Using MQTT APIs For IOT -Beginners Guide

MQTT is probably the most common IOT protocol and many IOT service providers and devices provide an MQTT based API.

In the tutorial we are going to use a simple MQTT API to manage several simulated devices.



In order to follow this tutorial you should have a basic understanding of:

 

IOT Test API and Testing Tools

To learn to use IOT MQTT APIs we also need a suitable simple IOT API to test against, and unfortunately there currently don’t appear to be any available on line.

Therefore I’ve created a basic API using node-red which is available as a flow.

If there is enough interest I will also make it available online.

You will require suitable testing tools.

For MQTT the most common tools are

Introducing Our Home Made API

Our API will be used for controlling three simulated IOT devices. These devices are also controllable using the http API.

  • A switch
  • A light Bulb
  • A temperature and humidity sensor.

The Switch

In MQTT endpoints are created using topics.

The topics used are:

myapi/command/device – for commands
myapi/response/device – for responses

There are two ways of sending commands

  • Using the topic only as in http Get request
  • Using the topic and payload as in the http POST request

Using The Topic only

Example Turn switch on/off:

myapi/command/switch1/state/on
myapi/command/switch1/state/off

The message payload is blank and will be ignored

Example:

mqtt-api-example

Using The Topic And Message Payload

Commands are sent using JSON encoded data in the payload.

Example: To turn the switch on use:

myapi/command/switch1

payload data =”{“state”:”ON”}”

Example using mosquitto_pub –: to turn the switch on/off use:

mosquitto_pub -h 192.168.1.77 -t myapi/command/switch1 -m "{\"state\":\"on\"}"

Note: Notice how the data is delimited using \

To view the response you need to subscribe to the topic myapi/response/#

The response should look like this:

mos-sub-mqtt-api-test

Switch Commands

Action Request Type Topic Payload
Get switch status Topic Only myapi/commands/switch1 None
Set switch state on Topic Only myapi/switch1/state/on None
Set switch state off Topic Only myapi/switch1/state/off None
Set switch timer on Topic Only myapi/switch1/timer/on None
Set switch timer off Topic Only myapi/switch1/timer/off None
Set switch state on Topic+Payload myapi/switch1 “{“state”:”on”}”
Set switch state off Topic+Payload myapi/switch1 “{“state”:”off”}”
Set switch timer on Topic+Payload myapi/switch1 “{“timer”:”on”}”
Set switch timer off Topic+Payload myapi/switch1 “{“timer”:”off”}”

For the bulb the topic is:

myapi/bulb1

and you can set the state to off and also the colour values to 000000-FFFFFF

Examples:

myapi/bulb1/state/on
myapi/bulb1/colour/ff00ff

Command Table

Action Request Type Topic Payload
Get bulb status Topic Only myapi/bulb1 None
Set bulb state on Topic Only myapi/bulb1/state/on None
Set bulb state off Topic Only myapi/bulb1/state/off None
Set bulb colour Topic Only myapi/bulb1/colour/xxxxxx None
Set bulb state on Topic+Payload myapi/bulb1 “{“state”:”on”}”
Set bulb state off Topic+Payload myapi/bulb1 “{“state”:”off”}”
Set bulb colour Topic+Payload myapi/bulb1 “{“colour”:”xxxxx”}”

Temperature Sensor

The temperature sensor is mainly read, but you can turn it on and off using both command types.

topic=myapi/sensor1/state/on

or

topic=myapi/sensor1

payload data =”{“state”:”ON”}”

Resources

Related Tutorials

 

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