AWS Step Functions SendSQSMessage: Dynamic MessageGroupId - amazon-web-services

Question

I am using the AWS CDK to create a state machine that sends a message to a fifo queue and waits for a callback from the lambda worker to continue execution.
I would like the messages that get sent to the fifo queue to have a dynamic MessageGroupId assigned to them so I can control the number of lambda workers processing the messages. The only way I can think of to have a dynamic MessageGroupId is to reference some parameter on the step function input with JsonPath, however I have not come across any documentation about it. My initial tests to use JsonPath to dynamically pass the MessageGroupId failed, simply passing the string "$.MessageGroupId" effectively giving each message the same message group id and thus one lambda worker.
Is it possible to dynamically assign a message group id to a sqs message when sent from a step function?
If so, how?

Answer 1

With the help AWS Support, I managed to do it by either using the Context Object or passing an ID from the initial input and reference it with $.
Here's an example:
{
"Comment": "Generate unique MessageGroupId",
"StartAt": "Start",
"States": {
"Start": {
"Type": "Task",
"TimeoutSeconds": 60,
"Resource": "arn:aws:states:::sqs:sendMessage.waitForTaskToken",
"Parameters": {
"QueueUrl": "<YOUR_QUEUE_URL>",
"MessageBody": {
"Input.$": "$",
"TaskToken.$": "$$.Task.Token"
},
"MessageGroupId.$": "$$.Execution.Id"
},
"ResultPath": "$",
"End": true
}
}
}
My problem was that I was trying to MessageGroupId like so:
"MessageGroupId": "$$.Execution.Id"
Where I should have done:
"MessageGroupId.$": "$$.Execution.Id"
Appending .$ would resolve the expression "$$.Execution.Id" instead of putting literally the string "$$.Execution.Id".