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MQTT

MQTT is an IoT friendly messaging protocol for publishing and subscribing to a shared MQTT broker from a wide range of devices.

This tutorial shows you how to connect to an MQTT broker using TCP (with or without TLS). We will use the public test.mosquitto.org server in this introduction.

Prerequisites

We assume that you have set up your development environment as described in the IDE tutorial.

We also assume that you have flashed your device with Jaguar and that you are familiar with running Toit programs on it. If not, have a look at the Hello world tutorial.

Note that you can do this tutorial without a device. In that case, you need to use the -d host option whenever you invoke jag run. The program will then run on your computer instead of on a device.

Packages

The MQTT functionality is not part of the core libraries and must be imported as a package. See the packages tutorial for details.

We are using the mqtt package. To install it, run the following command:

jag pkg install github.com/toitware/mqtt@v2

When connecting to TLS secured services we will also use the certificate-roots package:

jag pkg install github.com/toitware/toit-cert-roots@v1

Introduction

MQTT uses a long-running broker to which clients connect. The broker is responsible for routing messages between clients. Clients can publish messages to certain topics, and subscribe to topics to receive messages published to those topics.

When a client is not connected, the broker can store messages for the client until it reconnects.

Not all messages are stored, however. Each message is published with a QoS (Quality of Service) level. The QoS level determines how the broker handles the message. The QoS levels are:

  • QoS 0: At most once delivery
  • QoS 1: At least once delivery
  • QoS 2: Exactly once delivery

QoS 2 is not implemented in the Toit MQTT library, and we will not cover it in this tutorial.

QoS 0 messages are not stored by the broker. If a client is not connected when a QoS 0 message is published, the message is lost.

QoS 1 messages are stored by the broker until the client reconnects. When the client reconnects, the broker will resend all messages that were published with QoS 1.

QoS 1 furthermore has the property that the broker will acknowledge receipt of the message. This means that the client can be sure that the message was received by the broker.

Note that clients can also lower a QoS level when subscribing to a topic. This can be used to inform the broker not to store messages even if they were sent with QoS 1.

Identification

In order to store messages for a client, the broker needs to be able to identify the client. This is done using a CLIENT-ID. The client ID is sent when connecting to the broker, and identifies a session on the broker.

To avoid impersonation, the broker can require the client to provide a client certificate or a username and password when connecting.

When a client connects to the broker, it can also specify a CLEAN-SESSION flag. If this flag is set, the broker will discard any stored messages for the client. If the flag is not set, the broker will keep any stored messages for the client.

With the CLEAN-SESSION flag, a client may also decide not to provide a CLIENT-ID. In this case, the broker will generate a random ID for the client.

The MQTT library

The MQTT library comes with two client implementations:

  1. a simplified Client class,
  2. and a FullClient class that exposes more of the MQTT protocol.

We will use the Client class in this tutorial. If you need more control you can use the FullClient class instead. The documentation for the FullClient class can be found here.

Code

Open an mqtt.toit file, and watch it with Jaguar.

Insert the following code:

import mqtt

CLIENT-ID ::= ""
HOST ::= "test.mosquitto.org"

main:
  client := mqtt.Client --host=HOST
  client.start --client-id=CLIENT-ID
  // Client is now connected.

Note that the start call starts a task that connects to the broker. The task will keep running in the background. It will prevent the program from exiting until the client is disconnected. Alternatively, you can use the --background flag to start the task with a background priority, thus allowing the program to exit.

Some brokers require the client to provide a username and password. These can be passed to the client during the start call in an options object:

  options := mqtt.SessionOptions
      --client-id=CLIENT-ID
      --username=MY-USERNAME
      --password=MY-PASSWORD
  client.start --options=options

See the documentation or examples of the MQTT library for more information.

Publish

When publishing data to the broker, you simply need to specify which topic to publish to and provide a payload.

Let's change the example so it publishes a message. Since the mosquitto server is shared with other developers, we should choose a topic that avoids interference with other clients. For simplicity, we will use toit-mqtt/tutorial in the code below, but you should change it to a different topic name. Otherwise, you might receive messages from other clients.

Call the file publish.toit and insert the following code.

import mqtt
import encoding.json

CLIENT-ID ::= ""  // Use a random client ID.
HOST ::= "test.mosquitto.org"
TOPIC ::= "toit-mqtt/tutorial"

main:
  client := mqtt.Client --host=HOST
  client.start --client-id=CLIENT-ID

  payload := json.encode {
    "value": 42
  }
  client.publish TOPIC payload
  client.close

Note that the payload must be an io.Data, which is typically a string or a byte array. If your data is neither, you need to convert it first.

This program publishes a JSON object with the value 42 to the topic and then disconnects. Let's verify that the message was published by subscribing to the topic. You will need a second device, or run the program with jag run -d host ....

Subscribe

Subscribing to data requires two pieces of information: the topic (or filter) to subscribe to, and the callback that should be called when a message arrives.

Create the following subscribe.toit file. Don't forget to change the topic name to the one you used in the previous example.

import mqtt
import encoding.json

CLIENT-ID ::= ""  // Use a random client ID.
HOST ::= "test.mosquitto.org"
TOPIC ::= "toit-mqtt/tutorial"

main:
  client := mqtt.Client --host=HOST
  client.start --client-id=CLIENT-ID

  client.subscribe TOPIC:: | topic/string payload/ByteArray |
    decoded := json.decode payload
    print "Received value on '$topic': $decoded"

The client.subscribe function takes a topic (here TOPIC) and a callback (indicated by the :: and the | and | around the parameters).

The callback will be called every time a message is received on the topic.

Here, we decode the incoming payload with the JSON decoder. If the data payload should just be interpreted as a string, you can simply call payload.to-string instead.

Start this program first (optionally on your desktop with jag run -d host subscribe.toit), and then, in a second terminal, start the publish.toit program. You should see the following output:

Received value on 'toit-mqtt/tutorial': {value: 42}

Quality of Service

As discussed, brokers can store messages for clients when they are offline. For this to work the client must connect with a client ID. When the client disconnects and reconnects with the same client ID, the broker will deliver messages that were published while the client was offline.

Change the subscribe.toit program to use a non-empty client ID. For example:

CLIENT-ID ::= "toit-tutorial-ID-2023-07-06"

Make sure to use a unique ID, to avoid conflicts with other clients.

Now start the subscribe program, and then stop it, before starting the publish program.

Now run the publish.toit program again. Once it has terminated, start the subscribe.toit program again. If you are lucky you should see the following output. If not, see below.

Received value on 'toit-mqtt/tutorial': {value: 42}

This is the message that was published while the client was offline.

Since the subscribe program only subscribes to the topic after it has connected, there is a race condition between the connection being established and the subscription handler being registered. If the stored messages are delivered before the subscription handler is registered, they will be lost. To avoid this, you can register the subscription handler before connecting to the broker:

main:
  routes := {
    TOPIC: :: | topic/string payload/ByteArray |
      decoded := json.decode payload
      print "Received value on '$topic': $decoded"
  }
  client := mqtt.Client --host=HOST --routes=routes
  client.start --client-id=CLIENT-ID
  // Client is now connected and subscribed to the given routes.

TLS

For secure connections to the broker, we need to use TLS. For that we need root certificates which are then used to authenticate the server.

If you haven't already, install the certificate-roots package with

jag pkg install github.com/toitware/toit-cert-roots@v1

In some cases, the certificate might be given by the server and should be pasted into the sources.

Once we have the root certificates, the client can be created with the tls named constructor as follows:

import mqtt
import certificate-roots
import encoding.json

CLIENT-ID ::= "toit-tutorial-ID-2023-07-06"
HOST ::= "test.mosquitto.org"
TOPIC ::= "toit-mqtt/tutorial"

main:
  certificate-roots.install-common-trusted-roots

  routes := {
    TOPIC: :: | topic/string payload/ByteArray |
      decoded := json.decode payload
      print "Received value on '$topic': $decoded"
  }
  client := mqtt.Client.tls --host=HOST --routes=routes
  client.start --client-id=CLIENT-ID

  while true:
    payload := json.encode {
      "now": Time.now.utc.to-iso8601-string
    }
    client.publish TOPIC payload
    sleep --ms=1_000

This example combines all the previous examples. It connects to the broker using TLS, subscribes to the topic, and publishes a message every second.

Conclusion

In conclusion, this tutorial has introduced the MQTT protocol and its usage in Toit. We have covered how to set up and initiate an MQTT client, publish and subscribe to topics, and control message storage with Quality of Service levels. It is recommended that you experiment further with these features and consult the Toit MQTT library documentation for more advanced usage and options.

Exercises

  1. Try to publish and subscribe to different topics.
  2. Try to publish and subscribe to topics with different QoS levels.
    1. Send messages with QoS 0 and 1 and see what happens when the subscriber is offline.
    2. Send messages with QoS 1 but subscribe with --max-qos=0 and see what happens.