How do I pass the input of a step function startExecution to the executed function

0

I'm trying to use a step function startExecution state to break up a state machine into 2 parts and I want to pass the input of the startExecution function to the executed function, e.g.,

"Process chunk": {
            "Type": "Task",
            "Resource": "arn:aws:states:::states:startExecution",
            "Parameters": {
              "StateMachineArn": "arn:aws:states:us-west-1:12345:stateMachine:Ingestion-process-parts",
              "Input": "$.input.MessageDetails"  
            },
            "End": true,
            "ResultPath": "$.input.MessageDetails"
          }

where the startExecution input is

"input": {
    "MessageNumber": 0,
    "MessageDetails": "<an S3 URL>"
  },

but no matter what I try for the input to the executed state machine it just passes a literal value, i.e., in the case above, instead of passing the input of the startExecution state to the executed function it passes

"input": {
    "MessageDetails": "$.input.MessageDetails",
    "AWS_STEP_FUNCTIONS_STARTED_BY_EXECUTION_ID": "arn:aws:states:us-west-1:1234:execution:Ingestion-Chunk-CSV:055ede1a-dc03-4412-a341-d3b6611dbed4"
  },

Note that the same input parsing Does work for the Result:

{
  "name": "Process chunk",
  "output": {
    "MessageNumber": 0,
    "MessageDetails": "<an S3 URL>",

It doesn't appear the the normal approach of parsing the input using $... works for this purpose. Is it possible to do what I want?

thanks

asked a month ago42 views
1 Answer
1
Accepted Answer

As documented here,

For key-value pairs where the value is selected using a path, the key name must end in .$

What you need to do is the following:

"Process chunk": {
     "Type": "Task",
     "Resource": "arn:aws:states:::states:startExecution",
     "Parameters": {
          "StateMachineArn": "arn:aws:states:us-west-1:12345:stateMachine:Ingestion-process-parts",
          "Input.$": "$.input.MessageDetails"  
     },
     "End": true,
     "ResultPath": "$.input.MessageDetails"
 }
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Uri
answered a month ago
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EXPERT
reviewed a month ago
  • Thanks Uri, that worked great!

    I did, however, find that using "StateMachineArn.$": "$$.StateMachine.Id" resulted in an attempted recursion, rather than calling the function I wanted, so I left that as before.

  • My bad of course. I thought you are passing your own ARN. Of course you need to pass the ARN that you want to invoke so you are correct. Will edit my answer.

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