Amazon web services – How to post to a topic on AWS IoT using HTTP(S) POSTMAN

Based on your question (How to post to a topic on AWS IoT using HTTP(S) POSTMAN) you just want to publish a payload in a specific topic inside the AWS-IoT MQTT broker.

This task is very simple and you do not need to create a thing to achieve it or create certificates to be attached to POSTMAN.

This is a simple explanation of the process:

1 – The user get access to the HTTPs endpoint in your case trough POSTMAN.
2 – You can use in your POST body the data you want to publish in your payload. NOTE: you can have a more flexible code that would accept the data you want to publish and the topic you want to publish to. That being said you can send the data and the topic in your POST body.
3 – When you trig the HTTPs endpoint you will call a Lambda. This function will be connected to the API Gateway and will receive the payload that you sent from POSTMAN.
4 – Inside the lambda you will have a connection with the AWS IoT-Core where you will publish the payload that you got from POSTMAN in the desired topic.

In another words, to acompplish this task you will need to understand how to setup an endpoint through API API Gateway and how to create the lambda to connect to the MQTT broker. You can create this architecture in some different ways. I use to do that through SERVERLESS framework.

The important point here is the permission that you need to provide to the lambda to publish in the MQTT broker. Without that permission it will faill.

I will not go in details on how to create a HTTPs endpoint with SERVERLESS because that was not the question. The following code shows how to set a HTTPs endpoint to a lambda called fnPublishMqtt and what is the RoleStatements to allow that lambda the credentials to publish in the AWS IoT-Core MQTT broker.

publishmqtt:
    handler: fnPublishMqtt.fnPublishMqtt
    events:
    - http:
        path: publishmqtt
        method: post
        private: false
        cors: false
    memorySize: 256
    iamRoleStatementsName: "someRoleName"
    iamRoleStatements:
    - Effect: "Allow"
      Action:
      - "iot:*"
      Resource: '*'

The next step is the lambda itself using NodeJS SDK V3:

const { IoTDataPlaneClient, PublishCommand } = require("@aws-sdk/client-iot-data-plane")
const clientIoTDataPlane = new IoTDataPlaneClient ({ region: "us-east-1" })


const publishMQTT = async (params) => {
  try {
    const input = {
      topic: params.topic,
      payload: JSON.stringify(params.payload),
      qos:'0'
    }
    const command = new PublishCommand (input)
    const result = clientIoTDataPlane.send(command)
    return result
  } catch (error) {
    console.error(error.stack)
    throw error.stack
  }
}


module.exports.fnPublishMqtt = async (event) => {
  
    try {
        const jsonBody = JSON.parse(event.body)

        const mqttParams = {
            topic: jsonBody.topic,
            payload: jsonBody.payload
        }
        const resp = await publishMQTT(mqttParams)
        console.log('resp:', resp)
        return {
            statusCode: 200,
            body: JSON.stringify(resp)
        }

        
    } catch (error) {
        console.log('error:', error)
    }
    
}

Then just go to your POSTMAN call the endpoint https://xxxxxxx/publishmqtt passing the following POST body {topic:"topic/tes",payload: {status:74648}

You should be able to see {topic:"topic/tes",payload: {status:74648} published in your AWS IoT-Core MQTT broker.