event-bus-via-sns

event-bus-via-sns

This example demonstrates the implementation of the Event Bus pattern in Serverless architectures using SNS, SQS, and Lambda services. In particular, it focuses on the payment-service and the SNS topic (event-bus-topic) shown in the following diagram:

The application provisions following resources:

1. Lambda function incomingEventHandler

It uses the pys-inbound-queue queue as the event source. When a message is received by the queue with the attribute eventType set to either orderPlaced or orderCancelled, the Lambda service will invoke the function.

2. Queue pys-inbound-queue

It subscribes to the SNS topic and uses the FilterPolicy feature to exclusively receive messages from the order-service. Additionally, it has been configured with a dead-letter queue (DLQ) to forward messages that have failed to be processed after 3 attempts.

3. Queue pys-inbound-queue-dlq

It receives messages that have failed to be processed by the pys-inbound-queue queue. And it uses the queue's default retention period of 4 days to retain the messages. Once the retention period is reached, the messages are automatically deleted.

4. SNS topic event-bus-topic

It's the shared SNS topic that enables communication between different services within the system. While this service provisions the shared SNS topic, it's generally recommended to maintain shared resources separately to allow other services to access it.

Deployment

First, clone the project to local:

git clone [email protected]:m-sureshraj/serverless-examples.git
cd event-bus-via-sns

install the dependencies:

npm install

finally, deploy the project to AWS

sls deploy --verbose

Make sure to include the --verbose flag when running the sls deploy command. This will instruct the command to print the stack outputs.

A successful deployment prints a similar output:

✔ Service deployed to stack payment-service-dev (25s)

functions:
  incomingEventHandler: payment-service-dev-incomingEventHandler (170 kB)

Stack Outputs:
  IncomingEventHandlerLambdaFunctionQualifiedArn: arn:aws:lambda:eu-west-2:xxx:function:payment-service-dev-incomingEventHandler:4
  SNSTopicArn: arn:aws:sns:eu-west-2:xxx:event-bus-topic-dev
  ServerlessDeploymentBucketName: payment-service-dev-serverlessdeploymentbucket-xxx

Testing

The following aws cli command can be used to publish a message to the previously created SNS topic. This simulates how the order-service would communicate with the payment-service to inform about the orderPlaced event.

aws sns publish --topic-arn <the-topic-arn> \ 
 --message-attributes '{ "sourceSystem": {"DataType": "String", "StringValue": "order-service" }}' \
 --message '{ "eventType": "orderPlaced" }'

The following are the log outputs of the function that processed the message:

INFO	Received the orderPlaced event from order-service
INFO	{
  body: '{ "eventType": "orderPlaced" }',
  attributes: { ... },
  messageAttributes: {
    sourceSystem: {
      stringValue: 'order-service',
      dataType: 'String'
      ...
    }
  },
  ...
}