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amazon-web-servicesaws-step-functions

Is it possible to use intrinsic functions as part of a CHOICE state?

I am curious to know if it is possible to use certain intrinsic function as part of my state machine definition with respect to a choice node.

My use case is that I have a payload that comes from a queue that includes varying bits of information, however the important one is part of the "body" field. I fetch that field in my first step as follows:

FormatData": {
              "Type": "Pass",
              "Next": "ChooseWhichCampaignToCreate",
              "InputPath": "$[0].body",
              "OutputPath": "$"
            },

This is all fine and well, however, the actual output is a string that is formatted as JSON. Not a straight JSON object. As such, in my next state I have to convert the Output ("$") to a JSON object as so:

"CreateSessionInviteCampaign": {
              "Type": "Task",
              "Resource": "arn:aws:states:::lambda:invoke",
              "Parameters": {
                "FunctionName": "Coaching-EmailCampaign-Prepare",
                "Payload.$": "States.StringToJson($)"
              },
              "Next": "ChoseWhichCampaignToPrepare"
            },

I am rewriting it a bit to include some more choice nodes in order to make sure that each lambda has a specific bounded context and does the least amount of work it needs to do. As such I took a look at the specs (https://states-language.net/#state-types) and based on the table a Choice state cannot have a Parameters field, but can have an InputPath. Is it possible to apply the intrinsic function of StringToJson to the InputPath to have something like this:

States.StringToJson($).CampaignType or do I need to make another simple Pass through step after my format that would take the Input, JSONify it, and then proceed onwards like so (assuming it is correct, I would like to not have to define a basic lambda that json does a string to json):

"JSONIfyOutput": {
                "Type": "Pass",
                "Parameters": {
                    "Payload.$": "States.StringToJson($)"
                },
                "OutputPath": "$"
            },
Solution

  • According to the table of where intrinsic functions are supported, you cannot use them in a Choice, but can be used on Pass Parameters. Using your intrinsic function on Parameters via Transform input with Parameters will work.

    Where intrinsic functions are supported