MQTT Client Package - Lua language

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Lua MQTT client library (version 0.2 2012-06-01)

This project is part of the Eclipse Paho project


MQTT stands for “Message Queue Telemetry Transport”, a protocol authored by Dr. Andy Stanford-Clark and Arlen Nipper. The protocol is a message-based, publish/subscribe transport layer, which is optimized for simple telemetry applications running on small micro-controllers, such as an Arduino, over low-bandwidth connections. MQTT libraries exist for most popular programming languages, so you can utilize MQTT on whatever server or device that you require.

The Lua MQTT client library implements the client-side subset of the MQTT protocol specification 3.1.

Lua MQTT overview

A good use-case for this library is running on constrained systems, such as OpenWRT, and acting as a gateway between non-MQTT clients and MQTT servers. An advantage of using Lua is that only a text editor is required for rapid development of simple MQTT client applications on platforms such as OpenWRT. In constrast, working with the C programming language would comparatively require more effort, due to requiring a cross-platform development environment.

The Lua MQTT client library also runs (unmodified) on a Sony PlayStation Portable using the Lua Player HM (which requires your PSP to be able to run unsigned executables).

PlayStation Portable

Prerequisites ...

On Linux, Lua and LuaRocks can be installed via your Linux distribution package manager. On Mac OS X, Lua and LuaRocks can be installed viarDarwin ports. After that, LuaSocket and PenLight can be installed via LuaRocks.

Lua MQTT client library (source code) from GitHub ...

  • TODO

mqtt_test: Test publish and receive messages on different topics

This command periodically publishes a message on topic “test/1” and subscribes to the topic “test/2”. The command exits when the message “quit” is published on topic “test/2”.

  cd $(LUA_MQTT_LIB)              // where Lua MQTT library is installed
  example/mqtt_test -d localhost  // Assume MQTT server is on "localhost"

  -d,--debug                       Verbose console logging
  -i,--id     (default MQTT test)  MQTT client identifier
  -p,--port   (default 1883)       MQTT server port number
  <host>      (default localhost)  MQTT server hostname

mqtt_publish: Publish a single message to a specified topic

This command publishes a single message and then exits.

  example/mqtt_publish -d -t test/1 -m "Test message"

Only the --topic and --message parameters are required.

  -d,--debug                               Verbose console logging
  -H,--host         (default localhost)    MQTT server hostname
  -i,--id           (default MQTT client)  MQTT client identifier
  -m,--message      (string)               Message to be published
  -p,--port         (default 1883)         MQTT server port number
  -t,--topic        (string)               Topic on which to publish
  -w,--will_message                        Last will and testament message
  -w,--will_qos     (default 0)            Last will and testament QOS
  -w,--will_retain  (default 0)            Last will and testament retention
  -w,--will_topic                          Last will and testament topic

mqtt_subscribe: Subscribe to a topic

This command subscribes to a topic and listens indefinitely for messages. Use ^C (or similar) to stop execution.

  example/mqtt_subscribe -d -t test/1

Only the --topic parameter is required.

  -d,--debug                               Verbose console logging
  -H,--host         (default localhost)    MQTT server hostname
  -i,--id           (default MQTT client)  MQTT client identifier
  -k,--keepalive    (default 60)           Send MQTT PING period (seconds)
  -p,--port         (default 1883)         MQTT server port number
  -t,--topic        (string)               Subscription topic
  -w,--will_message                        Last will and testament message
  -w,--will_qos     (default 0)            Last will and testament QOS
  -w,--will_retain  (default 0)            Last will and testament retention
  -w,--will_topic                          Last will and testament topic
-- Define a function which is called by mqtt_client:handler(),
-- whenever messages are received on the subscribed topics

function callback(topic, message)
  print("Received: " .. topic .. ": " .. message)
  if (message == "quit") then running = false end

-- Create an MQTT client instance, connect to the MQTT server and
-- subscribe to the topic called "test/2"

MQTT = require "paho.mqtt"
mqtt_client = MQTT.client.create("localhost", nil, callback)
mqtt_client:connect("lua mqtt client"))

-- Continously invoke mqtt_client:handler() to process the MQTT protocol and
-- handle any received messages.  Also, publish a message on topic "test/1"

running = true

while (running) do
  mqtt_client:publish("test/1", "test message")
  socket.sleep(1.0)  -- seconds

-- Clean-up by unsubscribing from the topic and closing the MQTT connection


There are also a number of Lua MQTT client examples in the example/ directory. They can be run from the lua/ parent directory, as follow ...

cd mqtt_client/paho

Use the Lua require statement to load the MQTT client library ...

local MQTT = require 'paho.mqtt'

MQTT.Utility.set_debug(): Library debug console logging

The following statement enables debug console logging for diagnosis.


MQTT.client.create(): Create an MQTT client instance

Create an MQTT client that will be connected to the specified host.

mqtt_client = MQTT.client.create(hostname, port, callback)

The hostname must be provided, but both the port and callback function parameters are optional. This function returns an MQTT client instance that must be used for all subsequent MQTT operations for that server connection.

hostname string:   Host name or address of the MQTT broker
port     integer:  Port number of the MQTT broker (default: 1883)
callback function: Invoked when subscribed topic messages received

The callback function is defined as follows ...

function callback(topic, payload)
  -- application specific code

topic   -- string: Topic for the received message
payload -- string: Message data

MQTT.client:destroy(): Destroy an MQTT client instance

When finished with a server connection, this statement cleans-up all resources allocated by the client.


MQTT.client:connect(): Make a connection to an MQTT server

Before messages can be transmitted, the MQTT client must connect to the server.


Each individual client connection must use a unique identifier. Only the identifier parameter is required, the remaining parameters are optional.

mqtt_client:connect(identifier, will_topic, will_qos, will_retain, will_message)

MQTT also provides a “last will and testament” for clients, which is a message automatically sent by the server on behalf of the client, should the connection fail.

identifier   -- string: MQTT client identifier (maximum 23 characters)
will_topic   -- string: Last will and testament topic
will_qos     -- byte:   Last will and testament Quality Of Service
will_retain  -- byte:   Last will and testament retention status
will_message -- string: Last will and testament message

MQTT.client:disconnect(): Transmit MQTT Disconnect message

Transmit an MQTT disconnect message to the server.


MQTT.client:publish(): Transmit MQTT publish message

Transmit a message on a specified topic.

mqtt_client:publish(topic, payload)

topic   -- string: Topic for the published message
payload -- string: Message data

MQTT.client:subscribe(): Transmit MQTT Subscribe message

Subscribe to one or more topics. Whenever a message is published to one of those topics, the callback function (defined above) will be invoked.


topics -- table of strings, e.g. { "topic1", "topic2" }

MQTT.client:handler(): Handle received messages, maintain keep-alive messages

The handler() function must be called periodically to service incoming messages and to ensure that keep-alive messages (PING) are being sent when required.

The default KEEP_ALIVE_TIME is 60 seconds, therefore handler() must be invoked more often than once per minute.

Should any messages be received on the subscribed topics, then handler() will invoke the callback function (defined above).


MQTT.client:unsubscribe(): Transmit MQTT Unsubscribe message

Unsubscribe from one or more topics, so that messages published to those topics are no longer received.

  topics -- table of strings, e.g. { "topic1", "topic2" }
  • Occasional “MQTT.client:handler(): Message length mismatch” errors, particularly when subscribed topics are transmitting many messages.

  • Not really a problem, but if you find that the MQTT socket connection is being closed for no apparent reason, particularly for subscribers ... then check that all MQTT clients are using a unique client identifier.