README.markdown

Name
====

ngx_http_lua_module - Embed the power of Lua into Nginx HTTP Servers.

This module is a core component of [OpenResty](https://openresty.org). If you are using this module,
then you are essentially using OpenResty :)

*This module is not distributed with the Nginx source.* See
[the installation instructions](#installation).

Table of Contents
=================

* [Name](#name)
* [Status](#status)
* [Version](#version)
* [Videos](#videos)
* [Synopsis](#synopsis)
* [Description](#description)
* [Typical Uses](#typical-uses)
* [Nginx Compatibility](#nginx-compatibility)
* [Installation](#installation)
    * [Building as a dynamic module](#building-as-a-dynamic-module)
    * [C Macro Configurations](#c-macro-configurations)
* [Community](#community)
    * [English Mailing List](#english-mailing-list)
    * [Chinese Mailing List](#chinese-mailing-list)
* [Code Repository](#code-repository)
* [Bugs and Patches](#bugs-and-patches)
* [LuaJIT bytecode support](#luajit-bytecode-support)
* [System Environment Variable Support](#system-environment-variable-support)
* [HTTP 1.0 support](#http-10-support)
* [Statically Linking Pure Lua Modules](#statically-linking-pure-lua-modules)
* [Data Sharing within an Nginx Worker](#data-sharing-within-an-nginx-worker)
* [Known Issues](#known-issues)
    * [TCP socket connect operation issues](#tcp-socket-connect-operation-issues)
    * [Lua Coroutine Yielding/Resuming](#lua-coroutine-yieldingresuming)
    * [Lua Variable Scope](#lua-variable-scope)
    * [Locations Configured by Subrequest Directives of Other Modules](#locations-configured-by-subrequest-directives-of-other-modules)
    * [Cosockets Not Available Everywhere](#cosockets-not-available-everywhere)
    * [Special Escaping Sequences](#special-escaping-sequences)
    * [Mixing with SSI Not Supported](#mixing-with-ssi-not-supported)
    * [SPDY Mode Not Fully Supported](#spdy-mode-not-fully-supported)
    * [Missing data on short circuited requests](#missing-data-on-short-circuited-requests)
* [TODO](#todo)
* [Changes](#changes)
* [Test Suite](#test-suite)
* [Copyright and License](#copyright-and-license)
* [See Also](#see-also)
* [Directives](#directives)
* [Nginx API for Lua](#nginx-api-for-lua)
* [Obsolete Sections](#obsolete-sections)
    * [Special PCRE Sequences](#special-pcre-sequences)
    * [Lua/LuaJIT bytecode support](#lualuajit-bytecode-support)

Status
======

Production ready.

Version
=======

This document describes ngx_lua
[v0.10.25](https://github.com/openresty/lua-nginx-module/tags), which was released
on 19 June 2023.

Videos
======

* YouTube video "[Hello World HTTP Example with OpenResty/Lua](https://youtu.be/eSfYLvVQMxw)"

    [![Hello World HTTP Example with OpenResty/Lua](https://img.youtube.com/vi/eSfYLvVQMxw/0.jpg)](https://youtu.be/eSfYLvVQMxw)

* YouTube video "[Write Your Own Lua Modules in OpenResty/Nginx Applications](https://youtu.be/vfYxOMl5LVY)"

    [![Write Your Own Lua Modules in OpenResty/Nginx Applications](https://img.youtube.com/vi/vfYxOMl5LVY/0.jpg)](https://youtu.be/vfYxOMl5LVY)

* YouTube video "[OpenResty's resty Command-Line Utility Demo](https://youtu.be/L1c7aw4mSOo)"

    [![OpenResty's resty Command-Line Utility Demo](https://img.youtube.com/vi/L1c7aw4mSOo/0.jpg)](https://youtu.be/L1c7aw4mSOo)

* YouTube video "[Measure Execution Time of Lua Code Correctly in OpenResty](https://youtu.be/VkRYW_qLoME)"

    [![Measure Execution Time of Lua Code Correctly in OpenResty](https://img.youtube.com/vi/VkRYW_qLoME/0.jpg)](https://youtu.be/VkRYW_qLoME)

* YouTube video "[Precompile Lua Modules into LuaJIT Bytecode to Speedup OpenResty Startup](https://youtu.be/EP7c0BM2yNo)"

    [![Precompile Lua Modules into LuaJIT Bytecode to Speedup OpenResty Startup](https://img.youtube.com/vi/EP7c0BM2yNo/0.jpg)](https://youtu.be/EP7c0BM2yNo)

You are welcome to subscribe to our [official YouTube channel, OpenResty](https://www.youtube.com/channel/UCXVmwF-UCScv2ftsGoMqxhw).

[Back to TOC](#table-of-contents)

Synopsis
========
```nginx

 # set search paths for pure Lua external libraries (';;' is the default path):
 lua_package_path '/foo/bar/?.lua;/blah/?.lua;;';

 # set search paths for Lua external libraries written in C (can also use ';;'):
 lua_package_cpath '/bar/baz/?.so;/blah/blah/?.so;;';

 server {
     location /lua_content {
         # MIME type determined by default_type:
         default_type 'text/plain';

         content_by_lua_block {
             ngx.say('Hello,world!')
         }
     }

     location /nginx_var {
         # MIME type determined by default_type:
         default_type 'text/plain';

         # try access /nginx_var?a=hello,world
         content_by_lua_block {
             ngx.say(ngx.var.arg_a)
         }
     }

     location = /request_body {
         client_max_body_size 50k;
         client_body_buffer_size 50k;

         content_by_lua_block {
             ngx.req.read_body()  -- explicitly read the req body
             local data = ngx.req.get_body_data()
             if data then
                 ngx.say("body data:")
                 ngx.print(data)
                 return
             end

             -- body may get buffered in a temp file:
             local file = ngx.req.get_body_file()
             if file then
                 ngx.say("body is in file ", file)
             else
                 ngx.say("no body found")
             end
         }
     }

     # transparent non-blocking I/O in Lua via subrequests
     # (well, a better way is to use cosockets)
     location = /lua {
         # MIME type determined by default_type:
         default_type 'text/plain';

         content_by_lua_block {
             local res = ngx.location.capture("/some_other_location")
             if res then
                 ngx.say("status: ", res.status)
                 ngx.say("body:")
                 ngx.print(res.body)
             end
         }
     }

     location = /foo {
         rewrite_by_lua_block {
             res = ngx.location.capture("/memc",
                 { args = { cmd = "incr", key = ngx.var.uri } }
             )
         }

         proxy_pass http://blah.blah.com;
     }

     location = /mixed {
         rewrite_by_lua_file /path/to/rewrite.lua;
         access_by_lua_file /path/to/access.lua;
         content_by_lua_file /path/to/content.lua;
     }

     # use nginx var in code path
     # CAUTION: contents in nginx var must be carefully filtered,
     # otherwise there'll be great security risk!
     location ~ ^/app/([-_a-zA-Z0-9/]+) {
         set $path $1;
         content_by_lua_file /path/to/lua/app/root/$path.lua;
     }

     location / {
        client_max_body_size 100k;
        client_body_buffer_size 100k;

        access_by_lua_block {
            -- check the client IP address is in our black list
            if ngx.var.remote_addr == "132.5.72.3" then
                ngx.exit(ngx.HTTP_FORBIDDEN)
            end

            -- check if the URI contains bad words
            if ngx.var.uri and
                   string.match(ngx.var.request_body, "evil")
            then
                return ngx.redirect("/terms_of_use.html")
            end

            -- tests passed
        }

        # proxy_pass/fastcgi_pass/etc settings
     }
 }
```

[Back to TOC](#table-of-contents)

Description
===========

This module embeds [LuaJIT 2.0/2.1](https://luajit.org/luajit.html) into Nginx.
It is a core component of [OpenResty](https://openresty.org). If you are using
this module, then you are essentially using OpenResty.

Since version `v0.10.16` of this module, the standard Lua
interpreter (also known as "PUC-Rio Lua") is not supported anymore. This
document interchangeably uses the terms "Lua" and "LuaJIT" to refer to the
LuaJIT interpreter.

By leveraging Nginx's subrequests, this module allows the integration of the
powerful Lua threads (known as Lua "coroutines") into the Nginx event model.

Unlike [Apache's mod_lua](https://httpd.apache.org/docs/trunk/mod/mod_lua.html)
and [Lighttpd's mod_magnet](http://redmine.lighttpd.net/wiki/1/Docs:ModMagnet),
Lua code executed using this module can be *100% non-blocking* on network
traffic as long as the [Nginx API for Lua](#nginx-api-for-lua) provided by
this module is used to handle requests to upstream services such as MySQL,
PostgreSQL, Memcached, Redis, or upstream HTTP web services.

At least the following Lua libraries and Nginx modules can be used with this
module:

* [lua-resty-memcached](https://github.com/openresty/lua-resty-memcached)
* [lua-resty-mysql](https://github.com/openresty/lua-resty-mysql)
* [lua-resty-redis](https://github.com/openresty/lua-resty-redis)
* [lua-resty-dns](https://github.com/openresty/lua-resty-dns)
* [lua-resty-upload](https://github.com/openresty/lua-resty-upload)
* [lua-resty-websocket](https://github.com/openresty/lua-resty-websocket)
* [lua-resty-lock](https://github.com/openresty/lua-resty-lock)
* [lua-resty-logger-socket](https://github.com/cloudflare/lua-resty-logger-socket)
* [lua-resty-lrucache](https://github.com/openresty/lua-resty-lrucache)
* [lua-resty-string](https://github.com/openresty/lua-resty-string)
* [ngx_memc](http://github.com/openresty/memc-nginx-module)
* [ngx_postgres](https://github.com/FRiCKLE/ngx_postgres)
* [ngx_redis2](http://github.com/openresty/redis2-nginx-module)
* [ngx_redis](http://wiki.nginx.org/HttpRedisModule)
* [ngx_proxy](http://nginx.org/en/docs/http/ngx_http_proxy_module.html)
* [ngx_fastcgi](http://nginx.org/en/docs/http/ngx_http_fastcgi_module.html)

Almost any Nginx modules can be used with this ngx_lua module by means of
[ngx.location.capture](#ngxlocationcapture) or
[ngx.location.capture_multi](#ngxlocationcapture_multi) but it is
recommended to use those `lua-resty-*` libraries instead of creating
subrequests to access the Nginx upstream modules because the former is usually
much more flexible and memory-efficient.

The Lua interpreter (also known as "Lua State" or "LuaJIT VM instance") is
shared across all the requests in a single Nginx worker process to minimize
memory use. Request contexts are segregated using lightweight Lua coroutines.

Loaded Lua modules persist in the Nginx worker process level resulting in a
small memory footprint in Lua even when under heavy loads.

This module is plugged into Nginx's "http" subsystem so it can only speak
downstream communication protocols in the HTTP family (HTTP 0.9/1.0/1.1/2.0,
WebSockets, etc...).  If you want to do generic TCP communications with the
downstream clients, then you should use the
[ngx_stream_lua](https://github.com/openresty/stream-lua-nginx-module#readme)
module instead, which offers a compatible Lua API.

[Back to TOC](#table-of-contents)

Typical Uses
============

Just to name a few:

* Mashup'ing and processing outputs of various Nginx upstream outputs (proxy, drizzle, postgres, redis, memcached, etc.) in Lua,
* doing arbitrarily complex access control and security checks in Lua before requests actually reach the upstream backends,
* manipulating response headers in an arbitrary way (by Lua)
* fetching backend information from external storage backends (like redis, memcached, mysql, postgresql) and use that information to choose which upstream backend to access on-the-fly,
* coding up arbitrarily complex web applications in a content handler using synchronous but still non-blocking access to the database backends and other storage,
* doing very complex URL dispatch in Lua at rewrite phase,
* using Lua to implement advanced caching mechanism for Nginx's subrequests and arbitrary locations.

The possibilities are unlimited as the module allows bringing together various
elements within Nginx as well as exposing the power of the Lua language to the
user. The module provides the full flexibility of scripting while offering
performance levels comparable with native C language programs both in terms of
CPU time as well as memory footprint thanks to LuaJIT 2.x.

Other scripting language implementations typically struggle to match this
performance level.

[Back to TOC](#table-of-contents)

Nginx Compatibility
===================

The latest version of this module is compatible with the following versions of Nginx:

* 1.25.x  (last tested: 1.25.1)
* 1.21.x  (last tested: 1.21.4)
* 1.19.x  (last tested: 1.19.3)
* 1.17.x  (last tested: 1.17.8)
* 1.15.x  (last tested: 1.15.8)
* 1.14.x
* 1.13.x  (last tested: 1.13.6)
* 1.12.x
* 1.11.x  (last tested: 1.11.2)
* 1.10.x
* 1.9.x (last tested: 1.9.15)
* 1.8.x
* 1.7.x (last tested: 1.7.10)
* 1.6.x

Nginx cores older than 1.6.0 (exclusive) are *not* supported.

[Back to TOC](#table-of-contents)

Installation
============

It is *highly* recommended to use [OpenResty releases](https://openresty.org)
which bundle Nginx, ngx_lua (this module), LuaJIT, as well as other powerful
companion Nginx modules and Lua libraries.

It is discouraged to build this module with Nginx yourself since it is tricky
to set up exactly right.

Note that Nginx, LuaJIT, and OpenSSL official releases have various limitations
and long-standing bugs that can cause some of this module's features to be
disabled, not work properly, or run slower. Official OpenResty releases are
recommended because they bundle [OpenResty's optimized LuaJIT 2.1 fork](https://github.com/openresty/luajit2) and
[Nginx/OpenSSL
patches](https://github.com/openresty/openresty/tree/master/patches).

Alternatively, ngx_lua can be manually compiled into Nginx:

1. LuaJIT can be downloaded from the [latest release of OpenResty's LuaJIT fork](https://github.com/openresty/luajit2/releases). The official LuaJIT 2.x releases are also supported, although performance will be significantly lower for reasons elaborated above
1. Download the latest version of the ngx_devel_kit (NDK) module [HERE](https://github.com/simplresty/ngx_devel_kit/tags)
1. Download the latest version of ngx_lua [HERE](https://github.com/openresty/lua-nginx-module/tags)
1. Download the latest supported version of Nginx [HERE](https://nginx.org/) (See [Nginx Compatibility](#nginx-compatibility))
1. Download the latest version of the lua-resty-core [HERE](https://github.com/openresty/lua-resty-core)
1. Download the latest version of the lua-resty-lrucache [HERE](https://github.com/openresty/lua-resty-lrucache)

Build the source with this module:

```bash

 wget 'https://openresty.org/download/nginx-1.19.3.tar.gz'
 tar -xzvf nginx-1.19.3.tar.gz
 cd nginx-1.19.3/

 # tell nginx's build system where to find LuaJIT 2.0:
 export LUAJIT_LIB=/path/to/luajit/lib
 export LUAJIT_INC=/path/to/luajit/include/luajit-2.0

 # tell nginx's build system where to find LuaJIT 2.1:
 export LUAJIT_LIB=/path/to/luajit/lib
 export LUAJIT_INC=/path/to/luajit/include/luajit-2.1

 # Here we assume Nginx is to be installed under /opt/nginx/.
 ./configure --prefix=/opt/nginx \
         --with-ld-opt="-Wl,-rpath,/path/to/luajit/lib" \
         --add-module=/path/to/ngx_devel_kit \
         --add-module=/path/to/lua-nginx-module

 # Note that you may also want to add `./configure` options which are used in your
 # current nginx build.
 # You can get usually those options using command nginx -V

 # you can change the parallelism number 2 below to fit the number of spare CPU cores in your
 # machine.
 make -j2
 make install

 # Note that this version of lug-nginx-module not allow to set `lua_load_resty_core off;` any more.
 # So, you have to install `lua-resty-core` and `lua-resty-lrucache` manually as below.

 cd lua-resty-core
 make install PREFIX=/opt/nginx
 cd lua-resty-lrucache
 make install PREFIX=/opt/nginx

 # add necessary `lua_package_path` directive to `nginx.conf`, in the http context

 lua_package_path "/opt/nginx/lib/lua/?.lua;;";
```

[Back to TOC](#table-of-contents)

Building as a dynamic module
----------------------------

Starting from NGINX 1.9.11, you can also compile this module as a dynamic module, by using the `--add-dynamic-module=PATH` option instead of `--add-module=PATH` on the
`./configure` command line above. And then you can explicitly load the module in your `nginx.conf` via the [load_module](https://nginx.org/en/docs/ngx_core_module.html#load_module)
directive, for example,

```nginx

 load_module /path/to/modules/ndk_http_module.so;  # assuming NDK is built as a dynamic module too
 load_module /path/to/modules/ngx_http_lua_module.so;
```

[Back to TOC](#table-of-contents)

C Macro Configurations
----------------------

While building this module either via OpenResty or with the Nginx core, you can define the following C macros via the C compiler options:

* `NGX_LUA_USE_ASSERT`
	When defined, will enable assertions in the ngx_lua C code base. Recommended for debugging or testing builds. It can introduce some (small) runtime overhead when enabled. This macro was first introduced in the `v0.9.10` release.
* `NGX_LUA_ABORT_AT_PANIC`
	When the LuaJIT VM panics, ngx_lua will instruct the current nginx worker process to quit gracefully by default. By specifying this C macro, ngx_lua will abort the current nginx worker process (which usually results in a core dump file) immediately. This option is useful for debugging VM panics. This option was first introduced in the `v0.9.8` release.

To enable one or more of these macros, just pass extra C compiler options to the `./configure` script of either Nginx or OpenResty. For instance,


    ./configure --with-cc-opt="-DNGX_LUA_USE_ASSERT -DNGX_LUA_ABORT_AT_PANIC"


[Back to TOC](#table-of-contents)

Community
=========

[Back to TOC](#table-of-contents)

English Mailing List
--------------------

The [openresty-en](https://groups.google.com/group/openresty-en) mailing list is for English speakers.

[Back to TOC](#table-of-contents)

Chinese Mailing List
--------------------

The [openresty](https://groups.google.com/group/openresty) mailing list is for Chinese speakers.

[Back to TOC](#table-of-contents)

Code Repository
===============

The code repository of this project is hosted on GitHub at
[openresty/lua-nginx-module](https://github.com/openresty/lua-nginx-module).

[Back to TOC](#table-of-contents)

Bugs and Patches
================

Please submit bug reports, wishlists, or patches by

1. creating a ticket on the [GitHub Issue Tracker](https://github.com/openresty/lua-nginx-module/issues),
1. or posting to the [OpenResty community](#community).

[Back to TOC](#table-of-contents)

LuaJIT bytecode support
=======================

Watch YouTube video "[Measure Execution Time of Lua Code Correctly in OpenResty](https://youtu.be/VkRYW_qLoME)"

[![Precompile Lua Modules into LuaJIT Bytecode to Speedup OpenResty Startup](https://img.youtube.com/vi/EP7c0BM2yNo/0.jpg)](https://youtu.be/EP7c0BM2yNo)

As from the `v0.5.0rc32` release, all `*_by_lua_file` configure directives (such as [content_by_lua_file](#content_by_lua_file)) support loading LuaJIT 2.0/2.1 raw bytecode files directly:

```bash

 /path/to/luajit/bin/luajit -b /path/to/input_file.lua /path/to/output_file.ljbc
```

The `-bg` option can be used to include debug information in the LuaJIT bytecode file:

```bash

 /path/to/luajit/bin/luajit -bg /path/to/input_file.lua /path/to/output_file.ljbc
```

Please refer to the official LuaJIT documentation on the `-b` option for more details:

<https://luajit.org/running.html#opt_b>

Note that the bytecode files generated by LuaJIT 2.1 is *not* compatible with
LuaJIT 2.0, and vice versa. The support for LuaJIT 2.1 bytecode was first added
in ngx_lua v0.9.3.

Attempts to load standard Lua 5.1 bytecode files into ngx_lua instances linked
to LuaJIT 2.0/2.1 (or vice versa) will result in an Nginx error message such as
the one below:


    [error] 13909#0: *1 failed to load Lua inlined code: bad byte-code header in /path/to/test_file.luac


Loading bytecode files via the Lua primitives like `require` and
`dofile` should always work as expected.

[Back to TOC](#table-of-contents)

System Environment Variable Support
===================================

If you want to access the system environment variable, say, `foo`, in Lua via the standard Lua API [os.getenv](https://www.lua.org/manual/5.1/manual.html#pdf-os.getenv), then you should also list this environment variable name in your `nginx.conf` file via the [env directive](https://nginx.org/en/docs/ngx_core_module.html#env). For example,

```nginx

 env foo;
```

[Back to TOC](#table-of-contents)

HTTP 1.0 support
================

The HTTP 1.0 protocol does not support chunked output and requires an explicit `Content-Length` header when the response body is not empty in order to support the HTTP 1.0 keep-alive.
So when a HTTP 1.0 request is made and the [lua_http10_buffering](#lua_http10_buffering) directive is turned `on`, ngx_lua will buffer the
output of [ngx.say](#ngxsay) and [ngx.print](#ngxprint) calls and also postpone sending response headers until all the response body output is received.
At that time ngx_lua can calculate the total length of the body and construct a proper `Content-Length` header to return to the HTTP 1.0 client.
If the `Content-Length` response header is set in the running Lua code, however, this buffering will be disabled even if the [lua_http10_buffering](#lua_http10_buffering) directive is turned `on`.

For large streaming output responses, it is important to disable the [lua_http10_buffering](#lua_http10_buffering) directive to minimise memory usage.

Note that common HTTP benchmark tools such as `ab` and `http_load` issue HTTP 1.0 requests by default.
To force `curl` to send HTTP 1.0 requests, use the `-0` option.

[Back to TOC](#table-of-contents)

Statically Linking Pure Lua Modules
===================================

With LuaJIT 2.x, it is possible to statically link the bytecode of pure Lua
modules into the Nginx executable.

You can use the `luajit` executable to compile `.lua` Lua
module files to `.o` object files containing the exported bytecode
data, and then link the `.o` files directly in your Nginx build.

Below is a trivial example to demonstrate this. Consider that we have the following `.lua` file named `foo.lua`:

```lua

 -- foo.lua
 local _M = {}

 function _M.go()
     print("Hello from foo")
 end

 return _M
```

And then we compile this `.lua` file to `foo.o` file:

```bash

 /path/to/luajit/bin/luajit -bg foo.lua foo.o
```

What matters here is the name of the `.lua` file, which determines how you use this module later on the Lua land. The file name `foo.o` does not matter at all except the `.o` file extension (which tells `luajit` what output format is used). If you want to strip the Lua debug information from the resulting bytecode, you can just specify the `-b` option above instead of `-bg`.

Then when building Nginx or OpenResty, pass the `--with-ld-opt="foo.o"` option to the `./configure` script:

```bash

 ./configure --with-ld-opt="/path/to/foo.o" ...
```

Finally, you can just do the following in any Lua code run by ngx_lua:

```lua

 local foo = require "foo"
 foo.go()
```

And this piece of code no longer depends on the external `foo.lua` file any more because it has already been compiled into the `nginx` executable.

If you want to use dot in the Lua module name when calling `require`, as in

```lua

 local foo = require "resty.foo"
```

then you need to rename the `foo.lua` file to `resty_foo.lua` before compiling it down to a `.o` file with the `luajit` command-line utility.

It is important to use exactly the same version of LuaJIT when compiling `.lua` files to `.o` files as building nginx + ngx_lua. This is because the LuaJIT bytecode format may be incompatible between different LuaJIT versions. When the bytecode format is incompatible, you will see a Lua runtime error saying that the Lua module is not found.

When you have multiple `.lua` files to compile and link, then just specify their `.o` files at the same time in the value of the `--with-ld-opt` option. For instance,

```bash

 ./configure --with-ld-opt="/path/to/foo.o /path/to/bar.o" ...
```

If you have too many `.o` files, then it might not be feasible to name them all in a single command. In this case, you can build a static library (or archive) for your `.o` files, as in

```bash

 ar rcus libmyluafiles.a *.o
```

then you can link the `myluafiles` archive as a whole to your nginx executable:

```bash

 ./configure \
     --with-ld-opt="-L/path/to/lib -Wl,--whole-archive -lmyluafiles -Wl,--no-whole-archive"
```

where `/path/to/lib` is the path of the directory containing the `libmyluafiles.a` file. It should be noted that the linker option `--whole-archive` is required here because otherwise our archive will be skipped because no symbols in our archive are mentioned in the main parts of the nginx executable.

[Back to TOC](#table-of-contents)

Data Sharing within an Nginx Worker
===================================

To globally share data among all the requests handled by the same Nginx worker
process, encapsulate the shared data into a Lua module, use the Lua
`require` builtin to import the module, and then manipulate the
shared data in Lua. This works because required Lua modules are loaded only
once and all coroutines will share the same copy of the module (both its code
and data).

Note that the use of global Lua variables is *strongly discouraged*, as it may
lead to unexpected race conditions between concurrent requests.

Here is a small example on sharing data within an Nginx worker via a Lua module:

```lua

 -- mydata.lua
 local _M = {}

 local data = {
     dog = 3,
     cat = 4,
     pig = 5,
 }

 function _M.get_age(name)
     return data[name]
 end

 return _M
```

and then accessing it from `nginx.conf`:

```nginx

 location /lua {
     content_by_lua_block {
         local mydata = require "mydata"
         ngx.say(mydata.get_age("dog"))
     }
 }
```

The `mydata` module in this example will only be loaded and run on the first request to the location `/lua`,
and all subsequent requests to the same Nginx worker process will use the reloaded instance of the
module as well as the same copy of the data in it, until a `HUP` signal is sent to the Nginx master process to force a reload.
This data sharing technique is essential for high performance Lua applications based on this module.

Note that this data sharing is on a *per-worker* basis and not on a *per-server* basis. That is, when there are multiple Nginx worker processes under an Nginx master, data sharing cannot cross the process boundary between these workers.

It is usually recommended to share read-only data this way. You can also share changeable data among all the concurrent requests of each Nginx worker process as
long as there is *no* nonblocking I/O operations (including [ngx.sleep](#ngxsleep))
in the middle of your calculations. As long as you do not give the
control back to the Nginx event loop and ngx_lua's light thread
scheduler (even implicitly), there can never be any race conditions in
between. For this reason, always be very careful when you want to share changeable data on the
worker level. Buggy optimizations can easily lead to hard-to-debug
race conditions under load.

If server-wide data sharing is required, then use one or more of the following approaches:

1. Use the [ngx.shared.DICT](#ngxshareddict) API provided by this module.
1. Use only a single Nginx worker and a single server (this is however not recommended when there is a multi core CPU or multiple CPUs in a single machine).
1. Use data storage mechanisms such as `memcached`, `redis`, `MySQL` or `PostgreSQL`. [The OpenResty official releases](https://openresty.org) come with a set of companion Nginx modules and Lua libraries that provide interfaces with these data storage mechanisms.

[Back to TOC](#table-of-contents)

Known Issues
============

[Back to TOC](#table-of-contents)

TCP socket connect operation issues
-----------------------------------

The [tcpsock:connect](#tcpsockconnect) method may indicate `success` despite connection failures such as with `Connection Refused` errors.

However, later attempts to manipulate the cosocket object will fail and return the actual error status message generated by the failed connect operation.

This issue is due to limitations in the Nginx event model and only appears to affect Mac OS X.

[Back to TOC](#table-of-contents)

Lua Coroutine Yielding/Resuming
-------------------------------

* Because Lua's `dofile` and `require` builtins are currently implemented as C functions in LuaJIT 2.0/2.1, if the Lua file being loaded by `dofile` or `require` invokes [ngx.location.capture*](#ngxlocationcapture), [ngx.exec](#ngxexec), [ngx.exit](#ngxexit), or other API functions requiring yielding in the *top-level* scope of the Lua file, then the Lua error "attempt to yield across C-call boundary" will be raised. To avoid this, put these calls requiring yielding into your own Lua functions in the Lua file instead of the top-level scope of the file.

[Back to TOC](#table-of-contents)

Lua Variable Scope
------------------

Care must be taken when importing modules, and this form should be used:

```lua

 local xxx = require('xxx')
```

instead of the old deprecated form:

```lua

 require('xxx')
```

Here is the reason: by design, the global environment has exactly the same lifetime as the Nginx request handler associated with it. Each request handler has its own set of Lua global variables and that is the idea of request isolation. The Lua module is actually loaded by the first Nginx request handler and is cached by the `require()` built-in in the `package.loaded` table for later reference, and the `module()` builtin used by some Lua modules has the side effect of setting a global variable to the loaded module table. But this global variable will be cleared at the end of the request handler,  and every subsequent request handler all has its own (clean) global environment. So one will get Lua exception for accessing the `nil` value.

The use of Lua global variables is a generally inadvisable in the ngx_lua context as:

1. the misuse of Lua globals has detrimental side effects on concurrent requests when such variables should instead be local in scope,
1. Lua global variables require Lua table look-ups in the global environment which is computationally expensive, and
1. some Lua global variable references may include typing errors which make such difficult to debug.

It is therefore *highly* recommended to always declare such within an appropriate local scope instead.

```lua

 -- Avoid
 foo = 123
 -- Recommended
 local foo = 123

 -- Avoid
 function foo() return 123 end
 -- Recommended
 local function foo() return 123 end
```

To find all instances of Lua global variables in your Lua code, run the [lua-releng tool](https://github.com/openresty/nginx-devel-utils/blob/master/lua-releng) across all `.lua` source files:

    $ lua-releng
    Checking use of Lua global variables in file lib/foo/bar.lua ...
            1       [1489]  SETGLOBAL       7 -1    ; contains
            55      [1506]  GETGLOBAL       7 -3    ; setvar
            3       [1545]  GETGLOBAL       3 -4    ; varexpand

The output says that the line 1489 of file `lib/foo/bar.lua` writes to a global variable named `contains`, the line 1506 reads from the global variable `setvar`, and line 1545 reads the global `varexpand`.

This tool will guarantee that local variables in the Lua module functions are all declared with the `local` keyword, otherwise a runtime exception will be thrown. It prevents undesirable race conditions while accessing such variables. See [Data Sharing within an Nginx Worker](#data-sharing-within-an-nginx-worker) for the reasons behind this.

[Back to TOC](#table-of-contents)

Locations Configured by Subrequest Directives of Other Modules
--------------------------------------------------------------

The [ngx.location.capture](#ngxlocationcapture) and [ngx.location.capture_multi](#ngxlocationcapture_multi) directives cannot capture locations that include the [add_before_body](http://nginx.org/en/docs/http/ngx_http_addition_module.html#add_before_body), [add_after_body](http://nginx.org/en/docs/http/ngx_http_addition_module.html#add_after_body), [auth_request](https://nginx.org/en/docs/http/ngx_http_auth_request_module.html#auth_request), [echo_location](http://github.com/openresty/echo-nginx-module#echo_location), [echo_location_async](http://github.com/openresty/echo-nginx-module#echo_location_async), [echo_subrequest](http://github.com/openresty/echo-nginx-module#echo_subrequest), or [echo_subrequest_async](http://github.com/openresty/echo-nginx-module#echo_subrequest_async) directives.

```nginx

 location /foo {
     content_by_lua_block {
         res = ngx.location.capture("/bar")
     }
 }
 location /bar {
     echo_location /blah;
 }
 location /blah {
     echo "Success!";
 }
```

```nginx

 $ curl -i http://example.com/foo
```

will not work as expected.

[Back to TOC](#table-of-contents)

Cosockets Not Available Everywhere
----------------------------------

Due to internal limitations in the Nginx core, the cosocket API is disabled in the following contexts: [set_by_lua*](#set_by_lua), [log_by_lua*](#log_by_lua), [header_filter_by_lua*](#header_filter_by_lua), and [body_filter_by_lua](#body_filter_by_lua).

The cosockets are currently also disabled in the [init_by_lua*](#init_by_lua) and [init_worker_by_lua*](#init_worker_by_lua) directive contexts but we may add support for these contexts in the future because there is no limitation in the Nginx core (or the limitation might be worked around).

There exists a workaround, however, when the original context does *not* need to wait for the cosocket results. That is, creating a zero-delay timer via the [ngx.timer.at](#ngxtimerat) API and do the cosocket results in the timer handler, which runs asynchronously as to the original context creating the timer.

[Back to TOC](#table-of-contents)

Special Escaping Sequences
--------------------------

**NOTE** Following the `v0.9.17` release, this pitfall can be avoided by using the `*_by_lua_block {}` configuration directives.

PCRE sequences such as `\d`, `\s`, or `\w`, require special attention because in string literals, the backslash character, `\`, is stripped out by both the Lua language parser and by the Nginx config file parser before processing if not within a `*_by_lua_block {}` directive. So the following snippet will not work as expected:

```nginx

 # nginx.conf
 ? location /test {
 ?     content_by_lua '
 ?         local regex = "\d+"  -- THIS IS WRONG OUTSIDE OF A *_by_lua_block DIRECTIVE
 ?         local m = ngx.re.match("hello, 1234", regex)
 ?         if m then ngx.say(m[0]) else ngx.say("not matched!") end
 ?     ';
 ? }
 # evaluates to "not matched!"
```

To avoid this, *double* escape the backslash:

```nginx

 # nginx.conf
 location /test {
     content_by_lua '
         local regex = "\\\\d+"
         local m = ngx.re.match("hello, 1234", regex)
         if m then ngx.say(m[0]) else ngx.say("not matched!") end
     ';
 }
 # evaluates to "1234"
```

Here, `\\\\d+` is stripped down to `\\d+` by the Nginx config file parser and this is further stripped down to `\d+` by the Lua language parser before running.

Alternatively, the regex pattern can be presented as a long-bracketed Lua string literal by encasing it in "long brackets", `[[...]]`, in which case backslashes have to only be escaped once for the Nginx config file parser.

```nginx

 # nginx.conf
 location /test {
     content_by_lua '
         local regex = [[\\d+]]
         local m = ngx.re.match("hello, 1234", regex)
         if m then ngx.say(m[0]) else ngx.say("not matched!") end
     ';
 }
 # evaluates to "1234"
```

Here, `[[\\d+]]` is stripped down to `[[\d+]]` by the Nginx config file parser and this is processed correctly.

Note that a longer from of the long bracket, `[=[...]=]`, may be required if the regex pattern contains `[...]` sequences.
The `[=[...]=]` form may be used as the default form if desired.

```nginx

 # nginx.conf
 location /test {
     content_by_lua '
         local regex = [=[[0-9]+]=]
         local m = ngx.re.match("hello, 1234", regex)
         if m then ngx.say(m[0]) else ngx.say("not matched!") end
     ';
 }
 # evaluates to "1234"
```

An alternative approach to escaping PCRE sequences is to ensure that Lua code is placed in external script files and executed using the various `*_by_lua_file` directives.
With this approach, the backslashes are only stripped by the Lua language parser and therefore only need to be escaped once each.

```lua

 -- test.lua
 local regex = "\\d+"
 local m = ngx.re.match("hello, 1234", regex)
 if m then ngx.say(m[0]) else ngx.say("not matched!") end
 -- evaluates to "1234"
```

Within external script files, PCRE sequences presented as long-bracketed Lua string literals do not require modification.

```lua

 -- test.lua
 local regex = [[\d+]]
 local m = ngx.re.match("hello, 1234", regex)
 if m then ngx.say(m[0]) else ngx.say("not matched!") end
 -- evaluates to "1234"
```

As noted earlier, PCRE sequences presented within `*_by_lua_block {}` directives (available following the `v0.9.17` release) do not require modification.

```nginx

 # nginx.conf
 location /test {
     content_by_lua_block {
         local regex = [[\d+]]
         local m = ngx.re.match("hello, 1234", regex)
         if m then ngx.say(m[0]) else ngx.say("not matched!") end
     }
 }
 # evaluates to "1234"
```

**NOTE** You are recommended to use `by_lua_file` when the Lua code is very long.

[Back to TOC](#table-of-contents)

Mixing with SSI Not Supported
-----------------------------

Mixing SSI with ngx_lua in the same Nginx request is not supported at all. Just use ngx_lua exclusively. Everything you can do with SSI can be done atop ngx_lua anyway and it can be more efficient when using ngx_lua.

[Back to TOC](#table-of-contents)

SPDY Mode Not Fully Supported
-----------------------------

Certain Lua APIs provided by ngx_lua do not work in Nginx's SPDY mode yet: [ngx.location.capture](#ngxlocationcapture), [ngx.location.capture_multi](#ngxlocationcapture_multi), and [ngx.req.socket](#ngxreqsocket).

[Back to TOC](#table-of-contents)

Missing data on short circuited requests
----------------------------------------

Nginx may terminate a request early with (at least):

* 400 (Bad Request)
* 405 (Not Allowed)
* 408 (Request Timeout)
* 413 (Request Entity Too Large)
* 414 (Request URI Too Large)
* 494 (Request Headers Too Large)
* 499 (Client Closed Request)
* 500 (Internal Server Error)
* 501 (Not Implemented)

This means that phases that normally run are skipped, such as the rewrite or
access phase. This also means that later phases that are run regardless, e.g.
[log_by_lua](#log_by_lua), will not have access to information that is normally set in those
phases.

[Back to TOC](#table-of-contents)

TODO
====

* cosocket: implement LuaSocket's unconnected UDP API.
* cosocket: add support in the context of [init_by_lua*](#init_by_lua).
* cosocket: review and merge aviramc's [patch](https://github.com/openresty/lua-nginx-module/pull/290) for adding the `bsdrecv` method.
* cosocket: add configure options for different strategies of handling the cosocket connection exceeding in the pools.
* use `ngx_hash_t` to optimize the built-in header look-up process for [ngx.req.set_header](#ngxreqset_header), and etc.
* add `ignore_resp_headers`, `ignore_resp_body`, and `ignore_resp` options to [ngx.location.capture](#ngxlocationcapture) and [ngx.location.capture_multi](#ngxlocationcapture_multi) methods, to allow micro performance tuning on the user side.
* add automatic Lua code time slicing support by yielding and resuming the Lua VM actively via Lua's debug hooks.
* add `stat` mode similar to [mod_lua](https://httpd.apache.org/docs/trunk/mod/mod_lua.html).

[Back to TOC](#table-of-contents)

Changes
=======

The changes made in every release of this module are listed in the change logs of the OpenResty bundle:

<https://openresty.org/#Changes>

[Back to TOC](#table-of-contents)

Test Suite
==========

The following dependencies are required to run the test suite:

* Nginx version >= 1.4.2

* Perl modules:
	* Test::Nginx: <https://github.com/openresty/test-nginx>

* Nginx modules:
	* [ngx_devel_kit](https://github.com/simplresty/ngx_devel_kit)
	* [ngx_set_misc](https://github.com/openresty/set-misc-nginx-module)
	* [ngx_auth_request](http://mdounin.ru/files/ngx_http_auth_request_module-0.2.tar.gz) (this is not needed if you're using Nginx 1.5.4+.
	* [ngx_echo](https://github.com/openresty/echo-nginx-module)
	* [ngx_memc](https://github.com/openresty/memc-nginx-module)