Event Sourcing and CQRS in Distributed Systems

Traditional CRUD (Create, Read, Update, Delete) models work fine for simple apps, but distributed systems introduce new challenges. Keeping state consistent across services, maintaining audit trails, and replaying history become difficult. That’s where Event Sourcing and CQRS (Command Query Responsibility Segregation) shine.

What Is Event Sourcing?

Instead of storing only the current state, event sourcing stores every change as an immutable event in an event log.

• Example: Instead of “balance = $500,” the system stores a sequence of events:
  • Deposit $300
  • Deposit $200

The balance is derived by replaying these events.

Benefits

• Auditability: Every state change is recorded.
• Debugging & Replay: Reconstruct system state at any point in time.
• Integration: Other services can subscribe to the event stream.

Challenges

• Event Schema Evolution: How to handle old events when your schema changes.
• Storage Growth: Event logs grow indefinitely.
• Learning Curve: Developers must think differently than with CRUD.

What Is CQRS?

CQRS separates commands (writes) from queries (reads).

• Command side: Handles validations and state changes (often via event sourcing).
• Query side: Optimized for fast reads, often using denormalized views or caches.

👉 Example: In an e-commerce platform:

• Commands: PlaceOrder, CancelOrder, ShipOrder
• Queries: “Get all pending orders” (served from a read-optimized store)

Why Pair Event Sourcing and CQRS?

They complement each other:

• Event Sourcing ensures reliable, auditable state changes.
• CQRS makes reads fast by separating them from the write model.

Together, they’re powerful for systems like:

• Finance (ledgers, transactions).
• Logistics (package tracking).
• IoT (time-series event streams).

Trade-Offs

• Complexity: Overkill for simple CRUD apps.
• Eventual Consistency: Reads may lag slightly behind writes.
• Operational Overhead: Requires event buses, projections, and monitoring.

Best Practices

• Use idempotent event handlers (processing the same event twice should not break state).
• Plan for event versioning early.
• Monitor lag between command and query sides.
• Start small: don’t retrofit CQRS + Event Sourcing everywhere.

Key Takeaways

• Event Sourcing records history, not just state.
• CQRS separates reads from writes to optimize both.
• Combined, they unlock scalable, auditable, distributed systems — but at the cost of complexity.
• Use them where history and auditability matter as much as real-time state.