July 11, 2019

For the past month I’ve been working on a project that will help replace some of the legacy codebase. The goal is to use Aurora MySQL triggers to invoke a Lambda function whenever there’s any data change in a table. Aurora invokes Lambda asynchronously, which means the Lambda will automatically retry twice if the execution fails. Sometimes only two retries are not enough and losing the event is definitely not an option.

The first thing that comes to mind when trying to avoid losing events is to configure a Dead Letter Queue (DQL) on the Lambda function. AWS allows an SQS Queue to act as a DLQ. If you’re familiar with AWS SAM, the configuration looks like this:

DeleteListener : Type : AWS : : Serverless : : Function Properties : Role : !GetAtt LambdaExecutionRole.Arn CodeUri : . Handler : artisan Timeout : 60 MemorySize : 1024 Environment : Variables : ARTISAN_COMMAND : 'entity:deleted' Layers : - !Sub "arn:aws:lambda:${AWS::Region}:209497400698:layer:php-73:6" Runtime : provided VpcConfig : SecurityGroupIds : [ !ImportValue DefaultSecurityGroup ] SubnetIds : !Split [ ',' , !ImportValue PrivateSubnets ] DeadLetterQueue : Type : SQS TargetArn : !GetAtt [ RetryDeleteQueue , Arn ]

For my use case, the next step of the idea naturally came to mind immediately after. The concept goes like this: Aurora triggers my lambda, which fails to execute because of the order of events. If the event is stored in a DLQ, I can have another lambda use that DLQ as the source of Events.

RetryDeleteListener : Type : AWS : : Serverless : : Function Properties : Role : !GetAtt LambdaExecutionRole.Arn CodeUri : . Handler : artisan Timeout : 60 MemorySize : 1024 Environment : Variables : ARTISAN_COMMAND : 'retry:entity:deleted' Layers : - !Sub "arn:aws:lambda:${AWS::Region}:209497400698:layer:php-73:6" Runtime : provided VpcConfig : SecurityGroupIds : [ !ImportValue DefaultSecurityGroup ] SubnetIds : !Split [ ',' , !ImportValue PrivateSubnets ] Events : Retries : Type : SQS Properties : Queue : !GetAtt [ RetryDeleteQueue , Arn ] BatchSize : 1

To recap: Aurora Triggers DeleteListener Lambda. If it fails, the payload is preserved on a DLQ called RetryDeleteQueue. RetryDeleteListener Lambda will use the RetryDeleteQueue as an Event Source, which allows for more retries. Finally, to avoid poison-pill messages, we configure a DLQ on the DLQ so that after X amount of attempts, we can finally give up, but still store the message for further investigation. The DLQ configuration looks like this:

RetryDeleteQueue : Type : AWS : : SQS : : Queue Properties : VisibilityTimeout : 90 MessageRetentionPeriod : 604800 ReceiveMessageWaitTimeSeconds : 20 RedrivePolicy : deadLetterTargetArn : !GetAtt [ RetryDeleteDeadLetterQueue , Arn ] maxReceiveCount : 3 RetryDeleteDeadLetterQueue : Type : AWS : : SQS : : Queue Properties : MessageRetentionPeriod : 1209600

The complete process is described in the image below:

Conclusion

This kind of setup is relevant when your primary event source cannot push a message to SQS, which is the case for AWS Aurora. Aurora can only execute a Lambda function, which may or may not fail to accomplish it’s goal on the first attempt. Using a DLQ allows us to prevent the loss of the payload. Since the Payload is now on an AWS Queue, we can leverage Lambda Event Sourcing to retry these messages a few more times while still leveraging another Queue as the give-up moment. If a message lands on the RetryDeleteDeadLetterQueue we should investigate why the code is failing to accomplish it’s goal.

While working on this solution, two things were important to note:

The Queue Visibility Timeout cannot be smaller than the Lambda timeout.

The Aurora event payload is different than the SQS payload.

The first item seems pretty obvious once you realize what it means, but I had to get an error from Amazon before realizing it. If the message comes back to the Queue in 2 seconds, but the Lambda reserves the right to take 30 seconds to execute it’s job, we could end up processing the same message multiple times because it could always become visible again. The Queue visibility timeout has to take into account that the Lambda might take the full time to execute the message.

The second one is less obvious, but just a detail. The Aurora payload is the exact JSON that the trigger passes to the Lambda. The SQS, on the other hand, will be available at $event['Records'][0]['body'] and will require you to json_decode() it as it will be a plain string.

Reach out to me on Twitter if this helped you.

Cheers.