MQTT

Tip

New to MQTT? see the resources below:

To understand MQTT's publish and subscribe architecture, refer to

1. Introduction

Message Queueing Telemetry Transport (MQTT) is a lightweight messaging protocol for devices with limited resources. MQTT is built on top of TCP/IP and is ideal for machine-to-machine communication and the Internet of Things (IoT).

Webview Kiosk utilises the MQTT protocol to enable you to remotely:

receive events
update app settings
perform actions via commands
request information

2. MQTT Broker

To communicate with Webview Kiosk, you will need an MQTT Broker. The broker acts as an intermediary entity (central hub) that routes messages between devices and applications.

Our recommendation for open source, self-hostable brokers (server) and clients (to send messages to Webview Kiosk) are:

EMQX and MQTTX (or MQTTX Web)
Eclipse Mosquitto and Home Assistant - see the Integration Guide

There are also public brokers you can use for testing purposes, such as

HiveMQ: https://www.mqtt-dashboard.com
Mosquitto: https://test.mosquitto.org
EMQX: https://www.emqx.com/en/mqtt/public-mqtt5-broker

3. Global Variables

The following variables will be string-interpolated for supported settings, such as topic names and will payload:

Variable Name	Description
APP_INSTANCE_ID	Unique identifier for the app instance.
USERNAME	MQTT username used for authentication.

To use variables in your settings, use the syntax ${YOUR_VARIABLE}, e.g.

wk/${USERNAME}/${APP_INSTANCE_ID}/command

which may evaluate to a string such as

wk/kiosk_user_01/b8378168-ca69-11f0-af20-5a93ec0e6f90/command

Some settings will support additional variables.

5. Debug Logs

MQTT operations are recorded in the debug logs, which can be accessed from any MQTT settings screens. This includes:

connecting/connected/disconnect
subscribes
messages received or published
errors

A maximum of 100 messages will be stored for the duration of the app activity lifecycle. The logs will not persist between app restarts.

5. Settings

5.1. Enabled

Set to true to allow connections to your configured broker in MQTT -> Connections.

If you are currently in a disconnected state, you will need to manually click the Connect button after enabling this option (or simply restart the app).

Updated MQTT settings do not get applied immediately if you are already connected. Instead, you will need to reconnect by:

clicking the restart button in MQTT settings
manually restarting the application
send a Reconnect command

Default: false

5.2. Use Foreground Service

Start a Foreground Service to keep the MQTT connection alive.

This will maintain the connection even when Webview Kiosk goes to the background (e.g. another app is opened) or the device screen is turned off (by using PowerManager.PARTIAL_WAKE_LOCK).

Turn on notifications to see the current MQTT status being updated by the foreground service every second.

Also, it is highly recommended that you disable battery optimisation for Webview Kiosk to stop the service from being killed.

For more information, visit:

https://dontkillmyapp.com