Serverless Plugin: Lambda DeadLetterConfig

What is it?

A serverless plugin that can assign a DeadLetterConfig to a Lambda function and optionally create a new SQS queue or SNS Topic with a simple syntax.

Failed asynchronous messages for Amazon Lambda can be be sent to an SQS queue or an SNS topic by setting the DeadLetterConfig. Lambda Dead Letter Queues are documented here.

At the time this plugin was developed AWS Cloudformation (and serverless) did not support the DeadLetterConfig property of the Lambda so we have introduced a plugin that calls UpdateFunctionConfiguration on the lambda after serverless deploys the CloudFormation stack.

Installation

Requirements

• nodeJs > v4.0
• serverless > v1.4

Install the plugin.

npm install serverless-plugin-lambda-dead-letter

Install the plugin with npm and reference it in the serverless yaml file as documented here.

# serverless.yml file
plugins:
  - serverless-plugin-lambda-dead-letter

How do I use it?

Dead letter settings are assigned via a new deadLetter property nested under a function in a serverless.yml file.

There are several methods to configure the Lambda deadLetterConfig.

• Method-1: Create a new deadLetter SQS queue or SNS Topic
• Method-2: Use a pre-existing queue/topic.
• Method-3: Use a queue/topic created in the resources.
• Remove Dead Letter Resource: Remove any deadletter queue/topic that was previously assigned.

Method-1

DeadLetter Queue

Use the deadLetter.sqs to create a new dead letter queue for the function.

The resulting cloudformation stack will contain an SQS Queue and it's respective QueuePolicy.

Create new dead-letter queue by name

# 'functions' in serverless.yml
functions:
  createUser: # Function name
    handler: handler.createUser # Reference to function 'createUser' in code
    deadLetter:
      sqs:  createUser-dl-queue  # New Queue with this name

Create new dead-letter queue with properties

# 'functions' in serverless.yml
functions:
  createUser: # Function name
    handler: handler.createUser # Reference to function 'createUser' in code
    deadLetter:
      sqs:      # New Queue with these properties
        queueName: createUser-dl-queue
        delaySeconds: 60
        maximumMessageSize: 2048
        messageRetentionPeriod: 200000
        receiveMessageWaitTimeSeconds: 15
        visibilityTimeout: 300

DeadLetter Topic

Use the deadLetter.sns to create a new dead letter topic for the function.

The resulting cloudformation stack will contain an SQS Topic resource.

# 'functions' in serverless.yml
functions:
  createUser: # Function name
    handler: handler.createUser # Reference to function 'createUser' in code
    deadLetter:
      sns:  createUser-dl-topic

Method-2

Use the targetArn property to specify the exact SQS queue or SNS topic to use for Lambda dead letter messages. In this case the queue\topic must already exist as must the queue\topic policy.

Reference the ARN of an existing queue createUser-dl-queue

# 'functions' in serverless.yml
functions:
  createUser: # Function name
    handler: handler.createUser # Reference to function 'createUser' in code
    deadLetter:
      targetArn: arn:aws:sqs:us-west-2:123456789012:createUser-dl-queue

Method-3

If you created a queue\topic in the resource section you can reference it using the GetResourceArn pseudo method.

This will use the arn of the resource referenced by {logicalId}

    deadLetter:
      targetArn:
        GetResourceArn: {logicalId}

Note:

• At present this only works for SQS queues or SNS Topics.
• If a queue\topic is created in the resources section you will still need to add a resource for the respective queue\topic policy so that that lambda has permissions to write to the dead letter queue\topic.

In this example the createUser lambda function is using the new CreateUserDeadLetterQueue SQS queue defined in the resources section.

# 'functions' in serverless.yml
functions:
  createUser: # Function name
    handler: handler.createUser # Reference to function 'createUser' in code
    # ...
    deadLetter:
      targetArn:
        GetResourceArn: CreateUserDeadLetterQueue
resources:
    Resources:
      CreateUserDeadLetterQueue:
        Type: AWS::SQS::Queue
        Properties:
          QueueName: create-user-lambda-dl-queue
      CreateUserDeadLetterQueuePolicy:
        Type: AWS::SQS::QueuePolicy
        Properties:
          Queues:
            - Ref: CreateUserDeadLetterQueue
            # Policy properties abbreviated but you need more here ...

Remove DeadLetter Resource

If you previously had a DeadLetter target and want to remove it such that there is no dead letter queue or topic you can supply the deadLetter object with an empty targetArn. Upon deploy the plugin will run the Lambda UpdateFunctionConfiguration and set an empty TargetArn.

# 'functions' in serverless.yml
functions:
  createUser: # Function name
    handler: handler.createUser # Reference to function 'createUser' in code
    # ...
    # Set an empty targetArn to erase previous DLQ settings.
    deadLetter:
      targetArn: