AWS Compute Blog

Mocking service integrations with AWS Step Functions Local

This post is written by Sam Dengler, Principal Specialist Solutions Architect, and Dhiraj Mahapatro, Senior Specialist Solutions Architect.

AWS Step Functions now supports over 200 AWS Service integrations via AWS SDK Integration. Developers want to build and test control flow logic for workflows using branching logicerror handling, and retries. This allows for precise workflow execution with deterministic results. Additionally, developers use Step Functions’ input and output processing features to transform data as it enters and exits tasks.

Developers can test their state machines locally using Step Functions Local before deploying them to an AWS account. However state machines that use service integrations like AWS Lambda, Amazon SQS, or Amazon SNS require Step Functions Local to perform calls to AWS service endpoints. Often, developers want to test the control and data flows of their state machine executions in isolation, without any dependency on service integration availability.

Today, AWS is releasing Mocked Service Integrations for Step Functions Local. This allows developers to define sample outputs from AWS service integrations. You can combine them into test case scenarios to validate workflow control and data flow definitions. You can find the code used in this post in the Step Functions examples GitHub repository.

Sales lead generation sample workflow

In this example, new sales leads are created in a customer relationship management system. This triggers the sample workflow execution using input data, which provides information about the contact.

Using the sales lead data, the workflow first validates the contact’s identity and address. If valid, it uses Step Functions’ AWS SDK integration for Amazon Comprehend to call the DetectSentiment API. It uses the sales lead’s comments as input for sentiment analysis.

If the comments have a positive sentiment, it adds the sales leads information to a DynamoDB table for follow-up. The event is published to Amazon EventBridge to notify subscribers.

If the sales lead data is invalid or a negative sentiment is detected, it publishes events to EventBridge for notification. No record is added to the Amazon DynamoDB table. The following Step Functions Workflow Studio diagram shows the control logic:

The full workflow definition is available in the code repository. Note the workflow task names in the diagram, such as DetectSentiment, which are important when defining the mocked responses.

Sentiment analysis test case

In this example, you test a scenario in which:

  1. The identity and address are successfully validated using a Lambda function.
  2. A positive sentiment is detected using the Comprehend.DetectSentiment API after three retries.
  3. A contact item is written to a DynamoDB table successfully
  4. An event is published to an EventBridge event bus successfully

The execution path for this test scenario is shown in the following diagram (the red and green numbers have been added). 0 represents the first execution; 1, 2, and 3 represent the max retry attempts (MaxAttempts), in case of an InternalServerException.

Mocked response configuration

To use service integration mocking, create a mock configuration file with sections specifying mock AWS service responses. These are grouped into test cases that can be activated when executing state machines locally. The following example provides code snippets and the full mock configuration is available in the code repository.

To mock a successful Lambda function invocation, define a mock response that conforms to the Lambda.Invoke API response elements. Associate it to the first request attempt:

"CheckIdentityLambdaMockedSuccess": {
  "0": {
    "Return": {
      "StatusCode": 200,
      "Payload": {
        "statusCode": 200,
        "body": "{\"approved\":true,\"message\":\"identity validation passed\"}"

To mock the DetectSentiment retry behavior, define failure and successful mock responses that conform to the Comprehend.DetectSentiment API call. Associate the failure mocks to three request attempts, and associate the successful mock to the fourth attempt:

"DetectSentimentRetryOnErrorWithSuccess": {
  "0-2": {
    "Throw": {
      "Error": "InternalServerException",
      "Cause": "Server Exception while calling DetectSentiment API in Comprehend Service"
  "3": {
    "Return": {
      "Sentiment": "POSITIVE",
      "SentimentScore": {
        "Mixed": 0.00012647535,
        "Negative": 0.00008031699,
        "Neutral": 0.0051454515,
        "Positive": 0.9946478

Note that Step Functions Local does not validate the structure of the mocked responses. Ensure that your mocked responses conform to actual responses before testing. To review the structure of service responses, either perform the actual service calls using Step Functions or view the documentation for those services.

Next, associate the mocked responses to a test case identifier:

"RetryOnServiceExceptionTest": {
  "Check Identity": "CheckIdentityLambdaMockedSuccess",
  "Check Address": "CheckAddressLambdaMockedSuccess",
  "DetectSentiment": "DetectSentimentRetryOnErrorWithSuccess",
  "Add to FollowUp": "AddToFollowUpSuccess",
  "CustomerAddedToFollowup": "CustomerAddedToFollowupSuccess"

With the test case and mock responses configured, you can use them for testing with Step Functions Local.

Test case execution using Step Functions Local

The Step Functions Developer Guide describes the steps used to set up Step Functions Local on your workstation and create a state machine.

After these steps are complete, you can run a workflow locally using the start-execution AWS CLI command. Activate the mocked responses by appending a pound sign and the test case identifier to the state machine ARN:

aws stepfunctions start-execution \
  --endpoint http://localhost:8083 \
  --state-machine arn:aws:states:us-east-1:123456789012:stateMachine:LeadGenerationStateMachine#RetryOnServiceExceptionTest \
  --input file://events/sfn_valid_input.json

Test case validation

To validate the workflow executed correctly in the test case, examine the state machine execution events using the StepFunctions.GetExecutionHistory API. This ensures that the correct states are used. There are a variety of validation tools available. This post shows how to achieve this using the AWS CLI filtering feature using JMESPath syntax.

In this test case, you validate the TaskFailed and TaskSucceeded events match the retry definition for the DetectSentiment task, which specifies three retries. Use the following AWS CLI command to get the execution history and filter on the execution events:

aws stepfunctions get-execution-history \
  --endpoint http://localhost:8083 \
  --execution-arn <ExecutionArn>
  --query 'events[?(type==`TaskFailed` && contains(taskFailedEventDetails.cause, `Server Exception while calling DetectSentiment API in Comprehend Service`)) || (type==`TaskSucceeded` && taskSucceededEventDetails.resource==`comprehend:detectSentiment`)]'

The results include matching events:

  "timestamp": "2022-01-13T17:24:32.276000-05:00",
  "type": "TaskFailed",
  "id": 19,
  "previousEventId": 18,
  "taskFailedEventDetails": {
    "error": "InternalServerException",
    "cause": "Server Exception while calling DetectSentiment API in Comprehend Service"

These results should be compared to the test acceptance criteria to verify the execution behavior. Test cases, acceptance criteria, and validation expressions vary by customer and use case. These techniques are flexible to accommodate various happy path and error scenarios. To explore additional sample test cases and examples, visit the example code repository.


This post introduces a new robust way to test AWS Step Functions state machines in isolation. With mocking, developers get more control over the type of scenarios that a state machine can handle, leading to assertion of multiple behaviors. Testing a state machine with mocks can also be part of the software release. Asserting on behaviors like error handling, branching, parallel, dynamic parallel (map state) helps test the entire state machine’s behavior. For any new behavior in the state machine, such as a new type of exception from a state, you can mock and add as a test.

See the Step Functions Developer Guide for more information on service mocking with Step Functions Local. The sample application covers basic scenarios of testing a state machine. You can use a similar approach for complex scenarios including other Step Functions flows, like map and wait.

For more serverless learning resources, visit Serverless Land.