How to Invoke AWS API Gateway from Step Function using IAM Role Authentication

Learn how to invoke an AWS API Gateway from Step Functions using IAM role authentication, including practical solutions to common issues.
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How to Invoke AWS API Gateway from Step Function using IAM Role Authentication

When working with AWS services, integrating a variety of them can sometimes be challenging, especially when dealing with different accounts and authentication methods. In this guide, we will explore a common problem faced when trying to invoke an AWS API Gateway from a Step Function using IAM role authentication.

The Problem: Invoking API Gateway from a Step Function

Imagine you have a use case where you need to invoke a REST API endpoint hosted on API Gateway in another AWS account. You have been provided with permissions within that account, including a role that can be assumed. In such scenarios, you might find yourself debating between two strategies:

Generating the request from a Lambda function to call the API.

Using a Step Function to directly invoke the API.

While both options have their merits, the second option—using Step Functions—offers better maintainability and integration with other AWS workflows.

However, as some users have noted, invoking the API through Step Functions can lead to difficulties, especially concerning proper permissions and resource policies. If you face issues during execution, such as getting a 403 error indicating that you are not authorized to access the resource, you are not alone.

Exploring Step Functions and API Gateway Integration

To understand how to properly invoke API Gateway from Step Functions, let's break down the process into digestible steps:

Step 1: Define Your Step Function

Here's an example definition for your Step Function:

[[See Video to Reveal this Text or Code Snippet]]

Replace your_api_endpoint and your_path with the respective values of your API.

Step 2: Set Up IAM Role Trust Relationships

Ensure that your IAM role is correctly configured to allow the Step Functions to assume it. Here’s a sample policy configuration for trust relationships:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Policy to Assume the Role

You should also assign a policy that allows the Step Function’s execution role to assume the designated role. The policy can look like this:

[[See Video to Reveal this Text or Code Snippet]]

Replace account_id and role_name with your account details.

Step 4: Debugging Common Errors

If after these configurations you still encounter a 403 Forbidden error, it may indicate that the Step Function is having trouble assuming the designated role. This can occur for several reasons:

The role policy may not allow the Step Function to assume it.

There might be missing resource policies on the API Gateway that allow access from the Step Functions.

Step 5: Ensure Cross-Account Permissions

Currently, the integration between Step Functions and API Gateway does not support cross-account operations without specific resource policies on the API Gateway itself. Make sure that the API Gateway has permissions configured to allow the assumed role from the Step Function to access it.

Conclusion

Integrating Step Functions with API Gateway using IAM role authentication can be complex, especially when it involves cross-account operations. However, proper understanding and configuration of IAM roles, policies, and resource permissions can lead to a successful setup.

If you encounter issues similar to the one described, don't hesitate to review your role's permissions, ensure correct policies on your API Gateway, and verify that your Step Function is set up appropriately.

By following these steps, you should be well-equipped to invoke AWS API Gateway from a Step Function effectively!

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