How can I access preceding state inputs using AWS Step Functions?

Question

Here is my step function:

{
  "Comment": "A description of my state machine",
  "StartAt": "lambda1",
  "States": {
    "lambda1": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "OutputPath": "$.Payload",
      "Parameters": {
        "Payload.$": "$"
      },
      "Next": "lambda2"
    },
    "lambda2": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "OutputPath": "$.Payload",
      "Parameters": {
        "Payload.$": {
          "item_id.$": "$.item_id" 
        }
      },
      "Next": "lambda3"
    },
    "lambda3": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "OutputPath": "$.Payload",
      "Parameters": {
        "Payload.$": {
          "item_id.$": "" <------- How to access the id here
          "data.$": $.data
        }
      },
      "End": true
    }
  }
}

I am passing an item_id to lambda2, and that lambda returns some output data.

In lambda3, I need the output data of lambda2 as well as the item_id that was passed to lambda2 from lambda1.

I cannot modify the application code for lambda2 to also return the item_id passed to it.

Is it possible for me to pass the item_id from lambda1 all the way to lambda3?

Answer 1

In lambda3, I need the output data of lambda2 as well as the item_id that was passed to lambda2 from lambda1.

Yes, this is possible using ResultPath.

I've detailed the various outcomes of using ResultPath in my answer here.

In your case, you need a ResultPath of $.DataFromLambda1 set for lambda2 which would set the state input of lambda3 to:

1. the state input of lambda1 (which is the item_id)
2. + the state output of lambda2.

"lambda2": {
  ...
  "OutputPath": "$.Payload",
  "ResultPath": "$.DataFromLambda1",
  "Parameters": {
    "Payload.$": {
      "item_id.$": "$.item_id" 
    }
  },
  "Next": "lambda3"
}

Then change lambda3 to:

"lambda3": {
  "Type": "Task",
  "Resource": "arn:aws:states:::lambda:invoke",
  "OutputPath": "$.Payload",
  "Parameters": {
    "Payload.$": {
      "item_id.$": "$.DataFromLambda1.item_id"
      "data.$": $.data
    }
  },
  "End": true
}