Parallel workflow state - Amazon Step Functions
Parallel State ExampleParallel State Input and Output ProcessingError Handling
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Parallel workflow state

The Parallel state ("Type": "Parallel") can be used to add separate branches of execution in your state machine.

In addition to the common state fields, Parallel states include these additional fields.

Branches (Required)

An array of objects that specify state machines to execute in parallel. Each such state machine object must have fields named States and StartAt, whose meanings are exactly like those in the top level of a state machine.

ResultPath (Optional)

Specifies where (in the input) to place the output of the branches. The input is then filtered as specified by the OutputPath field (if present) before being used as the state's output. For more information, see Input and Output Processing.

ResultSelector (Optional)

Pass a collection of key value pairs, where the values are static or selected from the result. For more information, see ResultSelector.

Retry (Optional)

An array of objects, called Retriers, that define a retry policy in case the state encounters runtime errors. For more information, see State machine examples using Retry and using Catch.

Catch (Optional)

An array of objects, called Catchers, that define a fallback state that is executed if the state encounters runtime errors and its retry policy is exhausted or isn't defined. For more information, see Fallback States.

A Parallel state causes Amazon Step Functions to execute each branch, starting with the state named in that branch's StartAt field, as concurrently as possible, and wait until all branches terminate (reach a terminal state) before processing the Parallel state's Next field.

Parallel State Example

{
  "Comment": "Parallel Example.",
  "StartAt": "LookupCustomerInfo",
  "States": {
    "LookupCustomerInfo": {
      "Type": "Parallel",
      "End": true,
      "Branches": [
        {
         "StartAt": "LookupAddress",
         "States": {
           "LookupAddress": {
             "Type": "Task",
             "Resource":
               "arn:aws-cn:lambda:us-east-1:123456789012:function:AddressFinder",
             "End": true
           }
         }
       },
       {
         "StartAt": "LookupPhone",
         "States": {
           "LookupPhone": {
             "Type": "Task",
             "Resource":
               "arn:aws-cn:lambda:us-east-1:123456789012:function:PhoneFinder",
             "End": true
           }
         }
       }
      ]
    }
  }
}

In this example, the LookupAddress and LookupPhone branches are executed in parallel. Here is how the visual workflow looks in the Step Functions console.

Visual graph of an example parallel workflow.

Each branch must be self-contained. A state in one branch of a Parallel state must not have a Next field that targets a field outside of that branch, nor can any other state outside the branch transition into that branch.

Parallel State Input and Output Processing

A Parallel state provides each branch with a copy of its own input data (subject to modification by the InputPath field). It generates output that is an array with one element for each branch, containing the output from that branch. There is no requirement that all elements be of the same type. The output array can be inserted into the input data (and the whole sent as the Parallel state's output) by using a ResultPath field in the usual way (see Input and Output Processing).

{
  "Comment": "Parallel Example.",
  "StartAt": "FunWithMath",
  "States": {
    "FunWithMath": {
      "Type": "Parallel",
      "End": true,
      "Branches": [
        {
          "StartAt": "Add",
          "States": {
            "Add": {
              "Type": "Task",
              "Resource": "arn:aws-cn:states:us-east-1:123456789012:activity:Add",
              "End": true
            }
          }
        },
        {
          "StartAt": "Subtract",
          "States": {
            "Subtract": {
              "Type": "Task",
              "Resource": "arn:aws-cn:states:us-east-1:123456789012:activity:Subtract",
              "End": true
            }
          }
        }
      ]
    }
  }
}

If the FunWithMath state was given the array [3, 2] as input, then both the Add and Subtract states receive that array as input. The output of the Add and Subtract tasks would be the sum of and difference between the array elements 3 and 2, which is 5 and 1, while the output of the Parallel state would be an array.

[ 5, 1 ]
Tip

If the Parallel or Map state you use in your state machines returns an array of arrays, you can transform them into a flat array with the ResultSelector field. For more information, see Flattening an array of arrays.

Error Handling

If any branch fails, because of an unhandled error or by transitioning to a Fail state, the entire Parallel state is considered to have failed and all its branches are stopped. If the error is not handled by the Parallel state itself, Step Functions stops the execution with an error.

Note

When a parallel state fails, invoked Lambda functions continue to run and activity workers processing a task token are not stopped.

  • To stop long-running activities, use heartbeats to detect if its branch has been stopped by Step Functions, and stop workers that are processing tasks. Calling SendTaskHeartbeat, SendTaskSuccess, or SendTaskFailure will throw an error if the state has failed. See Heartbeat Errors.

  • Running Lambda functions cannot be stopped. If you have implemented a fallback, use a Wait state so that cleanup work happens after the Lambda function has finished.