Transactional Outbox in AWS Serverless

2. Key Concepts

• Atomicity: Ensures that a series of operations either all succeed or none do.
• Outbox Table: A database table that holds messages to be processed.
• Polling: A mechanism to read from the outbox table and send messages to the target system.
• Idempotency: Ensures that processing the same message multiple times does not lead to unintended side effects.

3. Implementation Steps

Follow these steps to implement the Transactional Outbox pattern on AWS Serverless:

1. Create an Outbox Table: Use Amazon DynamoDB or RDS to create a table for the outbox.

   
   CREATE TABLE Outbox (
       Id SERIAL PRIMARY KEY,
       EventType VARCHAR(255),
       Payload JSON,
       Status VARCHAR(50),
       CreatedAt TIMESTAMP DEFAULT CURRENT_TIMESTAMP
   );
                       

2. Modify Application Logic: When a transaction is successfully committed, insert a record into the outbox table.

   
   INSERT INTO Outbox (EventType, Payload, Status)
   VALUES ('UserCreated', '{"userId": 1, "name": "John Doe"}', 'PENDING');
                       

3. Set Up a Lambda Function: Create a Lambda function that polls the outbox table and processes new messages.

   
   const AWS = require('aws-sdk');
   const dynamoDB = new AWS.DynamoDB.DocumentClient();
   
   exports.handler = async (event) => {
       const params = {
           TableName: 'Outbox',
           FilterExpression: 'Status = :status',
           ExpressionAttributeValues: {
               ':status': 'PENDING',
           },
       };
   
       const data = await dynamoDB.scan(params).promise();
       for (const item of data.Items) {
           // Process the message (e.g., send to an event bus)
           // Update status to PROCESSED
       }
   };
                       

4. Schedule the Lambda Function: Use Amazon EventBridge or CloudWatch Events to trigger the Lambda function at regular intervals.
5. Implement Idempotency: Ensure that each message is processed only once, using unique identifiers.

4. Best Practices

• Use a unique identifier for each message to maintain idempotency.
• Implement error handling and retries for failed message processing.
• Monitor the outbox table for unprocessed messages.
• Keep the outbox table tidy by purging old records periodically.

5. FAQ

6. Flowchart of Process

graph TD;
    A[Transaction Occurs] --> B{Success?};
    B -->|Yes| C[Insert into Outbox];
    C --> D[Polling Lambda Function];
    D --> E[Process Message];
    E --> F[Update Status to PROCESSED];
    B -->|No| G[Rollback Transaction];