Manipulate parameters in Step Functions workflows

The InputPath, Parameters and ResultSelector fields provide a way to manipulate JSON as it moves through your workflow. InputPath can limit the input that is passed by filtering the JSON notation by using a path (see Using JSONPath paths). With the Parameters field, you can pass a collection of key-value pairs, using either static values or selections from the input using a path.

The ResultSelector field provides a way to manipulate the state’s result before ResultPath is applied.

AWS Step Functions applies the InputPath field first, and then the Parameters field. You can first filter your raw input to a selection you want using InputPath, and then apply Parameters to manipulate that input further, or add new values. You can then use the ResultSelector field to manipulate the state's output before ResultPath is applied.

InputPath

Use InputPath to select a portion of the state input.

For example, suppose the input to your state includes the following.

{
  "comment": "Example for InputPath.",
  "dataset1": {
    "val1": 1,
    "val2": 2,
    "val3": 3
  },
  "dataset2": {
    "val1": "a",
    "val2": "b",
    "val3": "c"
  }
}

You could apply the InputPath.

"InputPath": "$.dataset2",

With the previous InputPath, the following is the JSON that is passed as the input.

{
  "val1": "a",
  "val2": "b",
  "val3": "c"
}

{ "a": [1, 2, 3, 4] }

[ 1, 2 ]

Parameters

This section describes the different ways you can use the Parameters field.

Key-value pairs

Use the Parameters field to create a collection of key-value pairs that are passed as input. The values of each can either be static values that you include in your state machine definition, or selected from either the input or the Context object with a path. For key-value pairs where the value is selected using a path, the key name must end in .$.

For example, suppose you provide the following input.

{
  "comment": "Example for Parameters.",
  "product": {
    "details": {
       "color": "blue",
       "size": "small",
       "material": "cotton"
    },
    "availability": "in stock",
    "sku": "2317",
    "cost": "$23"
  }
}

To select some of the information, you could specify these parameters in your state machine definition.

"Parameters": {
        "comment": "Selecting what I care about.",
        "MyDetails": {
          "size.$": "$.product.details.size",
          "exists.$": "$.product.availability",
          "StaticValue": "foo"
        }
      },

Given the previous input and the Parameters field, this is the JSON that is passed.

{
  "comment": "Selecting what I care about.",
  "MyDetails": {
      "size": "small",
      "exists": "in stock",
      "StaticValue": "foo"
  }
},

In addition to the input, you can access a special JSON object, known as the Context object. The Context object includes information about your state machine execution. See Accessing execution data from the Context object in Step Functions .

Connected resources

The Parameters field can also pass information to connected resources. For example, if your task state is orchestrating an AWS Batch job, you can pass the relevant API parameters directly to the API actions of that service. For more information, see:

Amazon S3

If the Lambda function data you are passing between states might grow to more than 262,144 bytes, we recommend using Amazon S3 to store the data, and implement one of the following methods:

Alternatively, you can adjust your implementation to pass smaller payloads in your executions.

ResultSelector

Use the ResultSelector field to manipulate a state's result before ResultPath is applied. The ResultSelector field lets you create a collection of key value pairs, where the values are static or selected from the state's result. Using the ResultSelector field, you can choose what parts of a state's result you want to pass to the ResultPath field.

ResultSelector is an optional field in the following states:

For example, Step Functions service integrations return metadata in addition to the payload in the result. ResultSelector can select portions of the result and merge them with the state input with ResultPath. In this example, we want to select just the resourceType and ClusterId, and merge that with the state input from an Amazon EMR createCluster.sync. Given the following:

{
  "resourceType": "elasticmapreduce",
  "resource": "createCluster.sync",
  "output": {
    "SdkHttpMetadata": {
      "HttpHeaders": {
        "Content-Length": "1112",
        "Content-Type": "application/x-amz-JSON-1.1",
        "Date": "Mon, 25 Nov 2019 19:41:29 GMT",
        "x-amzn-RequestId": "1234-5678-9012"
      },
      "HttpStatusCode": 200
    },
    "SdkResponseMetadata": {
      "RequestId": "1234-5678-9012"
    },
    "ClusterId": "AKIAIOSFODNN7EXAMPLE"
  }
}

You can then select the resourceType and ClusterId using ResultSelector:

"Create Cluster": {
  "Type": "Task",
  "Resource": "arn:aws:states:::elasticmapreduce:createCluster.sync",
  "Parameters": {
    <some parameters>
  },
  "ResultSelector": {
    "ClusterId.$": "$.output.ClusterId",
    "ResourceType.$": "$.resourceType"
  },
  "ResultPath": "$.EMROutput",
  "Next": "Next Step"
}

With the given input, using ResultSelector produces:

{
  "OtherDataFromInput": {},
  "EMROutput": {
      "ClusterId": "AKIAIOSFODNN7EXAMPLE"
      "ResourceType": "elasticmapreduce",
  }
}

Flattening an array of arrays

If the Parallel workflow state or Map workflow state state in your state machines return an array of arrays, you can transform them into a flat array with the ResultSelector field. You can include this field inside the Parallel or Map state definition to manipulate the result of these states.

To flatten arrays, use the syntax: [*] in the ResultSelector field as shown in the following example.

"ResultSelector": {
    "flattenArray.$": "$[*][*]"
  }

For examples that show how to flatten an array, see Step 3 in the following tutorials: