Cross language

In the EMQ X Broker distribution, some plugins are provided for multiple language support. It allows you to extend the behavior of EMQ X Broker with other programming languages, and the architecture in the system is:

Multiple Language Suppoprt

  • Multiple language support appears as a plug-in, which is based on the hook feature provided by the emqx core project to get the Events/messages of EMQ X Broker.
  • For different language environments, different language support plugins are required.
  • This support plugin embeds all the environments of the language runtime.
  • The user only needs to write a script or library file in this language for the plugin to be called.

This is the basic logic for implementing multiple language support. When using this kind of plugin, please make sure you have some knowledge of hooks and plugins .


Support of Lua is achieved by emqx_lua_hook which includes:

  • A set of Lua runtime environment, implemented by luerl
  • Some control commands to manage the load and unload of Lua.


In the EMQ X Broker distribution, user-defined Lua script files should be placed in data/script/.

Take the sending content of the control message as an example, and add the file data/script/test.lua:

function on_message_publish(clientid, username, topic, payload, qos, retain)
    return topic, "hello", qos, retain

function register_hook()
    return "on_message_publish"

The script shows:

  • Implemented a callback function on_message_publish and changed the payload field of all published messages to hello.
  • Use register_hook to tell emqx_lua_hook the name list of callback function that need to be registered.

It is worth noting that the names, parameters, data types, and number of these callback functions are fixed and must be consistent with the examples provided.

After the script is written, you need to manually load it into the emqx_lua_hook plugin:

The emqx_lua_hook plugin is enabled at first:

./bin/emqx_ctl plugins load emqx_lua_hook

Load test.lua into emqx_lua_hook:

./bin/emqx_ctl luahook load test.lua

When the execution succeeds, it means that the script has been successfully loaded. Otherwise, check whether the syntax of the source file is correct.

After completion, you can start two MQTT clients, one to subscribe to any topic, and the other to publish any message to the topic that you just subscribed to. It can be found that the message content received by the subscriber is hello which proves that the test.lua script has taken effect.

Callback function

Supported callback functions and parameter type: emqx-web-hook -

Example: examples.lua


Load the specified Lua script:

## Script: Script file name
luahook load <Script>

Unload the specified Lua script:

luahook unload <Script>

Reload the specified Lua script:

luahook reload <Script>

Load the specified Lua script and set it to start with emqx_lua_hook:

luahook enable <Script>

Unload the specified Lua script and cancel it to start with emqx_lua_hook:

luahook disable <Script>

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