Messaging & Event Driven design

Source: Dev.to

Event‑Driven Architecture (EDA)

Event‑driven architecture is a modern pattern built from small, decoupled services that publish, consume, or route events.

• Event: Represents a change in state or an update (e.g., an item placed in a shopping cart, a file uploaded, an order ready to ship).
  • Events may carry the full state (item name, price, quantity) or only identifiers (e.g., “order #8942 was shipped”) that can be used to look up related information.

Unlike traditional request‑driven models, EDA promotes loose coupling between producer and consumer services, making it easier to scale, update, and independently deploy components.

Amazon SQS (Simple Queue Service)

• Fully managed message queue
• Used for decoupling application components

Queue Types

Amazon SNS (Simple Notification Service)

• Pub/Sub messaging service
• One message can be delivered to multiple subscribers

Common Subscribers

• SQS
• Lambda
• Email / HTTP endpoints

SNS vs. SQS

• SNS pushes messages to subscribers.
• SQS polls for messages.

Fan‑out pattern: SNS → multiple SQS queues.

Amazon EventBridge

Amazon EventBridge is a serverless event bus that facilitates event‑driven architectures. It connects different services and applications by routing events from sources (AWS services, custom applications, third‑party software) to targets such as AWS Lambda, Amazon SNS, Amazon SQS, and more.

EventBridge enables decoupled systems where components operate independently and react to events in real time, enhancing scalability, flexibility, and resilience.

When to Use Which Service

Lab 7: Messaging with Amazon SQS

Objective

• Decouple application components using SQS.
• Process messages asynchronously with Lambda.
• Configure a Dead‑Letter Queue (DLQ) for failure handling.

Architecture Overview

Producer (Lambda / CLI) → Amazon SQS → Consumer (Lambda)
                                 ↳ Dead Letter Queue

Steps

1. Create a Standard Queue

   • Navigate to SQS → Create queue.
   • Type: Standard
   • Visibility timeout: 60 seconds
   • Click Create queue.

2. Create a Dead Letter Queue (DLQ)

   • Create another SQS queue named order-processing-dlq.
   • In the main queue’s settings, configure the DLQ:
     • Max receives: 3

3. Create a Lambda Consumer

   • Navigate to Lambda → Create function.
   • Runtime: Python 3.12
   • Add an SQS trigger pointing to the queue created in step 1.
   • Use the following sample code:

   def lambda_handler(event, context):
       for record in event['Records']:
           print(record['body'])

4. Send a Message to the Queue

   aws sqs send-message \
       --queue-url <QUEUE_URL> \
       --message-body "Test message"

Validation

• Verify that the message is processed by the Lambda function.
• Confirm that messages that fail processing after the maximum receive count are moved to the DLQ.