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, eg
{
"firstKey": "firstValue",
"secondKey": "secondValue"
}
I would like to forward all of it without worrying about (sub) paths.
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"
}
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,
)
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.
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