Advanced Architecture: CQRS, Event Sourcing, and Sagas

When building at scale, traditional CRUD (Create, Read, Update, Delete) patterns often fall short. We need patterns that handle high concurrency and eventual consistency.

1. CQRS (Command Query Responsibility Segregation)

Concept: Separate the data model for writing from the data model for reading.

• Command: Optimized for validation and consistency.
• Query: Optimized for fast retrieval (often a flattened view in Elasticsearch or Redis).

2. Event Sourcing

Instead of storing the current state of an object, you store the history of events that led to that state.

• Why? Perfect audit logs, the ability to “time travel,” and high write performance.

3. The Saga Pattern (Distributed Transactions)

How do you maintain consistency across 3 microservices without a distributed lock?

• Orchestration Saga: A central controller manages the steps.
• Choreography Saga: Each service emits events that trigger the next service.
• Compensating Transactions: If step 3 fails, the Saga triggers “undo” actions in steps 1 and 2.

Summary

These patterns are powerful but complex. Use them only for the most critical parts of your system where high scale or rigorous auditing is required.