Variable flow in parallel execution of AWS step function

Question

I have a step function definition as below:

{
    "StartAt": "Assign Variables",
    "QueryLanguage": "JSONata",
    "States": {
        "Assign Variables": {
            "Comment": "Assign variables from input",
            "Type": "Pass",
            "Assign": {
                "var1": "{% $states.input.var1 %}",
                "var2": "{% $states.input.var2 %}"
            },
            "Next": "Parallel_Execution"
        },
        "Parallel_Execution": {
            "Type": "Parallel",
            "Branches": [
                {
                    "StartAt": "Task_A",
                    "States": {
                        "Task_A": {
                            "Comment": "Invoke Lambda Function A to update db",
                            "Type": "Task",
                            "Resource": "arn:aws:lambda:region:account-id:function:FunctionA",
                            "Assign": {
                                **"st_time": "{% $states.result.st_time %}"**
                            },
                            "Next": "Task_B"
                        },
                        "Task_B": {
                            "Comment": "Invoke Lambda Function B to send notification",
                            "Type": "Task",
                            "Resource": "arn:aws:lambda:region:account-id:function:FunctionB",
                            "End": true
                        }
                    }
                }
            ],
            "Catch": [
                {
                    "ErrorEquals": ["States.ALL"],
                    "Next": "Error_Handler"
                }
            ],
            "End": true
        },
        "Error_Handler": {
            "Comment": "Update db to reflect error in above process",
            "Type": "Task",
            "Resource": "arn:aws:lambda:region:account-id:function:ErrorHandlerFunction",
            "Next": "Final_State"
        },
        "Final_State": {
            "Type": "Fail"
        }
    }
}

In the "Error_Handler" state, I need to access the variable "st_time" assigned inside parallel execution using JSONata syntax. I am new to the step function and so to JSONata. So I am not sure how can I achieve this.

Pooya Paridel · Accepted Answer · 2025-09-15 19:17:49Z

0

In Step Functions, variables assigned inside Parallel and Map states are scoped to their respective branches and aren't directly accessible outside. Here are a way to handle your requirement:

Pass the variable through the output:
   - Modify Task_A to include st_time in its output
   - Make sure the Parallel state's output includes this value
   - Then you can access it in Error_Handler using:
   `$.Parallel_Execution[0].st_time` (assuming it's in the first branch)

answered Sep 15 at 19:17

Pooya Paridel

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