Lua Filter

Lua Filter

The luafilter is a filter that calls a set of functions in a Lua script.

Read the Lua language documentation for information on how to write Lua scripts.

Note: This module is experimental and must be built from source. The module is deprecated in MaxScale 23.08 and might be removed in a future release.

Filter Parameters

The luafilter has two parameters. They control which scripts will be called by the filter. Both parameters are optional but at least one should be defined. If both global_script and session_script are defined, the entry points in both scripts will be called.


The global Lua script. The parameter value is a path to a readable Lua script which will be executed.

This script will always be called with the same global Lua state and it can be used to build a global view of the whole service.


The session level Lua script. The parameter value is a path to a readable Lua script which will be executed once for each session.

Each session will have its own Lua state meaning that each session can have a unique Lua environment. Use this script to do session specific tasks.

Lua Script Calling Convention

The entry points for the Lua script expect the following signatures:

  • nil createInstance() - global script only, called when MaxScale is started

    • The global script will be loaded in this function and executed once on a global level before calling the createInstance function in the Lua script.
  • nil newSession(string, string) - new session is created

    • After the session script is loaded, the newSession function in the Lua scripts is called. The first parameter is the username of the client and the second parameter is the client's network address.
  • nil closeSession() - session is closed

    • The closeSession function in the Lua scripts will be called.
  • (nil | bool | string) routeQuery(string) - query is being routed

    • The Luafilter calls the routeQuery functions of both the session and the global script. The query is passed as a string parameter to the routeQuery Lua function and the return values of the session specific function, if any were returned, are interpreted. If the first value is bool, it is interpreted as a decision whether to route the query or to send an error packet to the client. If it is a string, the current query is replaced with the return value and the query will be routed. If nil is returned, the query is routed normally.
  • nil clientReply() - reply to a query is being routed

    • This function calls the clientReply function of the Lua scripts.
  • string diagnostic() - global script only, print diagnostic information

    • This will call the matching diagnostics entry point in the Lua script. If the Lua function returns a string, it will be printed to the client.

These functions, if found in the script, will be called whenever a call to the matching entry point is made.

Script Template

Here is a script template that can be used to try out the luafilter. Copy it into a file and add global_script=<path to script> into the filter configuration. Make sure the file is readable by the maxscale user.

function createInstance()


function newSession()


function closeSession()


function routeQuery(query)


function clientReply(query)


function diagnostic()


Functions Exposed by the Luafilter

The luafilter exposes three functions that can be called from the Lua script.

  • string lua_qc_get_type_mask()

  • Returns the type of the current query being executed as a string. The values are the string versions of the query types defined in query_classifier.h are separated by vertical bars (|).

    This function can only be called from the routeQuery entry point.

  • string lua_qc_get_operation()

  • Returns the current operation type as a string. The values are defined in query_classifier.h.

    This function can only be called from the routeQuery entry point.

  • string lua_get_canonical()

  • Returns the canonical version of a query by replacing all user-defined constant values with question marks.

    This function can only be called from the routeQuery entry point.

  • number id_gen()

  • This function generates unique integers that can be used to distinct sessions from each other.

Example Configuration and Script

Here is a minimal configuration entry for a luafilter definition.


And here is a script that opens a file in /tmp/ and logs output to it.

f ="/tmp/test.log", "a+")

function createInstance()

function newSession(a, b)
    f:write("newSession for: " .. a .. "@" .. b .. "\n")

function closeSession()

function routeQuery(string)
    f:write("routeQuery: " .. string .. " -- type: " .. lua_qc_get_type_mask() .. " operation: " .. lua_qc_get_operation() .. "\n")

function clientReply()

function diagnostic()
    return "Hello from Lua!"


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