Intermittent 403 Response from Serverless Framework API Gateway with Retrofit, while Postman Works Fine

Hello there,

Intermittent 403 responses from your Serverless Framework API Gateway, especially when Postman works fine, can be quite challenging to debug. Here are some steps and considerations to help you troubleshoot and resolve the issue:

1. API Gateway Configuration:

   • Double-check your API Gateway configuration, especially the authentication and authorization settings. Ensure that you have the correct authorization methods configured and that there are no conflicting settings.

2. API Gateway Usage Plans and Throttling:

   • If you are using API Gateway usage plans with throttling, make sure that your API calls are not hitting any rate limits or being throttled. This can sometimes result in intermittent 403 errors.

3. Integration Timeout:

   • Check the integration timeout settings in your API Gateway configuration. If the integration with your Lambda function takes longer than the configured timeout, it can result in 403 errors.

4. Lambda Function Execution:

   • Review the Lambda function associated with the API. Check for any code issues, including potential transient errors that might cause the function to fail intermittently.

5. Cold Starts:

   • Consider Lambda cold starts as a possible cause of the intermittent issue. Cold starts can introduce delays in processing the request. You can try adjusting your Lambda function's memory configuration to reduce cold start times if this is a concern.

6. Resource Permissions:

   • Ensure that the Lambda function, API Gateway, and other AWS resources have the necessary IAM permissions. Lack of permissions can lead to 403 errors.

7. CORS Configuration:

   • If your API has CORS (Cross-Origin Resource Sharing) configured, ensure that it's set up correctly. Inconsistent CORS headers can lead to 403 errors, particularly when making requests from a browser.

8. Client-Side Considerations:

   • Check your Retrofit client configuration, especially for any custom headers or authentication mechanisms. Ensure that there are no issues with how you are making requests from the client side.

9. Logs and Monitoring:

   • Implement detailed logging in your Lambda function and API Gateway to capture information about incoming requests, response codes, and any errors that occur. This can help you pinpoint the issue.

10. Retry Logic:

    • If you have any retry logic in your client application (e.g., Retrofit), make sure it's not inadvertently causing the intermittent 403 responses.

11. Network and Connectivity:

    • Investigate if there are any network issues or firewall rules that might be causing connectivity problems specifically with your client environment.

12. Testing with Different Environments:

    • Try running your Retrofit client from different environments or machines to see if the issue persists consistently. This can help isolate client-specific issues.

13. AWS Support:

    • If the issue persists after thorough investigation, consider reaching out to AWS Support for assistance. They can help diagnose and troubleshoot AWS-specific problems.

Remember that intermittent issues can be particularly challenging to diagnose, so be patient and methodical in your troubleshooting efforts. Capturing detailed logs and monitoring metrics will be crucial in identifying the root cause of the 403 errors.

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