AWS IoT - Failed to invoke lambda function

Question

I'm trying to get my IoT device to fire off a Lambda function. Been trying for a few hours to no avail.

My lambda function is simple:

exports.handler = async (event) => {
    // TODO implement
    const response = {
        statusCode: 200,
        body: JSON.stringify(event.message),
    };
    return response;
};

I've setup my IoT rule to trigger the lambda function with SELECT * FROM 'iot/trigger-lambda'. When I publish a message to the iot/trigger-lambda topic from AWS IoT's Test functionality, the lambda function triggers successfully. But, when I publish my message from my device, I get the following error:

{
    "ruleName": "ruleTriggerLambda",
    "topic": "iot/trigger-lambda",
    "cloudwatchTraceId": "xxx-yyy-zzz",
    "clientId": "ABC123",
    "base64OriginalPayload": "eydvcGVyYGTpb24nOiAncHV0T2JqZWN0JywgJ2J1Y2tldCc6ICdyZWR3YXZlLWFwcC1kZXYnLCAna2V5JzogJ3JlYWNoYmFjay56aXAnLQWncmVwbHlUbyc6ICdpb3QvdGhyGHT0aWQtMDAwMS9wcmUtc2lnbmVkLBNzLXVybC1yBBNwb25zZXMnfSBbMV0=",
    "failures": [
        {
            "failedAction": "LambdaAction",
            "failedResource": "arn:aws:lambda:us-east-1:xxx:function:lambdaTriggerLambda",
            "errorMessage": "Failed to invoke lambda function. Received Server error from Lambda. The error code is 400"
        }
    ]
}

When I added the lambda function to my rule, AWS indicated it would create the correction permissions. I've also tried updating the permissions of the lambda function.

Any suggestions on how to fix this? Any help/information is much appreciated.

Edit: after 4 hours or ripping my hair out

It turned out to be the format of the data that I was sending as a payload from my IoT device. Let me explain, if someone can tell me why this is, I'd love to know.

I was using my mac as my device and I implemented this code from the AWS IoT Python SDK. My message looked like this:

message = {"message": "test message"}

It seems like pretty straight forward json.

The code that is publishing the message looks like this:

mqtt_connection.publish(
                topic=args.topic,
                payload=args.message,
                qos=mqtt.QoS.AT_LEAST_ONCE)

The payload being received at AWS IoT Core looked like this:

{'message': 'test message'} [1]

And was accompanied by an error message like so: We cannot display the message as JSON, and are instead displaying it as UTF-8 String.

I guess I'm a little dense for not connecting the dots but it turns out I had to do the following to my code:

mqtt_connection.publish(
                topic=args.topic,
                payload=json.dumps(args.message).replace("\"", '').replace("[1]", '').replace("'", '"'),
                qos=mqtt.QoS.AT_LEAST_ONCE)

Hopefully this explanation will help someone in the future.

Seems like you can just use `JSON.stringify({message: "test message"})`. It's a built in JS function that takes a JS object and converts it to a string. Iot messages are expected to be strings that can be converted to json. — DCTID, Sep 16 '20 at 02:12

AWS IoT - Failed to invoke lambda function

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