How to make lambda return 429 on invocation limit

Question

I have implemented a lambda function that synchronously handles HTTP requests through AWS loadbalancer. 
Since lambdas have invocation limits the lambda might be overloaded/throttled when a lot of requests (>1000) are received at the same time.

Since these requests fail because of throttling, I assumed that the loadbalancer-lambda integration would return a 429 code, allowing the client to retry the request at a better time. But this is not the case, whenever the lambda fails because of heavy load the return code is a simple 500 and not a 429. Making the client either retry at a too fast rate or simply believing the service to be broken.

This makes it very difficult to guarantee a stable service, how can I make sure that the lambda returns the more correct 429 code whenever it is throttled?

Thank you!

Answer

Hi

Unfortunately, you can't directly force a Lambda function to return a 429 error upon throttling. Here's why, and potential solutions:
Lambda Throttling Behavior: When a Lambda function is throttled, it usually triggers an internal error within the Lambda service, often resulting in a 5xx type error response.  Lambda itself is not designed to emit a 429.

Alternative solutions/Workaround:

* Exponential Backoff and Retries: https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/
* Configuring provisioned concurrency - https://docs.aws.amazon.com/lambda/latest/dg/provisioned-concurrency.html
* Queuing - https://docs.aws.amazon.com/lambda/latest/dg/with-sqs.html

Answer

Hi,

Is there an important reason why you use ALB instead of standard pattern API GTW + Lambda ?

If the case of this standard pattern with GTW, you will get a 429 on throttling. See https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-request-throttling.html for example

Best,

Didier

How to make lambda return 429 on invocation limit

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