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socket: Automatic LuaSocket/cosockets compatibility module


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CentOS/RHEL 7 or Amazon Linux 2

yum -y install
yum -y install lua-resty-socket

CentOS/RHEL 8+, Fedora Linux, Amazon Linux 2023

yum -y install
yum -y install lua5.1-resty-socket

To use this Lua library with NGINX, ensure that nginx-module-lua is installed.

This document describes lua-resty-socket v1.0.0 released on Jan 18 2019.

Module Version cosocket/LuaSocket automatic compatibility module for lua-resty modules wanting to be compatible with plain Lua or OpenResty's init context.

The use case for this library is: you are developing a lua-resty module relying on cosockets, but you want it to also be usable in OpenResty's init context or even in plain Lua. This module aims at always providing your library with sockets that will be compatible in the current context, saving you time and effort, and extending LuaSocket's API to match that of cosockets, allowing you to always write your code as if you were in a cosocket-compatible OpenResty context.


  • Allows your lua-resty modules to automatically use cosockets/LuaSocket
  • Provides sslhandshake proxy when using LuaSocket, with a dependency on LuaSec
  • Does not get blocked to using LuaSocket in further contexts if loaded in the ngx_lua init (easy mistake to make)
  • Memoizes underlying socket methods for performance
  • Outputs a warning log for your users when spawning a socket using LuaSocket while in OpenResty


The aim of this module is to provide an automatic fallback to LuaSocket when ngx_lua's cosockets are not available. That is: - When not used in ngx_lua - In ngx_lua contexts where cosockets are not supported (init, init_worker, etc...)

When falling back to LuaSocket, it provides you with shims for cosocket-only functions such as getreusedtimes, setkeepalive etc...

It comes handy when one is developing a module/library that aims at being either compatible with both ngx_lua and plain Lua, or in ngx_lua contexts such as init.

Libraries using it

Here are some concrete examples uses of this module. You can see how we only write code as if we were constantly in an cosocket-compatible OpenResty context, which greatly simplifies our work and provides out of the box plain Lua compatibility.

  • lua-cassandra: see how the cassandra module is compatible in both OpenResty and plain Lua with no efforts or special code paths distinguishing cosockets and LuaSocket.

Important note

The use of LuaSocket inside ngx_lua is very strongly discouraged due to its blocking nature. However, it is fine to use it in the init context where blocking is not considered harmful.

In the future, only the init phase will allow falling back to LuaSocket.

It currently only support TCP sockets.


All of the available functions follow the same prototype as the cosocket API, allowing this example to run in any ngx_lua context or outside ngx_lua altogether:

local socket = require 'resty.socket'
local sock = socket.tcp()

getmetatable(sock) == socket.luasocket_mt ---> true/false depending on underlying socket

sock:settimeout(1000) ---> 1000ms translated to 1s if LuaSocket

sock:getreusedtimes(...) ---> 0 if LuaSocket

sock:setkeepalive(...) ---> calls close() if LuaSocket

sock:sslhandshake(...) ---> LuaSec dependency if LuaSocket

As such, one can write a module relying on TCP sockets such as:

local socket = require 'resty.socket'

local _M = {}

  local sock = socket.tcp() -- similar to ngx.socket.tcp()

  return setmetatable({
    sock = sock
  }, {__index = _M})

function _M:connect(host, port)
  local ok, err = self.sock:connect(host, port)
  if not ok then
    return nil, err

  local times, err = self.sock:getreusedtimes() -- cosocket API
  if not times then
    return nil, err
  elseif times == 0 then
    -- handle connection

return _M

The user of such a module could use it in contexts with cosocket support, or in the init phase of ngx_lua, with little effort from the developer.


Work licensed under the MIT License.


You may find additional configuration tips and documentation for this module in the GitHub repository for nginx-module-socket.