MQTT

MQTT (Message Queue Telemetry Transport) is an ISO standard publish-subscribe-based messaging protocol, which is becoming the de-facto standard protocol for accessing IoT data from the internet.

Overview

Publishing means that data is uploaded to a well-defined place (called topic) on a central server (called message-broker). Subscribing means that data is downloaded from a topic of the message broker once it was published there. Hence, MQTT is an event-driven communication protocol notifying a subscriber once a data message has arrived on its topic.

This article describes the properties of an MQTT client connection to an external broker. To get information about topic mapping on the internal Connectware MQTT broker see the article Cybus::Mapping.

Usage

To send or receive data via MQTT, a connection resource of the protocol MQTT has to be defined. The most simple configuration requires a host name of the broker and the port of the broker. The full list of connection parameters which can be configured is available at Connection Properties.

To receive data from a broker, an endpoint resource with the property subscribe has to be defined. In the other direction, to send data to the broker, an endpoint with the property write has to be defined. An endpoint can only have one of the two properties. The properties which define the respective configuration are listed under Endpoint Properties.

Connection Properties

Endpoint Properties

Output Format on Write

The MQTT protocol does not output results to the /res endpoint.

Output Format on Read

When data is read from MQTT, via a subscription Endpoint, the messages arriving to the target MQTT server are passed through unmodified.

Input Format on Write

When data is written to MQTT, via a write Endpoint, no special format is required and data is published to the target MQTT server unmodified.

Duplicates on overlapping subscriptions

In the case of multiple overlapping topic subscriptions with wildcards, there is a somewhat unexpected behaviour causing duplicate messages. As an example, if there is one subscription (such as a Cybus::Mapping) to topic topic/something and another to topic/#, each connected target (Mapping or Endpoint) will then receive two messages instead of one. This is caused by the behaviour of the internal MQTT broker and cannot be avoided, as it is an allowed behaviour of the MQTT 3.1 specification.

Example Configuration

The following example demonstrates how to configure a simple MQTT connection and endpoint that subscribes to the topic some/test1.

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mqtt-test1.yml

---
description: >

    MQTT Connection Test

metadata:
    name: MQTT Connection Test

parameters:
    mqttHost:
        type: string
        description: 'IP or hostname of the broker'

    mqttUsername:
        type: string
        description: 'MQTT Username'

    mqttPassword:
        type: string
        description: 'MQTT Password'

resources:
    mqttConnection:
        type: Cybus::Connection
        properties:
            protocol: Mqtt
            connection:
                host: !ref mqttHost
                port: 8883
                scheme: mqtts
                username: !ref mqttUsername
                password: !ref mqttPassword
                trustAllCertificates: false # choose true to allow self-signed certificates

    testEndpoint1:
        type: Cybus::Endpoint
        properties:
            protocol: Mqtt
            connection: !ref mqttConnection
            subscribe:
                topic: some/test1