OpenResty Scalable Web Platform by Extending NGINX with Lua

Frequently Asked Questions

Yichun Zhang , 31 Jul 2015 (created 13 Jun 2015)

Questions and Answers

Does OpenResty follow NGINX's mainline releases?

Yes, sure. OpenResty is currently employing a "tick-tock" release model. Each "tock" release (usually) upgrades the bundled NGINX core to the latest mainline release from the official NGINX team, followed by one ore more "tick" releases focusing on OpenResty's own features and enhancements without upgrading the NGINX core.

The first part of the OpenResty release version numbers is the version of the bundled NGINX core. For instance, OpenResty bundles NGINX 1.7.10 while OpenResty bundles NGINX 1.9.1.

How often does OpenResty make a new release?

We are trying to make a new release of OpenResty every one or two months. It may sometimes take longer because this project is mainly based on volunteers.

How should I report a problem?

Whenever you run into a problem, you are encouraged to report to the openresty-en (English) mailing list or the openresty (Chinese) mailing list (depending on your language). See the Community page for more details. But please don't cross-post.

You are highly recommended to provide as much details as possible while reporting a problem, for example,

  • anything interesting in your Nginx's error log file (logs/error.log), if any,
  • the exact versions of the related software you're using, including but not limited to the type/version of your operating system, the version of your OpenResty (or the versions of your Nginx, ngx_lua, Lua/LuaJIT and other modules used if you are not using the OpenResty bundle),
  • a minimal and standalone example that can reliably reproduce the issue on our side, and
  • enable the Nginx/OpenResty's debugging logs and provide the complete logs for the guilty request you performed (the same ./configure --with-debug command line also applies perfectly well to the OpenResty bundle).

The more details you provide, the more likely and faster we can help you out. Most of the time, you will find your own mistakes while minimizing your problematic example or looking at your Nginx error log files.

If you are absolutely certain that you have run into a real bug, then you are encouraged to file a ticket in the corresponding project's GitHub Issues page. For example, the GitHub Issues page for the ngx_lua component is The GitHub issues pages for OpenResty's components are all considered English-only. Never use other languages like Chinese there. It is still fine, however, to just send a mail to one of the mailing lists mentioned above; but again, please do not cross-post.

Why can't I use Lua 5.2 or later?

Lua 5.2+ are incompatible with Lua 5.1 on both the C API land and the Lua land (including various language semantics). If as you said there are quite a few people already using ngx_lua + Lua 5.1, then linking against Lua 5.2+ will probably break these people's existing Lua code. Lua 5.2+ are essentially incompatible different languages.

Supporting Lua 5.2+ requires nontrivial architectural changes in ngx_lua's basic infrastructure. The most troublesome thing is the quite different "environments" model in Lua 5.2+. At this point, we would hold back adding support for Lua 5.2+ to ngx_lua. Also, we do not want to create confusions and incompatibilities on the Lua land for applications running atop ngx_lua, as well as all the existing lua-resty-* libraries written in the Lua 5.1 language.

We believe it is better to stick with one Lua language in ngx_lua. Chasing the Lua language's version number has not many practical technical merits (if there were some political ones).

Can I use LuaJIT 2.0.x?

Yes sure. LuaJIT 2.0.x is always supported in OpenResty though LuaJIT 2.1+ is highly recommended.

Why does OpenResty use LuaJIT 2.1 by default?

As of this writing, LuaJIT 2.1 is still officially "beta" but OpenResty is using LuaJIT 2.1 by default and encourages use of it in production because

  1. We always run the latest LuaJIT v2.1 in various large-scale server-side business software infrastructures.
  2. All the recent performance improvements Cloudflare Inc. has sponsored only land in v2.1.
  3. For one of our typical Lua apps at the level of 10K+ LOC, LuaJIT v2.1 gives over 100% over-all speedup as compared to LuaJIT 2.0.x (when lua-resty-core is used).
  4. Many important bug fixes in the v2.1 branch have been back ported to 2.0.x series because these bugs were also in 2.0.x (just hidden for long). The 2.0.3 release is such a proof.

We highly recommend LuaJIT 2.1 because we really need speed in OpenResty though you always have the freedom to use LuaJIT 2.0.x or even the standard Lua 5.1 interpreter in OpenResty instead.

Why can't I use duplicate configuration directives?

Most of the ngx_lua module's configuration directives do not allow duplication in the same context. For example, the following nginx.conf snippet

    location / {
        content_by_lua_file conf/a.lua;
        content_by_lua_file conf/b.lua;

will yield the following error while starting nginx:

nginx: [emerg] "content_by_lua_file" directive is duplicate in

People may want to use duplicate directives to split complicated Lua code base into multiple separate .lua files. This is not allowed and it must be inefficient even if it were to be implemented in ngx_lua.

The recommended way to organize your Lua code base is to use Lua's own module mechanism:

You can put your unrelated Lua code into separate Lua module files, for example,

-- foo.lua
local _M = {}
function _M.go() ... end
return _M
-- bar.lua
local _M = {}
function _M.go() ... end
return _M

And finally, just require them in the entry point like this:

location / {
    content_by_lua '

You will need to add the path of your Lua modules to Lua's module search paths, for instance,

lua_package_path "$prefix/conf/?.lua;;";

in the http {} block in your nginx.conf.

You can take a look at OpenResty's standard Lua libraries for real-world examples, like openresty/lua-resty-redis.

Use of Lua's native module mechanism is also very efficient. Thanks to the built-in caching mechanism in Lua's built-in function require() (via the global package.loaded table anchored in the Lua registry, thus being shared by the whole Lua VM).

Can I use custom loggers in Lua?

Yes, sure. You have multiple options:

  1. Write a custom Lua logger library manipulating files with LuaJIT FFI directly. You need to access the low level file I/O syscalls like open, write, and close without going through libc's buffered I/O layer. This is important because this can ensure atomicity when multiple (worker) processes are appending data to the same file (in the appending mode). You can share the resulting file descriptors (fd) returned from the open syscall on the NGINX worker process level by the this technique. Maybe someday OpenResty will ship with a standard lua-resty-logger-file library; you are welcome to contribute such a library :)
  2. Avoid file I/O in the nginx server altogether by employing the lua-resty-logger-socket library. This library can send your log data to the remote endpoint (like a syslog-ng server) via sockets and 100% nonblocking I/O. Cloudflare, for example, has been using this library heavily across its global network. Unlike NGINX core's native syslog loggers, this one tries very hard to avoid data loss in erroneous conditions.

Socket-based logging is generally recommended for performance reasons because file I/O may almost always block the NGINX event loop (or some OS threads if enabling the new NGINX thread pool support for file I/O, still adding extra overhead).

Why am I seeing the "lua tcp socket connect timed out" error?

The error happens when the connect operation to the backend times out. The backend can be Redis, Memcached, MySQL, or any other thing you are currently using. This error log message is generated by the ngx_lua module's automatic error logger for cosockets. If you already handle all the cosocket errors properly in your own Lua code, then you can suppress the automatic logger by turning off the lua_socket_log_errors in your ngnx.conf:

lua_socket_log_errors off;

The cause of the connect timeout errors can be complicated but generally falls into the following categories:

  1. your redis server is too busy to accept new connections and the redis server's TCP stack is dropping packets when the accept() queue is full, or
  2. your nginx server is too busy to respond to new I/O events (like doing CPU intensive computations or blocking on system calls),
  3. your timeout threshold on the client side is too small to account for the latency over the wire and the network stack.

You can try the following to address the issue:

  1. Scale your backend to multiple nodes so as to utilize more (CPU) resources (for example, the redis server is single-threaded and can easily exhaust a single CPU core).
  2. Increase the backlog setting in your backend server (for example, there is a tcp-backlog parameter to tune in the redis.conf configuration file); the backlog setting determins the length limit of your accept() queue on your backend server.
  3. Check whether your nginx server is too busy doing CPU intensive work or blocking syscalls (like disk I/O syscalls) by using the flame graph tools,
  4. Increase the timeout threshold in your Lua code if it makes sense,
  5. Automatically retry connect() in your Lua code for one or two more time with an optional delay when your connect() call fails.

Why am I always getting 0 from getreusedtimes() or get_reused_times() calls?

This usually means that you fail to put your backend connections into the connection pool in the first place and the connection pool is always empty so that connect() calls always establish new connections instead of reusing existing ones.

The solution is to always check the return values of the setkeepalive() or set_keepalive() calls and handle errors properly if any. When you get errors from setkeepalive() or set_keepalive() calls, then we can work on solving the problem (like avoid calling setkeepalive() or set_keepalive() at the wrong times).

Can I set timeout threshold on subrequests?

Yes, you can, but not directy via an option or parameter on the subrequest calls like ngx.location.capture and echo_subrequest. Rather, you should specify the timeout configurations in the location targeted by your subrequest. For example,

location = /api {
    content_by_lua '
        local res = ngx.locatin.capture("/sub")

location = /sub {
    proxy_connect_timeout 100ms;
    proxy_read_timeout 100ms;
    proxy_send_timeout 100ms;
    proxy_pass http://backend;

Here you specify all the timeout thresholds provided by the ngx_proxy module in the (internal) location (= /sub) accessed by your subrequest.

Can I access remote URLs via ngx.location.capture or alike?

Yes, but not directly. The subrequest API targets nginx's "locations". So you need a dedicated location (be it "internal" or not) and configure the standard ngx_proxy module there.

Alternatively, you may consider using one of the lua-resty-http* libraries contributed by the community. To name a few:

Contributing to this FAQ

This FAQ document is hosted on GitHub and periodically updated to the site:

You can edit the file in the repository above and create pull requests so that I can incorporate your patches.