nodemcu-firmware/docs/en/modules/mqtt.md

MQTT Module

Since	Origin / Contributor	Maintainer	Source
2015-01-23	Stephen Robinson, Tuan PM	Vowstar	mqtt.c

The client adheres to version 3.1.1 of the MQTT protocol. Make sure that your broker supports and is correctly configured for version 3.1.1. The client is backwards incompatible with brokers running MQTT 3.1.

mqtt.Client()

Creates a MQTT client.

Syntax

mqtt.Client(clientid, keepalive[, username, password, cleansession, max_message_length])

Parameters

• clientid client ID
• keepalive keepalive seconds
• username user name
• password user password
• cleansession 0/1 for false/true. Default is 1 (true).
• max_message_length, how large messages to accept. Default is 1024.

Returns

MQTT client

Notes

According to MQTT specification the max PUBLISH length is 256Mb. This is too large for NodeMCU to realistically handle. To avoid an out-of-memory situation, there is a limit on how big messages to accept. This is controlled by the max_message_length parameter. In practice, this only affects incoming PUBLISH messages since all regular control packets are small. The default 1024 was chosen as this was the implicit limit in NodeMCU 2.2.1 and older (where this was not handled at all).

Note that "message length" refers to the full MQTT message size, including fixed & variable headers, topic name, packet ID (if applicable), and payload. For exact details, please see the MQTT specification.

Any message larger than max_message_length will be (partially) delivered to the overflow callback, if defined. The rest of the message will be discarded. Any subsequent messages should be handled as expected. Discarded messages will still be ACK'ed if QoS level 1 or 2 was requested, even if the application stack cannot handle them.

Heap memory will be used to buffer any message which spans more than a single TCP packet. A single allocation for the full message will be performed when the message header is first seen, to avoid heap fragmentation. If allocation fails, the MQTT session will be disconnected. Naturally, messages larger than max_message_length will not be stored.

Note that heap allocation may occur even if the individual messages are not larger than the configured max! For example, the broker may send multiple smaller messages in quick succession, which could go into the same TCP packet. If the last message in the TCP packet did not fit fully, a heap buffer will be allocated to hold the incomplete message while waiting for the next TCP packet.

The typical maximum size for a message to fit into a single TCP packet is 1460 bytes, but this depends on the network's MTU configuration, any packet fragmentation, and as described above, multiple messages in the same TCP packet.

Example

-- init mqtt client without logins, keepalive timer 120s
m = mqtt.Client("clientid", 120)

-- init mqtt client with logins, keepalive timer 120sec
m = mqtt.Client("clientid", 120, "user", "password")

-- setup Last Will and Testament (optional)
-- Broker will publish a message with qos = 0, retain = 0, data = "offline" 
-- to topic "/lwt" if client don't send keepalive packet
m:lwt("/lwt", "offline", 0, 0)

m:on("connect", function(client) print ("connected") end)
m:on("offline", function(client) print ("offline") end)

-- on publish message receive event
m:on("message", function(client, topic, data) 
  print(topic .. ":" ) 
  if data ~= nil then
    print(data)
  end
end)

-- on publish overflow receive event
m:on("overflow", function(client, topic, data)
  print(topic .. " partial overflowed message: " .. data )
end)

-- for TLS: m:connect("192.168.11.118", secure-port, 1)
m:connect("192.168.11.118", 1883, 0, function(client)
  print("connected")
  -- Calling subscribe/publish only makes sense once the connection
  -- was successfully established. You can do that either here in the
  -- 'connect' callback or you need to otherwise make sure the
  -- connection was established (e.g. tracking connection status or in
  -- m:on("connect", function)).

  -- subscribe topic with qos = 0
  client:subscribe("/topic", 0, function(client) print("subscribe success") end)
  -- publish a message with data = hello, QoS = 0, retain = 0
  client:publish("/topic", "hello", 0, 0, function(client) print("sent") end)
end,
function(client, reason)
  print("failed reason: " .. reason)
end)

m:close();
-- you can call m:connect again

MQTT Client

mqtt.client:close()

Closes connection to the broker.

Syntax

mqtt:close()

Parameters

none

Returns

true on success, false otherwise

mqtt.client:connect()

Connects to the broker specified by the given host, port, and secure options.

Syntax

mqtt:connect(host[, port[, secure[, autoreconnect]]][, function(client)[, function(client, reason)]])

Parameters

• host host, domain or IP (string)
• port broker port (number), default 1883
• secure 0/1 for false/true, default 0. Take note of constraints documented in the net module.
• autoreconnect 0/1 for false/true, default 0. This option is deprecated.
• function(client) callback function for when the connection was established
• function(client, reason) callback function for when the connection could not be established. No further callbacks should be called.

Returns

true on success, false otherwise

Notes

Don't use autoreconnect. Let me repeat that, don't use autoreconnect. You should handle the errors explicitly and appropriately for your application. In particular, the default for cleansession above is true, so all subscriptions are destroyed when the connection is lost for any reason.

In order to acheive a consistent connection, handle errors in the error callback. For example:

function handle_mqtt_error(client, reason) 
  tmr.create():alarm(10 * 1000, tmr.ALARM_SINGLE, do_mqtt_connect)
end

function do_mqtt_connect()
  mqtt:connect("server", function(client) print("connected") end, handle_mqtt_error)
end

In reality, the connected function should do something useful!

This is the description of how the autoreconnect functionality may (or may not) work.

When autoreconnect is set, then the connection will be re-established when it breaks. No error indication will be given (but all the subscriptions may be lost if cleansession is true). However, if the very first connection fails, then no reconnect attempt is made, and the error is signalled through the callback (if any). The first connection is considered a success if the client connects to a server and gets back a good response packet in response to its MQTT connection request. This implies (for example) that the username and password are correct.

Connection failure callback reason codes:

Constant	Value	Description
mqtt.CONN_FAIL_SERVER_NOT_FOUND	-5	There is no broker listening at the specified IP Address and Port
mqtt.CONN_FAIL_NOT_A_CONNACK_MSG	-4	The response from the broker was not a CONNACK as required by the protocol
mqtt.CONN_FAIL_DNS	-3	DNS Lookup failed
mqtt.CONN_FAIL_TIMEOUT_RECEIVING	-2	Timeout waiting for a CONNACK from the broker
mqtt.CONN_FAIL_TIMEOUT_SENDING	-1	Timeout trying to send the Connect message
mqtt.CONNACK_ACCEPTED	0	No errors. Note: This will not trigger a failure callback.
mqtt.CONNACK_REFUSED_PROTOCOL_VER	1	The broker is not a 3.1.1 MQTT broker.
mqtt.CONNACK_REFUSED_ID_REJECTED	2	The specified ClientID was rejected by the broker. (See mqtt.Client())
mqtt.CONNACK_REFUSED_SERVER_UNAVAILABLE	3	The server is unavailable.
mqtt.CONNACK_REFUSED_BAD_USER_OR_PASS	4	The broker refused the specified username or password.
mqtt.CONNACK_REFUSED_NOT_AUTHORIZED	5	The username is not authorized.

mqtt.client:lwt()

Setup Last Will and Testament (optional). A broker will publish a message with qos = 0, retain = 0, data = "offline" to topic "/lwt" if client does not send keepalive packet.

As the last will is sent to the broker when connecting, lwt() must be called BEFORE calling connect().  

The broker will publish a client's last will message once he NOTICES that the connection to the client is broken. The broker will notice this when:  - The client fails to send a keepalive packet for as long as specified in mqtt.Client()  - The tcp-connection is properly closed (without closing the mqtt-connection before)

• The broker tries to send data to the client and fails to do so, because the tcp-connection is not longer open.

This means if you specified 120 as keepalive timer, just turn off the client device and the broker does not send any data to the client, the last will message will be published 120s after turning off the device.

Syntax

mqtt:lwt(topic, message[, qos[, retain]])

Parameters

• topic the topic to publish to (string)
• message the message to publish, (buffer or string)
• qos QoS level, default 0
• retain retain flag, default 0

Returns

nil

mqtt.client:on()

Registers a callback function for an event.

Syntax

mqtt:on(event, function(client[, topic[, message]]))

Parameters

• event can be "connect", "message", "offline" or "overflow"
• function(client[, topic[, message]]) callback function. The first parameter is the client. If event is "message", the 2nd and 3rd param are received topic and message (strings).

Returns

nil

mqtt.client:publish()

Publishes a message.

Syntax

mqtt:publish(topic, payload, qos, retain[, function(client)])

Parameters

• topic the topic to publish to (topic string)
• message the message to publish, (buffer or string)
• qos QoS level
• retain retain flag
• function(client) optional callback fired when PUBACK received. NOTE: When calling publish() more than once, the last callback function defined will be called for ALL publish commands.

Returns

true on success, false otherwise

mqtt.client:subscribe()

Subscribes to one or several topics.

Syntax

mqtt:subscribe(topic, qos[, function(client)]) mqtt:subscribe(table[, function(client)])

Parameters

• topic a topic string
• qos QoS subscription level, default 0
• table array of 'topic, qos' pairs to subscribe to
• function(client) optional callback fired when subscription(s) succeeded. NOTE: When calling subscribe() more than once, the last callback function defined will be called for ALL subscribe commands.

Returns

true on success, false otherwise

Example

-- subscribe topic with qos = 0
m:subscribe("/topic",0, function(conn) print("subscribe success") end)

-- or subscribe multiple topic (topic/0, qos = 0; topic/1, qos = 1; topic2 , qos = 2)
m:subscribe({["topic/0"]=0,["topic/1"]=1,topic2=2}, function(conn) print("subscribe success") end)

!!! caution

Rather than calling `subscribe` multiple times you should use the multiple topics syntax shown in the above example if you want to subscribe to more than one topic at once.

mqtt.client:unsubscribe()

Unsubscribes from one or several topics.

Syntax

mqtt:unsubscribe(topic[, function(client)]) mqtt:unsubscribe(table[, function(client)])

Parameters

• topic a topic string
• table array of 'topic, anything' pairs to unsubscribe from
• function(client) optional callback fired when unsubscription(s) succeeded. NOTE: When calling unsubscribe() more than once, the last callback function defined will be called for ALL unsubscribe commands.

Returns

true on success, false otherwise

Example

-- unsubscribe topic
m:unsubscribe("/topic", function(conn) print("unsubscribe success") end)

-- or unsubscribe multiple topic (topic/0; topic/1; topic2)
m:unsubscribe({["topic/0"]=0,["topic/1"]=0,topic2="anything"}, function(conn) print("unsubscribe success") end)