🧠CQRS / Event Sourcing (Overview)

Separate write and read concerns; optionally persist domain history as events.

📌 Key IdeaCQRS separates command and query models for scalability and clarity. Event sourcing stores state changes as immutable events, enabling replay/audit but increasing operational complexity.

✨

Why It Matters

Read and write workloads often diverge, and one model cannot optimize both well.
Event sourcing adds auditability and replay capability for complex domains.
Understanding trade-offs helps avoid over-engineering simple CRUD systems.

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Visual

🧩 CQRS + Event Sourcing high-level flow

Write command -> validate -> append Event
                                 |
                                 v
                         project/read-model updates
                                 |
                                 v
                         query side serves reads

Write/read models can evolve independently.

✅

When to Use

Complex domains with divergent read/write workloads.
Strong audit/history requirements and replay value.
Systems benefiting from independently scaled read models.

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When NOT to Use

Simple CRUD domains where complexity offers little gain.
Teams lacking operational maturity for eventual consistency + projection maintenance.

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Code Variations

Event append + projection sketchjava

eventStore.append(new OrderPlaced(orderId, items));
projection.apply(new OrderPlaced(orderId, items));

Real systems handle idempotency, ordering, retries, and projection lag.

🔀

Common Confusions Cleared Up

CQRS vs Event Sourcing

CQRS is model split (commands/queries). Event sourcing is persistence model (events as source of truth). They are often used together but independent choices.

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Interview Q&A

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Quick Cheat Sheet

CQRSSeparate write and read models.

Event sourcingPersist events; rebuild state by replay.

CostComplexity and eventual consistency management.

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