Invoke an AWS Lambda function with Step Functions - AWS Step Functions
Supported APIsExamplesIAM policies

Invoke an AWS Lambda function with Step Functions

Learn how to use Step Functions to invoke Lambda functions either synchronously or asynchronously as part of an event-driven serverless application.

To learn about integrating with AWS services in Step Functions, see Integrating services and Passing parameters to a service API in Step Functions.

Key features of Optimized Lambda integration

  • The Payload field of the response is parsed from escaped Json to Json.

  • If the response contains the field FunctionError or an exception is raised within the Lambda function, the task fails.

For more information about managing state input, output, and results, see Processing input and output in Step Functions.

Optimized Lambda APIs

Workflow Examples

The following includes a Task state that invokes a Lambda function.

{  
   "StartAt":"CallLambda",
   "States":{  
      "CallLambda":{  
         "Type":"Task",
         "Resource":"arn:aws:states:::lambda:invoke",
         "Arguments":{  
            "FunctionName":"arn:aws:lambda:region:account-id:function:MyFunction"
         },
         "End":true
      }
   }
}

The following includes a Task state that implements the callback service integration pattern.

{  
   "StartAt":"GetManualReview",
   "States":{  
      "GetManualReview":{  
         "Type":"Task",
         "Resource":"arn:aws:states:::lambda:invoke.waitForTaskToken",
         "Arguments":{  
            "FunctionName":"arn:aws:lambda:region:account-id:function:get-model-review-decision",
            "Payload":{  
               "model":"{% $states.input.my-model %}",
               "TaskToken": "{% $states.context.Task.Token %}"
            },
            "Qualifier":"prod-v1"
         },
         "End":true
      }
   }
}

When you invoke a Lambda function, the execution will wait for the function to complete. If you invoke the Lambda function with a callback task, the heartbeat timeout does not start counting until after the Lambda function has completed executing and returned a result. As long as the Lambda function executes, the heartbeat timeout is not enforced.

It is also possible to call Lambda asynchronously using the InvocationType parameter, as seen in the following example:

Note

For asynchronous invocations of Lambda functions, the heartbeat timeout period starts immediately.

{

  "Comment": "A Hello World example of the Amazon States Language using Pass states",
  "StartAt": "Hello",
  "States": {
    "Hello": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "Arguments": {
        "FunctionName": "arn:aws:lambda:region:account-id:function:echo",
        "InvocationType": "Event"
      },
      "End": true
    }
  }
}

When the Task result is returned, the function output is nested inside a dictionary of metadata. For example:

{

   "ExecutedVersion":"$LATEST",
   "Payload":"FUNCTION OUTPUT",
   "SdkHttpMetadata":{
      "HttpHeaders":{
         "Connection":"keep-alive",
         "Content-Length":"4",
         "Content-Type":"application/json",
         "Date":"Fri, 26 Mar 2021 07:42:02 GMT",
         "X-Amz-Executed-Version":"$LATEST",
         "x-amzn-Remapped-Content-Length":"0",
         "x-amzn-RequestId":"0101aa0101-1111-111a-aa55-1010aaa1010",
         "X-Amzn-Trace-Id":"root=1-1a1a000a2a2-fe0101aa10ab;sampled=0"
      },
      "HttpStatusCode":200
   },
   "SdkResponseMetadata":{
      "RequestId":"6b3bebdb-9251-453a-ae45-512d9e2bf4d3"
   },
   "StatusCode":200
}

Alternatively, you can invoke a Lambda function by specifying a function ARN directly in the "Resource" field. When you invoke a Lambda function in this way, you can't specify .waitForTaskToken, and the task result contains only the function output.

{  
   "StartAt":"CallFunction",
   "States":{  
      "CallFunction": {  
         "Type":"Task",
         "Resource":"arn:aws:lambda:region:account-id:function:HelloFunction",
         "End": true
      }
   }
}

You can invoke a specific Lambda function version or alias by specifying those options in the ARN in the Resource field. See the following in the Lambda documentation:

IAM policies for calling AWS Lambda

The following example templates show how AWS Step Functions generates IAM policies based on the resources in your state machine definition. For more information, see How Step Functions generates IAM policies for integrated services and Discover service integration patterns in Step Functions.

In the following example, a state machine with two AWS Lambda task states which call function1 and function2, the autogenerated policy includes lambda:Invoke permission for both functions.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "lambda:InvokeFunction"
            ],
            "Resource": [
                "arn:aws:lambda:region:account-id:function:function-1",
                "arn:aws:lambda:region:account-id:function:function-2"
            ]
        }
    ]
}