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Discover the best practices for re-queuing messages in AWS SQS when using Lambda functions after API call failures. Learn how to handle error responses effectively.
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This video is based on the question https://stackoverflow.com/q/63970718/ asked by the user 'Kodeffeminate' ( https://stackoverflow.com/u/4232872/ ) and on the answer https://stackoverflow.com/a/63971029/ provided by the user 'Mark B' ( https://stackoverflow.com/u/13070/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering AWS SQS with Lambda: How to Re-Queue Messages on API Failures

As you dive into the world of AWS SQS (Simple Queue Service) and AWS Lambda, you may encounter some practical challenges. One common issue is handling message failures, especially when your Lambda function processes messages from SQS and makes external API calls. In this post, we will explore how to effectively re-queue messages when an API call fails, ensuring that your system remains robust and reliable.

The Problem

You have set up an integration where a Lambda function processes messages from SQS one at a time. Within your Lambda function, you call an external API using these messages. However, you notice that when an API fails (let’s say it returns a non-200 status code), those messages are deemed "processed" and don't re-enter the queue. This behavior can lead to data loss or, at the very least, hinder the ability to retry message processing.

How can you capture error responses from your API calls and ensure that these failed messages are retried? Let’s break down the solution step by step.

The Solution

To ensure that failed messages are retried and placed back on the queue, follow these steps:

Step 1: Modify Your Lambda Function

Adjust your Lambda code to throw an exception when the API returns an error. By throwing an exception, you signal to the Lambda/SQS integration that the processing of the message has failed. Here's a basic outline of what changes are needed:

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

Step 2: Configure Maximum Receives

To provide an effective retry mechanism, you should also configure the Maximum Receives setting on your SQS queue. This setting determines how many times a message can be received before it is sent to the Dead Letter Queue (DLQ).

Set Maximum Receives: Choose a number greater than 1 depending on how many times you want to retry processing the message before moving it to the DLQ. For example:

If you set it to 3, the message will be retried 3 times upon failure before being sent to the DLQ.

Step 3: Utilize Dead Letter Queue (DLQ)

Having a Dead Letter Queue (DLQ) configured is a best practice when working with SQS. This allows you to isolate and analyze messages that repeatedly fail processing.

Setup DLQ: Ensure that you have a DLQ configured where messages that fail processing can be sent after reaching the maximum receive limit.

Monitor and Analyze: Regularly monitor the DLQ to understand the failures and adjust your processing or error handling logic accordingly.

Conclusion

By following the above steps, you can capture error responses from your API calls and reliably put back messages onto your SQS queue for retrying. This approach not only reduces the risk of data loss but also enhances the overall resilience of your serverless architecture.

With a clear understanding of how to handle message failures in AWS SQS, you can confidently implement effective error-handling strategies in your Lambda functions. Happy coding!