Call Your AWS Step Functions with API Gateway in Just A Few Lines with SAM

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Last weekend I played with AWS Step Functions! I wanted to refactor a Lambda function I already coded for the sake of maintainability and evolutivity.

After having some fun with AWS Serverless Application Model (AWS SAM) refactoring and deploying my workflow on AWS, I wanted to call it from outside AWS with a simple endpoint.

Because it took me more than 2 minutes on the Internet to figure out how to do it, I thought it was necessary to have this blog post to help you.

Let's start!

Overview

In this post, we will deploy a basic Step Functions workflow behind an API Gateway that a simple POST request as below will trigger:

Architecture Overview

And because we use AWS SAM, it will only take us a few lines of YAML. Nothing more!

Amazon API Gateway

In the template.yaml, let's first define our API Gateway resource:

Resources:
  HelloWorldApi:
    Type: AWS::Serverless::Api
    Properties:
      StageName: dev

Here we created a new Amazon API Gateway called HelloWorldApi with the most basic information: the stage's name.

AWS Step Functions state machine

Now, let's reference our API Gateway in the state machine! To do so, create the Events section as defined below:

  HelloWorldStateMachine:
    Type: AWS::Serverless::StateMachine
    Properties:
      DefinitionUri: statemachine/hello_world.asl.json
      DefinitionSubstitutions:
        HelloWorldFunctionArn: !GetAtt HelloWorldFunction.Arn
      Events:
        HelloWorldApiEvent:
          Type: Api
          Properties:
            Method: post
            Path: /
            RestApiId: 
              Ref: HelloWorldApi
      Policies:
        - LambdaInvokePolicy:
            FunctionName: !Ref HelloWorldFunction

The Events section contains all the triggers for our state machine. In our case, we defined an API event called HelloWorldApiEvent referencing our API Gateway created previously with also a POST request attached to it.

Outputs

For convenience, let's define an additional output to know which endpoint use to trigger our workflow:

Outputs:
  HelloWorldApi:
    Description: "API Gateway endpoint URL to call Hello World State Machine"
    Value: !Sub "https://${HelloWorldApi}.execute-api.${AWS::Region}.amazonaws.com/dev/"

We're now all settled to deploy our work. Let's see how in the next section!

Deployment with AWS SAM

Now we finished editing our template.yaml which normally looks like this:

AWSTemplateFormatVersion: "2010-09-09"
Transform: AWS::Serverless-2016-10-31
Description: >
  sam-step-functions-api-tutorial

  Sample SAM Template for sam-step-functions-api-tutorial

Resources:
  HelloWorldApi:
    Type: AWS::Serverless::Api
    Properties:
      StageName: dev

  HelloWorldStateMachine:
    Type: AWS::Serverless::StateMachine
    Properties:
      DefinitionUri: statemachine/hello_world.asl.json
      DefinitionSubstitutions:
        HelloWorldFunctionArn: !GetAtt HelloWorldFunction.Arn
      Events:
        HelloWorldApiEvent:
          Type: Api
          Properties:
            Method: post
            Path: /
            RestApiId: 
              Ref: HelloWorldApi
      Policies:
        - LambdaInvokePolicy:
            FunctionName: !Ref HelloWorldFunction

  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      CodeUri: functions/hello-world/
      Handler: app.lambdaHandler
      Runtime: nodejs14.x

Outputs:
  HelloWorldApi:
    Description: "API Gateway endpoint URL to call Hello World State Machine"
    Value: !Sub "https://${HelloWorldApi}.execute-api.${AWS::Region}.amazonaws.com/dev/"
  HelloWorldStateMachineArn:
    Description: "Hello World state machine ARN"
    Value: !Ref HelloWorldStateMachine
  HelloWorldStateMachineRole:
    Description: "IAM Role created for Hello World state machine based on the specified SAM Policy Templates"
    Value: !GetAtt HelloWorldStateMachineRole.Arn

Let's first build our code with sam build command:

C:\sam-step-functions-api-tutorial>sam build
Building codeuri: C:\sam-step-functions-api-tutorial\functions\hello-world runtime: nodejs14.x metadata: {} functions: ['HelloWorldFunction']     
Running NodejsNpmBuilder:NpmPack
Running NodejsNpmBuilder:CopyNpmrc
Running NodejsNpmBuilder:CopySource
Running NodejsNpmBuilder:NpmInstall
Running NodejsNpmBuilder:CleanUpNpmrc

Build Succeeded

Built Artifacts  : .aws-sam\build
Built Template   : .aws-sam\build\template.yaml

Commands you can use next
=========================
[*] Invoke Function: sam local invoke
[*] Deploy: sam deploy --guided

After that, let's deploy our work with sam deploy -g command and the following options:

C:\sam-step-functions-api-tutorial>sam deploy -g

Configuring SAM deploy
======================

        Looking for config file [samconfig.toml] :  Not found

        Setting default arguments for 'sam deploy'
        ========================================= 
        Stack Name [sam-app]: sam-step-functions-api-tutorial
        AWS Region [us-east-1]: eu-west-1
        #Shows you resources changes to be deployed and require a 'Y' to initiate deploy
        Confirm changes before deploy [y/N]: 
        #SAM needs permission to be able to create roles to connect to the resources in your template
        Allow SAM CLI IAM role creation [Y/n]:
        Save arguments to configuration file [Y/n]: 
        SAM configuration file [samconfig.toml]: 
        SAM configuration environment [default]: 

        Looking for resources needed for deployment: Not found.
        Creating the required resources...

After a few minutes, you'll get something similar to the following output:


CloudFormation outputs from deployed stack
------------------------------------------------------------------------------------------------------------------------------------------------------------------   
Outputs
------------------------------------------------------------------------------------------------------------------------------------------------------------------   
Key                 HelloWorldStateMachineArn
Description         Hello World state machine ARN
Value               arn:aws:states:eu-west-1:717193739363:stateMachine:HelloWorldStateMachine-Ppeu4ioPCz1e

Key                 HelloWorldStateMachineRole
Description         IAM Role created for Hello World state machine based on the specified SAM Policy Templates
Value               arn:aws:iam::717193739363:role/sam-step-functions-api-tu-HelloWorldStateMachineRo-H8VZV35HFQB9

Key                 HelloWorldApi
Description         API Gateway endpoint URL to call Hello World State Machine
Value               https://sn46ephbv2.execute-api.eu-west-1.amazonaws.com/dev/
------------------------------------------------------------------------------------------------------------------------------------------------------------------   

Successfully created/updated stack - sam-step-functions-api-tutorial in eu-west-1

Our work is now deployed on AWS and ready to be triggered!

For instance, with Postman:

Postman

And the associated result in AWS:

Workflow triggered

Conclusion

You attached an API Gateway to a Step Functions workflow with AWS SAM! 🎉🎉

It took us only a few lines of YAML to combine these services. Very convenient!

You can find the GitHub repository with the complete code here: https://github.com/johanrin/sam-step-functions-api-tutorial

That's it for me, hope you learned something! If you have any questions, find me on Twitter and feel free to ask me anything 🙏

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