfile is enabled. The difference between these two flags is the location of filter modules which set them. Filters called before the postpone filter in filter chain, set filter_need_in_memory requesting that only the current request output should come in memory buffers. Filters called later in filter chain set main_filter_need_in_memory requiring that both the main request and all the subrequest read files in memory while sending output
• filter_need_temporary — flag showing that the request output should be produced in temporary buffers, but not in readonly memory buffers or file buffers. This is used by filters which may change output directly in the buffers, where it's sent

Configuration

Each HTTP module may have three types of configuration:

• Main configuration. This configuration applies to the entire nginx http{} block. This is global configuration. It stores global settings for a module
• Server configuration. This configuration applies to a single nginx server{}. It stores server-specific settings for a module
• Location configuration. This configuration applies to a single location{}, if{} or limit_except() block. This configuration stores settings specific to a location

Configuration structures are created at nginx configuration stage by calling functions, which allocate these structures, initialize them and merge. The following example shows how to create a simple module location configuration. The configuration has one setting foo of unsiged integer type.

typedef struct {
    ngx_uint_t  foo;
} ngx_http_foo_loc_conf_t;


static ngx_http_module_t  ngx_http_foo_module_ctx = {
    NULL,                                  /* preconfiguration */
    NULL,                                  /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    ngx_http_foo_create_loc_conf,          /* create location configuration */
    ngx_http_foo_merge_loc_conf            /* merge location configuration */
};


static void *
ngx_http_foo_create_loc_conf(ngx_conf_t *cf)
{
    ngx_http_foo_loc_conf_t  *conf;

    conf = ngx_pcalloc(cf->pool, sizeof(ngx_http_foo_loc_conf_t));
    if (conf == NULL) {
        return NULL;
    }

    conf->foo = NGX_CONF_UNSET_UINT;

    return conf;
}


static char *
ngx_http_foo_merge_loc_conf(ngx_conf_t *cf, void *parent, void *child)
{
    ngx_http_foo_loc_conf_t *prev = parent;
    ngx_http_foo_loc_conf_t *conf = child;

    ngx_conf_merge_uint_value(conf->foo, prev->foo, 1);
}

As seen in the example, ngx_http_foo_create_loc_conf() function creates a new configuration structure and ngx_http_foo_merge_loc_conf() merges a configuration with another configuration from a higher level. In fact, server and location configuration do not only exist at server and location levels, but also created for all the levels above. Specifically, a server configuration is created at the main level as well and location configurations are created for main, server and location levels. These configurations make it possible to specify server and location-specific settings at any level of nginx configuration file. Eventually configurations are merged down. To indicate a missing setting and ignore it while merging, nginx provides a number of macros like NGX_CONF_UNSET and NGX_CONF_UNSET_UINT. Standard nginx merge macros like ngx_conf_merge_value() and ngx_conf_merge_uint_value() provide a convenient way to merge a setting and set the default value if none of configurations provided an explicit value. For complete list of macros for different types see src/core/ngx_conf_file.h.

To access configuration of any HTTP module at configuration time, the following macros are available. They receive ngx_conf_t reference as the first argument.

• ngx_http_conf_get_module_main_conf(cf, module)
• ngx_http_conf_get_module_srv_conf(cf, module)
• ngx_http_conf_get_module_loc_conf(cf, module)

The following example gets a pointer to a location configuration of standard nginx core module ngx_http_core_module and changes location content handler kept in the handler field of the structure.

static ngx_int_t ngx_http_foo_handler(ngx_http_request_t *r);


static ngx_command_t  ngx_http_foo_commands[] = {

    { ngx_string("foo"),
      NGX_HTTP_LOC_CONF|NGX_CONF_NOARGS,
      ngx_http_foo,
      0,
      0,
      NULL },

      ngx_null_command
};


static char *
ngx_http_foo(ngx_conf_t *cf, ngx_command_t *cmd, void *conf)
{
    ngx_http_core_loc_conf_t  *clcf;

    clcf = ngx_http_conf_get_module_loc_conf(cf, ngx_http_core_module);
    clcf->handler = ngx_http_bar_handler;

    return NGX_CONF_OK;
}

In runtime the following macros are available to get configurations of HTTP modules.

• ngx_http_get_module_main_conf(r, module)
• ngx_http_get_module_srv_conf(r, module)
• ngx_http_get_module_loc_conf(r, module)

These macros receive a reference to an HTTP request ngx_http_request_t. Main configuration of a request never changes. Server configuration may change from a default one after choosing a virtual server for a request. Request location configuration may change multiple times as a result of a rewrite or internal redirect. The following example shows how to access HTTP configuration in runtime.

static ngx_int_t
ngx_http_foo_handler(ngx_http_request_t *r)
{
    ngx_http_foo_loc_conf_t  *flcf;

    flcf = ngx_http_get_module_loc_conf(r, ngx_http_foo_module);

    ...
}

Phases

Each HTTP request passes through a list of HTTP phases. Each phase is specialized in a particular type of processing. Most phases allow installing handlers. The phase handlers are called successively once the request reaches the phase. Many standard nginx modules install their phase handlers as a way to get called at a specific request processing stage. Following is the list of nginx HTTP phases.

• NGX_HTTP_POST_READ_PHASE is the earliest phase. The ngx_http_realip_module installs its handler at this phase. This allows to substitute client address before any other module is invoked
• NGX_HTTP_SERVER_REWRITE_PHASE is used to run rewrite script, defined at the server level, that is out of any location block. The ngx_http_rewrite_module installs its handler at this phase
• NGX_HTTP_FIND_CONFIG_PHASE — a special phase used to choose a location based on request URI. This phase does not allow installing any handlers. It only performs the default action of choosing a location. Before this phase, the server default location is assigned to the request. Any module requesting a location configuration, will receive the default server location configuration. After this phase a new location is assigned to the request
• NGX_HTTP_REWRITE_PHASE — same as NGX_HTTP_SERVER_REWRITE_PHASE, but for a new location, chosen at the previous phase
• NGX_HTTP_POST_REWRITE_PHASE — a special phase, used to redirect the request to a new location, if the URI was changed during rewrite. The redirect is done by going back to NGX_HTTP_FIND_CONFIG_PHASE. No handlers are allowed at this phase
• NGX_HTTP_PREACCESS_PHASE — a common phase for different types of handlers, not associated with access check. Standard nginx modules ngx_http_limit_conn_module and ngx_http_limit_req_module register their handlers at this phase
• NGX_HTTP_ACCESS_PHASE — used to check access permissions for the request. Standard nginx modules such as ngx_http_access_module and ngx_http_auth_basic_module register their handlers at this phase. If configured so by the satisfy directive, only one of access phase handlers may allow access to the request in order to continue processing
• NGX_HTTP_POST_ACCESS_PHASE — a special phase for the satisfy any case. If some access phase handlers denied the access and none of them allowed, the request is finalized. No handlers are supported at this phase
• NGX_HTTP_TRY_FILES_PHASE — a special phase, for the try_files feature. No handlers are allowed at this phase
• NGX_HTTP_CONTENT_PHASE — a phase, at which the response is supposed to be generated. Multiple nginx standard modules register their handers at this phase, for example ngx_http_index_module or ngx_http_static_module. All these handlers are called sequentially until one of them finally produces the output. It's also possible to set content handlers on a per-location basis. If the ngx_http_core_module's location configuration has handler set, this handler is called as the content handler and content phase handlers are ignored
• NGX_HTTP_LOG_PHASE is used to perform request logging. Currently, only the ngx_http_log_module registers its handler at this stage for access logging. Log phase handlers are called at the very end of request processing, right before freeing the request

Following is the example of a preaccess phase handler.

static ngx_http_module_t  ngx_http_foo_module_ctx = {
    NULL,                                  /* preconfiguration */
    ngx_http_foo_init,                     /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    NULL,                                  /* create location configuration */
    NULL                                   /* merge location configuration */
};


static ngx_int_t
ngx_http_foo_handler(ngx_http_request_t *r)
{
    ngx_str_t  *ua;

    ua = r->headers_in->user_agent;

    if (ua == NULL) {
        return NGX_DECLINED;
    }

    /* reject requests with "User-Agent: foo" */
    if (ua->value.len == 3 && ngx_strncmp(ua->value.data, "foo", 3) == 0) {
        return NGX_HTTP_FORBIDDEN;
    }

    return NGX_DECLINED;
}


static ngx_int_t
ngx_http_foo_init(ngx_conf_t *cf)
{
    ngx_http_handler_pt        *h;
    ngx_http_core_main_conf_t  *cmcf;

    cmcf = ngx_http_conf_get_module_main_conf(cf, ngx_http_core_module);

    h = ngx_array_push(&cmcf->phases[NGX_HTTP_PREACCESS_PHASE].handlers);
    if (h == NULL) {
        return NGX_ERROR;
    }

    *h = ngx_http_foo_handler;

    return NGX_OK;
}

Phase handlers are expected to return specific codes:

• NGX_OK — proceed to the next phase
• NGX_DECLINED — proceed to the next handler of the current phase. If current handler is the last in current phase, move to the next phase
• NGX_AGAIN, NGX_DONE — suspend phase handling until some future event. This can be for example asynchronous I/O operation or just a delay. It is supposed, that phase handling will be resumed later by calling ngx_http_core_run_phases()
• Any other value returned by the phase handler is treated as a request finalization code, in particular, HTTP response code. The request is finalized with the code provided

Some phases treat return codes in a slightly different way. At content phase, any return code other that NGX_DECLINED is considered a finalization code. As for the location content handlers, any return from them is considered a finalization code. At access phase, in satisfy any mode, returning a code other than NGX_OK, NGX_DECLINED, NGX_AGAIN, NGX_DONE is considered a denial. If none of future access handlers allow access or deny with a new code, the denial code will become the finalization code.

Variables

Accessing existing variables

Variables may be referenced using index (this is the most common method) or names (see below in the section about creating variables). Index is created at configuration stage, when a variable is added to configuration. The variable index can be obtained using ngx_http_get_variable_index():

ngx_str_t  name;  /* ngx_string("foo") */
ngx_int_t  index;

index = ngx_http_get_variable_index(cf, &name);

Here, the cf is a pointer to nginx configuration and the name points to a string with the variable name. The function returns NGX_ERROR on error or valid index otherwise, which is typically stored somewhere in a module configuration for future use.

All HTTP variables are evaluated in the context of HTTP request and results are specific to and cached in HTTP request. All functions that evaluate variables return ngx_http_variable_value_t type, representing the variable value:

typedef ngx_variable_value_t  ngx_http_variable_value_t;

typedef struct {
    unsigned    len:28;

    unsigned    valid:1;
    unsigned    no_cacheable:1;
    unsigned    not_found:1;
    unsigned    escape:1;

    u_char     *data;
} ngx_variable_value_t;

where:

• len — length of a value
• data — value itself
• valid — value is valid
• not_found — variable was not found and thus the data and len fields are irrelevant; this may happen, for example, with such variables as $arg_foo when a corresponding argument was not passed in a request
• no_cacheable — do not cache result
• escape — used internally by the logging module to mark values that require escaping on output

The ngx_http_get_flushed_variable() and ngx_http_get_indexed_variable() functions are used to obtain the variable value. They have the same interface - accepting a HTTP request r as a context for evaluating the variable and an index, identifying it. Example of typical usage:

ngx_http_variable_value_t  *v;

v = ngx_http_get_flushed_variable(r, index);

if (v == NULL || v->not_found) {
    /* we failed to get value or there is no such variable, handle it */
    return NGX_ERROR;
}

/* some meaningful value is found */

The difference between functions is that the ngx_http_get_indexed_variable() returns cached value and ngx_http_get_flushed_variable() flushes cache for non-cacheable variables.

There are cases when it is required to deal with variables which names are not known at configuration time and thus they cannot be accessed using indexes, for example in modules like SSI or Perl. The ngx_http_get_variable(r, name, key) function may be used in such cases. It searches for the variable with a given name and its hash key.

Creating variables

To create a variable ngx_http_add_variable() function is used. It takes configuration (where variable is registered), variable name and flags that control its behaviour:

• NGX_HTTP_VAR_CHANGEABLE — allows redefining the variable; If another module will define a variable with such name, no conflict will happen. For example, this allows user to override variables using the set directive.
• NGX_HTTP_VAR_NOCACHEABLE — disables caching, is useful for such variables as $time_local
• NGX_HTTP_VAR_NOHASH — indicates that this variable is only accessible by index, not by name. This is a small optimization which may be used when it is known that the variable is not needed in modules like SSI or Perl.
• NGX_HTTP_VAR_PREFIX — the name of this variable is a prefix. A handler must implement additional logic to obtain value of specific variable. For example, all “arg_” variables are processed by the same handler which performs lookup in request arguments and returns value of specific argument.

The function returns NULL in case of error or a pointer to ngx_http_variable_t:

struct ngx_http_variable_s {
    ngx_str_t                     name;
    ngx_http_set_variable_pt      set_handler;
    ngx_http_get_variable_pt      get_handler;
    uintptr_t                     data;
    ngx_uint_t                    flags;
    ngx_uint_t                    index;
};

The get and set handlers are called to obtain or set the variable value, data will be passed to variable handlers, index will hold assigned variable index, used to reference the variable.

Usually, a null-terminated static array of such structures is created by a module and processed at the preconfiguration stage to add variables into configuration:

static ngx_http_variable_t  ngx_http_foo_vars[] = {

    { ngx_string("foo_v1"), NULL, ngx_http_foo_v1_variable, NULL, 0, 0 },

    { ngx_null_string, NULL, NULL, 0, 0, 0 }
};

static ngx_int_t
ngx_http_foo_add_variables(ngx_conf_t *cf)
{
    ngx_http_variable_t  *var, *v;

    for (v = ngx_http_foo_vars; v->name.len; v++) {
        var = ngx_http_add_variable(cf, &v->name, v->flags);
        if (var == NULL) {
            return NGX_ERROR;
        }

        var->get_handler = v->get_handler;
        var->data = v->data;
    }

    return NGX_OK;
}

This function is used to initialize the preconfiguration field of the HTTP module context and is called before parsing HTTP configuration, so it could refer to these variables.

The get handler is responsible for evaluating the variable in a context of specific request, for example:

static ngx_int_t
ngx_http_variable_connection(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    u_char  *p;

    p = ngx_pnalloc(r->pool, NGX_ATOMIC_T_LEN);
    if (p == NULL) {
        return NGX_ERROR;
    }

    v->len = ngx_sprintf(p, "%uA", r->connection->number) - p;
    v->valid = 1;
    v->no_cacheable = 0;
    v->not_found = 0;
    v->data = p;

    return NGX_OK;
}

It returns NGX_ERROR in case of internal error (for example, failed memory allocation) or NGX_OK otherwise. The status of variable evaluation may be understood by inspecting flags of the ngx_http_variable_value_t (see description above).

The set handler allows setting the property referred by the variable. For example, the $limit_rate variable set handler modifies the request's limit_rate field:

...
{ ngx_string("limit_rate"), ngx_http_variable_request_set_size,
  ngx_http_variable_request_get_size,
  offsetof(ngx_http_request_t, limit_rate),
  NGX_HTTP_VAR_CHANGEABLE|NGX_HTTP_VAR_NOCACHEABLE, 0 },
...

static void
ngx_http_variable_request_set_size(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data)
{
    ssize_t    s, *sp;
    ngx_str_t  val;

    val.len = v->len;
    val.data = v->data;

    s = ngx_parse_size(&val);

    if (s == NGX_ERROR) {
        ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
                      "invalid size \"%V\"", &val);
        return;
    }

    sp = (ssize_t *) ((char *) r + data);

    *sp = s;

    return;
}

Complex values

A complex value, despite its name, provides an easy way to evaluate expressions that may contain text, variables, and their combination.

The complex value description in ngx_http_compile_complex_value is compiled at the configuration stage into ngx_http_complex_value_t which is used at runtime to obtain evaluated expression results.

ngx_str_t                         *value;
ngx_http_complex_value_t           cv;
ngx_http_compile_complex_value_t   ccv;

value = cf->args->elts; /* directive arguments */

ngx_memzero(&ccv, sizeof(ngx_http_compile_complex_value_t));

ccv.cf = cf;
ccv.value = &value[1];
ccv.complex_value = &cv;
ccv.zero = 1;
ccv.conf_prefix = 1;

if (ngx_http_compile_complex_value(&ccv) != NGX_OK) {
    return NGX_CONF_ERROR;
}

Here, ccv holds all parameters that are required to initialize the complex value cv:

• cf — configuration pointer
• value — string for parsing (input)
• complex_value — compiled value (output)
• zero — flag that enables zero-terminating value
• conf_prefix — prefixes result with configuration prefix (the directory where nginx is currently looking for configuration)
• root_prefix — prefixes result with root prefix (this is the normal nginx installation prefix)

The zero flag is usable when results are to be passed to libraries that require zero-terminated strings, and prefixes are handy when dealing with filenames.

Upon successful compilation, cv.lengths may be inspected to get information about the presence of variables in the expression. The NULL value means that the expression contained static text only, and there is no need in storing it as a complex value, so a simple string can be used.

The ngx_http_set_complex_value_slot() is a convenient function used to initialize complex value completely right in the directive declaration.

At runtime, a complex value may be calculated using the ngx_http_complex_value() function:

ngx_str_t  res;

if (ngx_http_complex_value(r, &cv, &res) != NGX_OK) {
    return NGX_ERROR;
}

Given the request r and previously compiled value cv the function will evaluate expression and put result into res.

Request redirection

An HTTP request is always connected to a location via the loc_conf field of the ngx_http_request_t structure. This means that at any point the location configuration of any module can be retrieved from the request by calling ngx_http_get_module_loc_conf(r, module). Request location may be changed several times throughout its lifetime. Initially, a default server location of the default server is assigned to a request. Once a request switches to a different server (chosen by the HTTP “Host” header or SSL SNI extension), the request switches to the default location of that server as well. The next change of the location takes place at the NGX_HTTP_FIND_CONFIG_PHASE request phase. At this phase a location is chosen by request URI among all non-named locations configured for the server. The ngx_http_rewrite_module may change the request URI at the NGX_HTTP_REWRITE_PHASE request phase as a result of rewrite and return to the NGX_HTTP_FIND_CONFIG_PHASE phase for choosing a new location based on the new URI.

It is also possible to redirect a request to a new location at any point by calling one of the functions ngx_http_internal_redirect(r, uri, args) or ngx_http_named_location(r, name).

The function ngx_http_internal_redirect(r, uri, args) changes the request URI and returns the request to the NGX_HTTP_SERVER_REWRITE_PHASE phase. The request proceeds with a server default location. Later at NGX_HTTP_FIND_CONFIG_PHASE a new location is chosen based on the new request URI.

The following example performs an internal redirect with the new request arguments.

ngx_int_t
ngx_http_foo_redirect(ngx_http_request_t *r)
{
    ngx_str_t  uri, args;

    ngx_str_set(&uri, "/foo");
    ngx_str_set(&args, "bar=1");

    return ngx_http_internal_redirect(r, &uri, &args);
}

The function ngx_http_named_location(r, name) redirects a request to a named location. The name of the location is passed as the argument. The location is looked up among all named locations of the current server, after which the requests switches to the NGX_HTTP_REWRITE_PHASE phase.

The following example performs a redirect to a named location @foo.

ngx_int_t
ngx_http_foo_named_redirect(ngx_http_request_t *r)
{
    ngx_str_t  name;

    ngx_str_set(&name, "foo");

    return ngx_http_named_location(r, &name);
}

Both functions ngx_http_internal_redirect(r, uri, args) and ngx_http_named_location(r, name) may be called when a request already has some contexts saved in its ctx field by nginx modules. These contexts could become inconsistent with the new location configuration. To prevent inconsistency, all request contexts are erased by both redirect functions.

Redirected and rewritten requests become internal and may access the internal locations. Internal requests have the internal flag set.

Subrequests

Subrequests are primarily used to include output of one request into another, possibly mixed with other data. A subrequest looks like a normal request, but shares some data with its parent. Particularly, all fields related to client input are shared since a subrequest does not receive any other input from client. The request field parent for a subrequest keeps a link to its parent request and is NULL for the main request. The field main keeps a link to the main request in a group of requests.

A subrequest starts with NGX_HTTP_SERVER_REWRITE_PHASE phase. It passes through the same phases as a normal request and is assigned a location based on its own URI.

Subrequest output header is always ignored. Subrequest output body is placed by the ngx_http_postpone_filter into the right position in relation to other data produced by the parent request.

Subrequests are related to the concept of active requests. A request r is considered active if c->data == r, where c is the client connection object. At any point, only the active request in a request group is allowed to output its buffers to the client. A non-active request can still send its data to the filter chain, but they will not pass beyond the ngx_http_postpone_filter and will remain buffered by that filter until the request becomes active. Here are some rules of request activation:

• Initially, the main request is active
• The first subrequest of an active request becomes active right after creation
• The ngx_http_postpone_filter activates the next request in active request's subrequest list, once all data prior to that request are sent
• When a request is finalized, its parent is activated

A subrequest is created by calling the function ngx_http_subrequest(r, uri, args, psr, ps, flags), where r is the parent request, uri and args are URI and arguments of the subrequest, psr is the output parameter, receiving the newly created subrequest reference, ps is a callback object for notifying the parent request that the subrequest is being finalized, flags is subrequest creation flags bitmask. The following flags are available:

• NGX_HTTP_SUBREQUEST_IN_MEMORY - subrequest output should not be sent to the client, but rather stored in memory. This only works for proxying subrequests. After subrequest finalization its output is available in r->upstream->buffer buffer of type ngx_buf_t
• NGX_HTTP_SUBREQUEST_WAITED - the subrequest done flag is set even if it is finalized being non-active. This subrequest flag is used by the SSI filter
• NGX_HTTP_SUBREQUEST_CLONE - the subrequest is created as a clone of its parent. It is started at the same location and proceeds from the same phase as the parent request

The following example creates a subrequest with the URI of "/foo".

ngx_int_t            rc;
ngx_str_t            uri;
ngx_http_request_t  *sr;

...

ngx_str_set(&uri, "/foo");

rc = ngx_http_subrequest(r, &uri, NULL, &sr, NULL, 0);
if (rc == NGX_ERROR) {
    /* error */
}

This example clones the current request and sets a finalization callback for the subrequest.

ngx_int_t
ngx_http_foo_clone(ngx_http_request_t *r)
{
    ngx_http_request_t          *sr;
    ngx_http_post_subrequest_t  *ps;

    ps = ngx_palloc(r->pool, sizeof(ngx_http_post_subrequest_t));
    if (ps == NULL) {
        return NGX_ERROR;
    }

    ps->handler = ngx_http_foo_subrequest_done;
    ps->data = "foo";

    return ngx_http_subrequest(r, &r->uri, &r->args, &sr, ps,
                               NGX_HTTP_SUBREQUEST_CLONE);
}


ngx_int_t
ngx_http_foo_subrequest_done(ngx_http_request_t *r, void *data, ngx_int_t rc)
{
    char  *msg = (char *) data;

    ngx_log_error(NGX_LOG_INFO, r->connection->log, 0,
                  "done subrequest r:%p msg:%s rc:%i", r, msg, rc);

    return rc;
}

Subrequests are normally created in a body filter. In this case subrequest output can be treated as any other explicit request output. This means that eventually the output of a subrequest is sent to the client after all explicit buffers passed prior to subrequest creation and before any buffers passed later. This ordering is preserved even for large hierarchies of subrequests. The following example inserts a subrequest output after all request data buffers, but before the final buffer with the last_buf flag.

ngx_int_t
ngx_http_foo_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    ngx_int_t                   rc;
    ngx_buf_t                  *b;
    ngx_uint_t                  last;
    ngx_chain_t                *cl, out;
    ngx_http_request_t         *sr;
    ngx_http_foo_filter_ctx_t  *ctx;

    ctx = ngx_http_get_module_ctx(r, ngx_http_foo_filter_module);
    if (ctx == NULL) {
        return ngx_http_next_body_filter(r, in);
    }

    last = 0;

    for (cl = in; cl; cl = cl->next) {
        if (cl->buf->last_buf) {
            cl->buf->last_buf = 0;
            cl->buf->last_in_chain = 1;
            cl->buf->sync = 1;
            last = 1;
        }
    }

    /* Output explicit output buffers */

    rc = ngx_http_next_body_filter(r, in);

    if (rc == NGX_ERROR || !last) {
        return rc;
    }

    /*
     * Create the subrequest.  The output of the subrequest
     * will automatically be sent after all preceding buffers,
     * but before the last_buf buffer passed later in this function.
     */

    if (ngx_http_subrequest(r, ctx->uri, NULL, &sr, NULL, 0) != NGX_OK) {
        return NGX_ERROR;
    }

    ngx_http_set_ctx(r, NULL, ngx_http_foo_filter_module);


    /* Output the final buffer with the last_buf flag */

    b = ngx_calloc_buf(r->pool);
    if (b == NULL) {
        return NGX_ERROR;
    }

    b->last_buf = 1;

    out.buf = b;
    out.next = NULL;

    return ngx_http_output_filter(r, &out);
}

A subrequest may also be created for other purposes than data output. For example, the ngx_http_auth_request_module creates a subrequest at NGX_HTTP_ACCESS_PHASE phase. To disable any output at this point, the subrequest header_only flag is set. This prevents subrequest body from being sent to the client. Its header is ignored anyway. The result of the subrequest can be analyzed in the callback handler.

Request finalization

An HTTP request is finalized by calling the function ngx_http_finalize_request(r, rc). It is usually finalized by the content handler after sending all output buffers to the filter chain. At this point the output may not be completely sent to the client, but remain buffered somewhere along the filter chain. If it is, the ngx_http_finalize_request(r, rc) function will automatically install a special handler ngx_http_writer(r) to finish sending the output. A request is also finalized in case of an error or if a standard HTTP response code needs to be returned to the client.

The function ngx_http_finalize_request(r, rc) expects the following rc values:

• NGX_DONE - fast finalization. Decrement request count and destroy the request if it reaches zero. The client connection may still be used for more requests after that
• NGX_ERROR, NGX_HTTP_REQUEST_TIME_OUT (408), NGX_HTTP_CLIENT_CLOSED_REQUEST (499) - error finalization. Terminate the request as soon as possible and close the client connection.
• NGX_HTTP_CREATED (201), NGX_HTTP_NO_CONTENT (204), codes greater than or equal to NGX_HTTP_SPECIAL_RESPONSE (300) - special response finalization. For these values nginx either sends a default code response page to the client or performs the internal redirect to an error_page location if it's configured for the code
• Other codes are considered success finalization codes and may activate the request writer to finish sending the response body. Once body is completely sent, request count is decremented. If it reaches zero, the request is destroyed, but the client connection may still be used for other requests. If count is positive, there are unfinished activities within the request, which will be finalized at a later point.

Request body

For dealing with client request body, nginx provides the following functions: ngx_http_read_client_request_body(r, post_handler) and ngx_http_discard_request_body(r). The first function reads the request body and makes it available via the request_body request field. The second function instructs nginx to discard (read and ignore) the request body. One of these functions must be called for every request. Normally, it is done in the content handler.

Reading or discarding client request body from a subrequest is not allowed. It should always be done in the main request. When a subrequest is created, it inherits the parent request_body object which can be used by the subrequest if the main request has previously read the request body.

The function ngx_http_read_client_request_body(r, post_handler) starts the process of reading the request body. When the body is completely read, the post_handler callback is called to continue processing the request. If request body is missing or already read, the callback is called immediately. The function ngx_http_read_client_request_body(r, post_handler) allocates the request_body request field of type ngx_http_request_body_t. The field bufs of this object keeps the result as a buffer chain. The body can be saved in memory buffers or file buffers, if client_body_buffer_size is not enough to fit the entire body in memory.

The following example reads client request body and returns its size.

ngx_int_t
ngx_http_foo_content_handler(ngx_http_request_t *r)
{
    ngx_int_t  rc;

    rc = ngx_http_read_client_request_body(r, ngx_http_foo_init);

    if (rc >= NGX_HTTP_SPECIAL_RESPONSE) {
        /* error */
        return rc;
    }

    return NGX_DONE;
}


void
ngx_http_foo_init(ngx_http_request_t *r)
{
    off_t         len;
    ngx_buf_t    *b;
    ngx_int_t     rc;
    ngx_chain_t  *in, out;

    if (r->request_body == NULL) {
        ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    len = 0;

    for (in = r->request_body->bufs; in; in = in->next) {
        len += ngx_buf_size(in->buf);
    }

    b = ngx_create_temp_buf(r->pool, NGX_OFF_T_LEN);
    if (b == NULL) {
        ngx_http_finalize_request(r, NGX_HTTP_INTERNAL_SERVER_ERROR);
        return;
    }

    b->last = ngx_sprintf(b->pos, "%O", len);
    b->last_buf = (r == r->main) ? 1: 0;
    b->last_in_chain = 1;

    r->headers_out.status = NGX_HTTP_OK;
    r->headers_out.content_length_n = b->last - b->pos;

    rc = ngx_http_send_header(r);

    if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
        ngx_http_finalize_request(r, rc);
        return;
    }

    out.buf = b;
    out.next = NULL;

    rc = ngx_http_output_filter(r, &out);

    ngx_http_finalize_request(r, rc);
}

The following fields of the request affect the way request body is read:

• request_body_in_single_buf - read body to a single memory buffer
• request_body_in_file_only - always read body to a file, even if fits the memory buffer
• request_body_in_persistent_file - do not unlink the file right after creation. Such a file can be moved to another directory
• request_body_in_clean_file - unlink the file the when the request is finalized. This can be useful when a file was supposed to be moved to another directory but eventually was not moved for some reason
• request_body_file_group_access - enable file group access. By default a file is created with 0600 access mask. When the flag is set, 0660 access mask is used
• request_body_file_log_level - log file errors with this log level
• request_body_no_buffering - read request body without buffering

When the request_body_no_buffering flag is set, the unbuffered mode of reading the request body is enabled. In this mode, after calling ngx_http_read_client_request_body(), the bufs chain may keep only a part of the body. To read the next part, the ngx_http_read_unbuffered_request_body(r) function should be called. The return value of NGX_AGAIN and the request flag reading_body indicate that more data is available. If bufs is NULL after calling this function, there is nothing to read at the moment. The request callback read_event_handler will be called when the next part of request body is available.

Response

An HTTP response in nginx is produced by sending the response header followed by the optional response body. Both header and body are passed through a chain of filters and eventually get written to the client socket. An nginx module can install its handler into the header or body filter chain and process the output coming from the previous handler.

Response header

Output header is sent by the function ngx_http_send_header(r). Prior to calling this function, r->headers_out should contain all the data required to produce the HTTP response header. It's always required to set the status field of r->headers_out. If the response status suggests that a response body follows the header, content_length_n can be set as well. The default value for this field is -1, which means that the body size is unknown. In this case, chunked transfer encoding is used. To output an arbitrary header, headers list should be appended.

static ngx_int_t
ngx_http_foo_content_handler(ngx_http_request_t *r)
{
    ngx_int_t         rc;
    ngx_table_elt_t  *h;

    /* send header */

    r->headers_out.status = NGX_HTTP_OK;
    r->headers_out.content_length_n = 3;

    /* X-Foo: foo */

    h = ngx_list_push(&r->headers_out.headers);
    if (h == NULL) {
        return NGX_ERROR;
    }

    h->hash = 1;
    ngx_str_set(&h->key, "X-Foo");
    ngx_str_set(&h->value, "foo");

    rc = ngx_http_send_header(r);

    if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
        return rc;
    }

    /* send body */

    ...
}

Header filters

The ngx_http_send_header(r) function invokes the header filter chain by calling the top header filter handler ngx_http_top_header_filter. It's assumed that every header handler calls the next handler in chain until the final handler ngx_http_header_filter(r) is called. The final header handler constructs the HTTP response based on r->headers_out and passes it to the ngx_http_writer_filter for output.

To add a handler to the header filter chain, one should store its address in ngx_http_top_header_filter global variable at configuration time. The previous handler address is normally stored in a module's static variable and is called by the newly added handler before exiting.

The following is an example header filter module, adding the HTTP header "X-Foo: foo" to every output with the status 200.

#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_http.h>


static ngx_int_t ngx_http_foo_header_filter(ngx_http_request_t *r);
static ngx_int_t ngx_http_foo_header_filter_init(ngx_conf_t *cf);


static ngx_http_module_t  ngx_http_foo_header_filter_module_ctx = {
    NULL,                                   /* preconfiguration */
    ngx_http_foo_header_filter_init,        /* postconfiguration */

    NULL,                                   /* create main configuration */
    NULL,                                   /* init main configuration */

    NULL,                                   /* create server configuration */
    NULL,                                   /* merge server configuration */

    NULL,                                   /* create location configuration */
    NULL                                    /* merge location configuration */
};


ngx_module_t  ngx_http_foo_header_filter_module = {
    NGX_MODULE_V1,
    &ngx_http_foo_header_filter_module_ctx, /* module context */
    NULL,                                   /* module directives */
    NGX_HTTP_MODULE,                        /* module type */
    NULL,                                   /* init master */
    NULL,                                   /* init module */
    NULL,                                   /* init process */
    NULL,                                   /* init thread */
    NULL,                                   /* exit thread */
    NULL,                                   /* exit process */
    NULL,                                   /* exit master */
    NGX_MODULE_V1_PADDING
};


static ngx_http_output_header_filter_pt  ngx_http_next_header_filter;


static ngx_int_t
ngx_http_foo_header_filter(ngx_http_request_t *r)
{
    ngx_table_elt_t  *h;

    /*
     * The filter handler adds "X-Foo: foo" header
     * to every HTTP 200 response
     */

    if (r->headers_out.status != NGX_HTTP_OK) {
        return ngx_http_next_header_filter(r);
    }

    h = ngx_list_push(&r->headers_out.headers);
    if (h == NULL) {
        return NGX_ERROR;
    }

    h->hash = 1;
    ngx_str_set(&h->key, "X-Foo");
    ngx_str_set(&h->value, "foo");

    return ngx_http_next_header_filter(r);
}


static ngx_int_t
ngx_http_foo_header_filter_init(ngx_conf_t *cf)
{
    ngx_http_next_header_filter = ngx_http_top_header_filter;
    ngx_http_top_header_filter = ngx_http_foo_header_filter;

    return NGX_OK;
}

Response body

Response body is sent by calling the function ngx_http_output_filter(r, cl). The function can be called multiple times. Each time it sends a part of the response body passed as a buffer chain. The last body buffer should have the last_buf flag set.

The following example produces a complete HTTP output with "foo" as its body. In order for the example to work not only as a main request but as a subrequest as well, the last_in_chain flag is set in the last buffer of the output. The last_buf flag is set only for the main request since a subrequest's last buffers does not end the entire output.

static ngx_int_t
ngx_http_bar_content_handler(ngx_http_request_t *r)
{
    ngx_int_t     rc;
    ngx_buf_t    *b;
    ngx_chain_t   out;

    /* send header */

    r->headers_out.status = NGX_HTTP_OK;
    r->headers_out.content_length_n = 3;

    rc = ngx_http_send_header(r);

    if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
        return rc;
    }

    /* send body */

    b = ngx_calloc_buf(r->pool);
    if (b == NULL) {
        return NGX_ERROR;
    }

    b->last_buf = (r == r->main) ? 1: 0;
    b->last_in_chain = 1;

    b->memory = 1;

    b->pos = (u_char *) "foo";
    b->last = b->pos + 3;

    out.buf = b;
    out.next = NULL;

    return ngx_http_output_filter(r, &out);
}

Body filters

The function ngx_http_output_filter(r, cl) invokes the body filter chain by calling the top body filter handler ngx_http_top_body_filter. It's assumed that every body handler calls the next handler in chain until the final handler ngx_http_write_filter(r, cl) is called.

A body filter handler receives a chain of buffers. The handler is supposed to process the buffers and pass a possibly new chain to the next handler. It's worth noting that the chain links ngx_chain_t of the incoming chain belong to the caller. They should never be reused or changed. Right after the handler completes, the caller can use its output chain links to keep track of the buffers it has sent. To save the buffer chain or to substitute some buffers before sending further, a handler should allocate its own chain links.

Following is the example of a simple body filter counting the number of body bytes. The result is available as the $counter variable which can be used in the access log.

#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_http.h>


typedef struct {
    off_t  count;
} ngx_http_counter_filter_ctx_t;


static ngx_int_t ngx_http_counter_body_filter(ngx_http_request_t *r,
    ngx_chain_t *in);
static ngx_int_t ngx_http_counter_variable(ngx_http_request_t *r,
    ngx_http_variable_value_t *v, uintptr_t data);
static ngx_int_t ngx_http_counter_add_variables(ngx_conf_t *cf);
static ngx_int_t ngx_http_counter_filter_init(ngx_conf_t *cf);


static ngx_http_module_t  ngx_http_counter_filter_module_ctx = {
    ngx_http_counter_add_variables,        /* preconfiguration */
    ngx_http_counter_filter_init,          /* postconfiguration */

    NULL,                                  /* create main configuration */
    NULL,                                  /* init main configuration */

    NULL,                                  /* create server configuration */
    NULL,                                  /* merge server configuration */

    NULL,                                  /* create location configuration */
    NULL                                   /* merge location configuration */
};


ngx_module_t  ngx_http_counter_filter_module = {
    NGX_MODULE_V1,
    &ngx_http_counter_filter_module_ctx,   /* module context */
    NULL,                                  /* module directives */
    NGX_HTTP_MODULE,                       /* module type */
    NULL,                                  /* init master */
    NULL,                                  /* init module */
    NULL,                                  /* init process */
    NULL,                                  /* init thread */
    NULL,                                  /* exit thread */
    NULL,                                  /* exit process */
    NULL,                                  /* exit master */
    NGX_MODULE_V1_PADDING
};


static ngx_http_output_body_filter_pt  ngx_http_next_body_filter;

static ngx_str_t  ngx_http_counter_name = ngx_string("counter");


static ngx_int_t
ngx_http_counter_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    ngx_chain_t                    *cl;
    ngx_http_counter_filter_ctx_t  *ctx;

    ctx = ngx_http_get_module_ctx(r, ngx_http_counter_filter_module);
    if (ctx == NULL) {
        ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_counter_filter_ctx_t));
        if (ctx == NULL) {
            return NGX_ERROR;
        }

        ngx_http_set_ctx(r, ctx, ngx_http_counter_filter_module);
    }

    for (cl = in; cl; cl = cl->next) {
        ctx->count += ngx_buf_size(cl->buf);
    }

    return ngx_http_next_body_filter(r, in);
}


static ngx_int_t
ngx_http_counter_variable(ngx_http_request_t *r, ngx_http_variable_value_t *v,
    uintptr_t data)
{
    u_char                         *p;
    ngx_http_counter_filter_ctx_t  *ctx;

    ctx = ngx_http_get_module_ctx(r, ngx_http_counter_filter_module);
    if (ctx == NULL) {
        v->not_found = 1;
        return NGX_OK;
    }

    p = ngx_pnalloc(r->pool, NGX_OFF_T_LEN);
    if (p == NULL) {
        return NGX_ERROR;
    }

    v->data = p;
    v->len = ngx_sprintf(p, "%O", ctx->count) - p;
    v->valid = 1;
    v->no_cacheable = 0;
    v->not_found = 0;

    return NGX_OK;
}


static ngx_int_t
ngx_http_counter_add_variables(ngx_conf_t *cf)
{
    ngx_http_variable_t  *var;

    var = ngx_http_add_variable(cf, &ngx_http_counter_name, 0);
    if (var == NULL) {
        return NGX_ERROR;
    }

    var->get_handler = ngx_http_counter_variable;

    return NGX_OK;
}


static ngx_int_t
ngx_http_counter_filter_init(ngx_conf_t *cf)
{
    ngx_http_next_body_filter = ngx_http_top_body_filter;
    ngx_http_top_body_filter = ngx_http_counter_body_filter;

    return NGX_OK;
}

Building filter modules

When writing a body or header filter, a special care should be taken of the filters order. There's a number of header and body filters registered by nginx standard modules. It's important to register a filter module in the right place in respect to other filters. Normally, filters are registered by modules in their postconfiguration handlers. The order in which filters are called is obviously the reverse of when they are registered.

A special slot HTTP_AUX_FILTER_MODULES for third-party filter modules is provided by nginx. To register a filter module in this slot, the ngx_module_type variable should be set to the value of HTTP_AUX_FILTER in module's configuration.

The following example shows a filter module config file assuming it only has one source file ngx_http_foo_filter_module.c

ngx_module_type=HTTP_AUX_FILTER
ngx_module_name=ngx_http_foo_filter_module
ngx_module_srcs="$ngx_addon_dir/ngx_http_foo_filter_module.c"

. auto/module

Buffer reuse

When issuing or altering a stream of buffers, it's often desirable to reuse the allocated buffers. A standard approach widely adopted in nginx code is to keep two buffer chains for this purpose: free and busy. The free chain keeps all free buffers. These buffers can be reused. The busy chain keeps all buffers sent by the current module which are still in use by some other filter handler. A buffer is considered in use if its size is greater than zero. Normally, when a buffer is consumed by a filter, its pos (or file_pos for a file buffer) is moved towards last (file_last for a file buffer). Once a buffer is completely consumed, it's ready to be reused. To update the free chain with newly freed buffers, it's enough to iterate over the busy chain and move the zero size buffers at the head of it to free. This operation is so common that there is a special function ngx_chain_update_chains(free, busy, out, tag) which does this. The function appends the output chain out to busy and moves free buffers from the top of busy to free. Only the buffers with the given tag are reused. This lets a module reuse only the buffers allocated by itself.

The following example is a body filter inserting the “foo” string before each incoming buffer. The new buffers allocated by the module are reused if possible. Note that for this example to work properly, it's also required to set up a header filter and reset content_length_n to -1, which is beyond the scope of this section.

typedef struct {
    ngx_chain_t  *free;
    ngx_chain_t  *busy;
}  ngx_http_foo_filter_ctx_t;


ngx_int_t
ngx_http_foo_body_filter(ngx_http_request_t *r, ngx_chain_t *in)
{
    ngx_int_t                   rc;
    ngx_buf_t                  *b;
    ngx_chain_t                *cl, *tl, *out, **ll;
    ngx_http_foo_filter_ctx_t  *ctx;

    ctx = ngx_http_get_module_ctx(r, ngx_http_foo_filter_module);
    if (ctx == NULL) {
        ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_foo_filter_ctx_t));
        if (ctx == NULL) {
            return NGX_ERROR;
        }

        ngx_http_set_ctx(r, ctx, ngx_http_foo_filter_module);
    }

    /* create a new chain "out" from "in" with all the changes */

    ll = &out;

    for (cl = in; cl; cl = cl->next) {

        /* append "foo" in a reused buffer if possible */

        tl = ngx_chain_get_free_buf(r->pool, &ctx->free);
        if (tl == NULL) {
            return NGX_ERROR;
        }

        b = tl->buf;
        b->tag = (ngx_buf_tag_t) &ngx_http_foo_filter_module;
        b->memory = 1;
        b->pos = (u_char *) "foo";
        b->last = b->pos + 3;

        *ll = tl;
        ll = &tl->next;

        /* append the next incoming buffer */

        tl = ngx_alloc_chain_link(r->pool);
        if (tl == NULL) {
            return NGX_ERROR;
        }

        tl->buf = cl->buf;
        *ll = tl;
        ll = &tl->next;
    }

    *ll = NULL;

    /* send the new chain */

    rc = ngx_http_next_body_filter(r, out);

    /* update "busy" and "free" chains for reuse */

    ngx_chain_update_chains(r->pool, &ctx->free, &ctx->busy, &out,
                            (ngx_buf_tag_t) &ngx_http_foo_filter_module);

    return rc;
}

Load balancing

The ngx_http_upstream_module provides basic functionality to pass requests to remote servers. This functionality is used by modules that implement specific protocols, such as HTTP or FastCGI. The module also provides an interface for creating custom load balancing modules and implements a default round-robin balancing method.

Examples of modules that implement alternative load balancing methods are least_conn and hash. Note that these modules are actually implemented as extensions of the upstream module and share a lot of code, such as representation of a server group. The keepalive module is an example of an independent module, extending upstream functionality.

The ngx_http_upstream_module may be configured explicitly by placing the corresponding upstream block into the configuration file, or implicitly by using directives that accept a URL evaluated at some point to the list of servers, for example, proxy_pass. Only explicit configurations may use an alternative load balancing method. The upstream module configuration has its own directive context NGX_HTTP_UPS_CONF. The structure is defined as follows:

struct ngx_http_upstream_srv_conf_s {
    ngx_http_upstream_peer_t         peer;
    void                           **srv_conf;

    ngx_array_t                     *servers;  /* ngx_http_upstream_server_t */

    ngx_uint_t                       flags;
    ngx_str_t                        host;
    u_char                          *file_name;
    ngx_uint_t                       line;
    in_port_t                        port;
    ngx_uint_t                       no_port;  /* unsigned no_port:1 */

#if (NGX_HTTP_UPSTREAM_ZONE)
    ngx_shm_zone_t                  *shm_zone;
#endif
};

• srv_conf — configuration context of upstream modules
• servers — array of ngx_http_upstream_server_t, the result of parsing a set of server directives in the upstream block
• flags — flags that mostly mark which features (configured as parameters of the server directive) are supported by the particular load balancing method.
  • NGX_HTTP_UPSTREAM_CREATE — used to distinguish explicitly defined upstreams from automatically created by proxy_pass and “friends” (FastCGI, SCGI, etc.)
  • NGX_HTTP_UPSTREAM_WEIGHT — “weight” is supported
  • NGX_HTTP_UPSTREAM_MAX_FAILS — “max_fails” is supported
  • NGX_HTTP_UPSTREAM_FAIL_TIMEOUT — “fail_timeout” is supported
  • NGX_HTTP_UPSTREAM_DOWN — “down” is supported
  • NGX_HTTP_UPSTREAM_BACKUP — “backup” is supported
  • NGX_HTTP_UPSTREAM_MAX_CONNS — “max_conns” is supported
• host — the name of an upstream
• file_name, line — the name of the configuration file and the line where the upstream block is located
• port and no_port — unused by explicit upstreams
• shm_zone — a shared memory zone used by this upstream, if any
• peer — an object that holds generic methods for initializing upstream configuration:

  typedef struct {
      ngx_http_upstream_init_pt        init_upstream;
      ngx_http_upstream_init_peer_pt   init;
      void                            *data;
  } ngx_http_upstream_peer_t;
  

  A module that implements a load balancing algorithm must set these methods and initialize private data. If init_upstream was not initialized during configuration parsing, ngx_http_upstream_module sets it to default ngx_http_upstream_init_round_robin.
  • init_upstream(cf, us) — configuration-time method responsible for initializing a group of servers and initializing the init() method in case of success. A typical load balancing module uses a list of servers in the upstream block to create some efficient data structure that it uses and saves own configuration to the data field.
  • init(r, us) — initializes per-request ngx_http_upstream_peer_t.peer (not to be confused with the ngx_http_upstream_srv_conf_t.peer described above which is per-upstream) structure that is used for load balancing. It will be passed as data argument to all callbacks that deal with server selection.

When nginx has to pass a request to another host for processing, it uses a configured load balancing method to obtain an address to connect to. The method is taken from the ngx_http_upstream_t.peer object of type ngx_peer_connection_t:

struct ngx_peer_connection_s {
    [...]

    struct sockaddr                 *sockaddr;
    socklen_t                        socklen;
    ngx_str_t                       *name;

    ngx_uint_t                       tries;

    ngx_event_get_peer_pt            get;
    ngx_event_free_peer_pt           free;
    ngx_event_notify_peer_pt         notify;
    void                            *data;

#if (NGX_SSL || NGX_COMPAT)
    ngx_event_set_peer_session_pt    set_session;
    ngx_event_save_peer_session_pt   save_session;
#endif

    [..]
};

The structure has the following fields:

• sockaddr, socklen, name — address of an upstream server to connect to; this is the output parameter of a load balancing method
• data — per-request load balancing method data; keeps the state of selection algorithm and usually includes the link to upstream configuration. It will be passed as an argument to all methods that deal with server selection (see below)
• tries — allowed number of attempts to connect to an upstream.
• get, free, notify, set_session, and save_session - methods of the load balancing module, see description below

All methods accept at least two arguments: peer connection object pc and the data created by ngx_http_upstream_srv_conf_t.peer.init(). Note that in general case it may differ from pc.data due to “chaining” of load balancing modules.

• get(pc, data) — the method is called when the upstream module is ready to pass a request to an upstream server and needs to know its address. The method is responsible to fill in the sockaddr, socklen, and name fields of ngx_peer_connection_t structure. The return value may be one of:
  • NGX_OK — server was selected
  • NGX_ERROR — internal error occurred
  • NGX_BUSY — there are no available servers at the moment. This can happen due to many reasons, such as: dynamic server group is empty, all servers in the group are in the failed state, all servers in the group are already handling the maximum number of connections or similar.
  • NGX_DONE — this is set by the keepalive module to indicate that the underlying connection was reused and there is no need to create a new connection to the upstream server.
• free(pc, data, state) — the method is called when an upstream module has finished work with a particular server. The state argument is the status of upstream connection completion. This is a bitmask, the following values may be set: NGX_PEER_FAILED — this attempt is considered unsuccessful, NGX_PEER_NEXT — a special case with codes 403 and 404 (see link above), which are not considered a failure. NGX_PEER_KEEPALIVE. Also, tries counter is decremented by this method.
• notify(pc, data, type) — currently unused in the OSS version.
• set_session(pc, data) and save_session(pc, data) — SSL-specific methods that allow to cache sessions to upstream servers. The implementation is provided by the round-robin balancing method.