How AWS Lambda Retry really works - Guide

By Enrico Portolan

Enrico Portolan

Whenever you are a newbie or an expert in the Serverless world, AWS lambda retry mechanism will cause a headache.

A distributed system usually have different nodes triggered by asynchronous actions. Each node must be designed as a single black block unity. When you design a distributed system, you have to consider a fallback system with a robust error-handling mechanism which may include automatic retries.

In this post, we’ll analyze the AWS Lambda Retry Policy, and the different techniques and best practice to handle errors.

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  1. Synchronous events (such as API Gateway): will not trigger any auto-retry policy. It’s the application responsibility to implement the fallback system.
  2. Async Events (such as SQS and SNS): will trigger two retries (by default). If all retries have failed, it’s important to save the event somewhere. For example, in a Dead Letter Queue (DLQ).
  3. Stream Based Events (such as Dynamo DB Streams): will retry the event until the data expires or is processed successfully.

Not for all requests make sense to retry. In some cases, retrying is only a waste of time and money. If a request failed, it has no chance of succeeding in subsequent attempts. So, how do you stop your Lambda to retry?

The quick and dirty approach is to set the Maximum Retry Attempt value to 0. This feature is a recent add on the AWS Lambda platform (Nov 2019)

A more elegant way is to implement a Global Error Handler in your function.

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The idea is that, since all possible exceptions are handled successfully, the Lambda will respond with a valid JSON object and will not retry any request.

To get the most out of the retry logic, we have to understand the idempotency concept:

Gotcha! Hold on, how can I understand if the current execution is a retry or new request?

Each Lamba has a unique request ID. Only when there is a Lambda retry you will get the same ID. In NodeJS, the value is in the context.awsRequestId property. There is a downside though. You need to store this data somewhere. The natural solution is using DynamoDB. Every new request, the Lambda function has to decide to add a new record on the DB or not.

The dead letter queue lets you redirect failed events to an SQS queue or SNS topic. From there, you can decide to add another Lambda function that will process the failed events and send them to a notification system. For example, send a message on a Slack channel.

You can configure the DLQ directly on the Lambda interface or using CloudFormation.

Step Functions is an orchestration service that allows you to model workflows as state machines. One can argue that this solution is cumbersome and too verbose but it comes with a series of benefits.

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Firstly, Step Functions help you to build a microservice-oriented architecture. One of the first I have done when I started developing with AWS Lambda, was to execute several actions inside a single Lambda. Following this approach, the result was that I had created a perfect example of Serverless Monolith 😓

On the other hand, following the state-machine approach, it comes more natural to run each operation in a different state (which are, indeed, different Lambdas).

Using Step Functions, the developer can decide the transition between states and retry behaviour (number of retries and delay duration). Each task can have his timeout value (unlimited). If the task is not completed in time, a StateTimeouterror is generated. Make sure to configure the Task timeout to be equal to the Lambda’s timeout.

Currently, Step Functions can only be triggered by a limited number of events (ApiGateway or from the SDK). The most common approach is to create a Lambda proxy function that acts as a trigger. For example, if you want to trigger your Step function using SQS, your proxy Lambda will be triggered by the SQS queue. Then, the proxy Lambda has to parse the SQS message and make the appropriate call to the Step Functions StartExecution API.

To be honest, I think that error handling in AWS Lambda can be confusing and not clear at first glance.

Personally, I prefer to use the DLQ method for easy tasks and small project,

In more complex scenarios, when I need granular control in the entire workflow and more control over retry behaviour, I go with Step Functions. This introduces additional cost for state transitions, but it gives you more flexibilities in return (control number of retries and timeout).

There are also other techniques, such as using a middleware (Middy for example), that can help to handle errors.

Thanks for reading this post, I hope it was both fun and useful.