Passthrough input to output in AWS Step Functions

Question

How can I passthrough the input to a Task state in an AWS Step Functions to the output?

After reading the Input and Output Processing page in the AWS docs, I have played with various combinations of InputPath, ResultPath and OutputPath.

State definition:

"First State": {
    "Type": "Task",
    "Resource": "[My Lambda ARN]",
    "Next": "Second State",
    "InputPath": "$.someKey",
    "OutputPath": "$"
}

Input:

{
    "someKey": "someValue"
}

Expected Result

I would like the output of the First State (and thus the input of Second State) to be

{
    "someKey": "someValue"
}

Actual Result

[empty]

What if the input is more complicated, e.g.

{
    "firstKey": "firstValue",
    "secondKey": "secondValue"
}

I would like to forward all of it without worrying about (sub) paths.

Hello, @matsev. Can you please answer my question? It is similar to what you asked. Thanks — Gustavo Lopes
– Gustavo Lopes, Commented Oct 11, 2019 at 17:13

matsev · Accepted Answer · 2020-10-10 20:03:04Z

43

In the Amazon States Language spec it is stated that:

If the value of ResultPath is null, that means that the state’s own raw output is discarded and its raw input becomes its result.

Consequently, I updated my state definition to

"First State": {
    "Type": "Task",
    "Resource": "[My Lambda ARN]",
    "Next": "Second State",
    "ResultPath": null
}

As a result, when passing the input example Task input payload will be copied to the output, even for rich objects like:

{
    "firstKey": "firstValue",
    "secondKey": "secondValue"
}

edited Oct 10, 2020 at 20:03

answered Dec 5, 2017 at 10:40

matsev

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Comments

Alex · Accepted Answer · 2021-01-07 09:38:26Z

13

For those who find themselves here using CDK, the solution is to use the explicit aws_stepfunctions.JsonPath.DISCARD enum rather than None/null.

from aws_cdk import (
    aws_stepfunctions,
    aws_stepfunctions_tasks,
)

aws_stepfunctions_tasks.LambdaInvoke(
    self,
    "my_function",
    lambda_function=lambda_function,
    result_path=aws_stepfunctions.JsonPath.DISCARD,
)

https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-stepfunctions.JsonPath.html#static-discard

answered Jan 7, 2021 at 9:38

Alex

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Comments

Jobu · Accepted Answer · 2023-05-09 02:06:24Z

2

For those who don't want to just pass input to output but to add result of Task to input and forward all of it as output you can use "ResultPath": "$.addedOutput" as described in https://docs.aws.amazon.com/step-functions/latest/dg/input-output-resultpath.html

answered May 9, 2023 at 2:06

Jobu

3212 silver badges7 bronze badges

Comments

spatel · Accepted Answer · 2020-10-09 05:50:05Z

-2

I was looking for a solution from passing input from one parallel state to another parallel state and the above option worked really good. For example my step function is like this...tas1->parallel task2 -> parallel trask3 -> task4. So when it start with parallel task3, the input values are wiped out, so ptask3 is failing. With the above option, i was able to pass in same input from ptask2 to ptas3.

answered Oct 9, 2020 at 5:50

spatel

111 silver badge2 bronze badges

1 Comment

NHol Over a year ago

Hi spatel, this isn't an answer in it's own right, more of a comment on the answer you are referring to. Please delete this Answer and post as a comment on the matsev answer

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