# MQTT Module
| Since  | Origin / Contributor  | Maintainer  | Source  |
| :----- | :-------------------- | :---------- | :------ |
| 2015-01-23 | [Stephen Robinson](https://github.com/esar/contiki-mqtt), [Tuan PM](https://github.com/tuanpmt/esp_mqtt) | [Vowstar](https://github.com/vowstar) | [mqtt.c](../../../app/modules/mqtt.c)|


The client adheres to version 3.1.1 of the [MQTT](https://en.wikipedia.org/wiki/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](http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc398718037).

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
```lua
-- 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](net.md).
- `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](http://www.hivemq.com/blog/mqtt-essentials-part-9-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](http://www.hivemq.com/blog/mqtt-essentials-part-5-mqtt-topics-best-practices))
- `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](http://www.hivemq.com/blog/mqtt-essentials-part-5-mqtt-topics-best-practices)
- `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
```lua
-- 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](http://www.hivemq.com/blog/mqtt-essentials-part-5-mqtt-topics-best-practices)
- `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
```lua
-- 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)
```