Orchestrate Lambda functions with AWS Step Functions - AWS Lambda

Orchestrate Lambda functions with AWS Step Functions

You can use AWS Step Functions to create state machines that orchestrate Lambda functions to create an application. Step Functions manages the state of your application and provides a visual interface for defining workflows that involve Lambda functions, AWS Batch jobs, Amazon SNS topics, Amazon SQS queues and other common AWS resources. Instead of defining your application logic in a program, you assemble components visually or with a JSON-based, structured language called Amazon States Language. Lambda functions in Step Functions are self-contained, reusable services with a well-defined interface that you can share with less technical users.

Step Functions invokes your function with an event document that you define. You create a task state that invokes your function with the input to the state machine or any JSON document.

Example event.json – Input to random-error function

{ "max-depth": 10, "current-depth": 0, "error-rate": 0.05 }

Step Functions makes it easy to retry failed executions or branch your application logic based on the result of an invocation.

Configuring a state machine as an event source

You can use the Step Functions console to generate a state that invokes a Lambda function, or define states directly in JSON. The following example shows a task state that invokes version 1 of a function named my-function with an event payload that has three keys. When the function returns a successful response, the state machine continues to the next task.

Example state machine task

"Invoke": { "Type": "Task", "Resource": "arn:aws:states:::lambda:invoke", "Parameters": { "FunctionName": "arn:aws:lambda:us-east-2:123456789012:function:my-function:1", "Payload": { "max-depth": 10, "current-depth": 0, "error-rate": 0.05 } }, "Next": "NEXT_STATE", "TimeoutSeconds": 25 }
Permissions

Your state machine needs permission to call the Lambda API to invoke a function. To grant it permission, add the AWSLambdaRole managed policy or a function-scoped inline policy to its role. For more information, see How AWS Step Functions Works with IAM in the AWS Step Functions Developer Guide.

The FunctionName and Payload parameters map to parameters in the Invoke API operation. In addition to these, you can also specify the InvocationType and ClientContext parameters. For example, to invoke the function asynchronously and continue to the next state without waiting for a result, you can set InvocationType to Event as shown following:

"InvocationType": "Event"

Instead of hard coding the event payload in the state machine definition, you can use the input from the state machine execution. The following example uses the input specified when you run the state machine as the event payload:

"Payload.$": "$"

You can also invoke a function asynchronously and wait for it to make a callback with the AWS SDK. To do this, you set the state's resource to arn:aws:states:::lambda:invoke.waitForTaskToken. For more information, see Invoke Lambda with Step Functions in the AWS Step Functions Developer Guide.

Handling function and service errors

When your function or the Lambda service returns an error, you can retry the invocation or continue to a different state based on the error type.

The following example shows an invoke task that retries on 5XX series Lambda API exceptions (ServiceException), throttles (TooManyRequestsException), runtime errors (Lambda.Unknown), and a function-defined error named function.MaxDepthError. It also catches an error named function.DoublesRolledError and continues to a state named CaughtException when it occurs.

Example invoke task – Retry and catch

"Invoke": { "Type": "Task", "Resource": "arn:aws:states:::lambda:invoke", "Retry": [ { "ErrorEquals": [ "function.MaxDepthError", "Lambda.TooManyRequestsException", "Lambda.ServiceException", "Lambda.Unknown" ], "MaxAttempts": 5 } ], "Catch": [ { "ErrorEquals": [ "function.DoublesRolledError" ], "Next": "CaughtException" } ], "Parameters": { "FunctionName": "arn:aws:lambda:us-east-2:123456789012:function:my-function:1", ...

To catch or retry on function errors, create a custom error type. The name of the error type must match the errorType in the formatted error response that Lambda returns when you throw an error. For details on throwing errors in each support language, see the following topics:

For more information on error handling in Step Functions, see Handling error conditions using a state machine in the AWS Step Functions Developer Guide.

AWS CloudFormation and AWS SAM

You can create a serverless application that includes a Step Functions state machine in a AWS CloudFormation template with the AWS Serverless Application Model (AWS SAM). With AWS SAM, you can define the state machine inline in the template or in a separate file. The following example shows a state machine that demonstrates invoking a Lambda function and handling errors. It refers to a function resource defined in the same template (not shown).

Example template.yml

AWSTemplateFormatVersion: '2010-09-09' Transform: 'AWS::Serverless-2016-10-31' Description: An AWS Lambda application that uses AWS Step Functions. Resources: statemachine: Type: AWS::Serverless::StateMachine Properties: DefinitionSubstitutions: FunctionArn: !GetAtt function.Arn Payload: | { "max-depth": 5, "current-depth": 0, "error-rate": 0.2 } Definition: StartAt: Invoke States: Invoke: Type: Task Resource: arn:aws:states:::lambda:invoke Parameters: FunctionName: "${FunctionArn}" Payload: "${Payload}" InvocationType: Event Retry: - ErrorEquals: - function.MaxDepthError - function.MaxDepthError - Lambda.TooManyRequestsException - Lambda.ServiceException - Lambda.Unknown IntervalSeconds: 1 MaxAttempts: 5 Catch: - ErrorEquals: - function.DoublesRolledError Next: CaughtException - ErrorEquals: - States.ALL Next: UncaughtException Next: Success CaughtException: Type: Pass Result: The function returned an error. End: true UncaughtException: Type: Pass Result: Invocation failed. End: true Success: Type: Pass Result: Invocation succeeded! End: true Events: scheduled: Type: Schedule Properties: Description: Run every minute Schedule: rate(1 minute) Type: STANDARD Policies: - AWSLambdaRole ...

This creates a state machine with the following structure:


        State machine with branching logic.