cqrs-event-sourcing

Event sourcing captures every state change as an immutable event in an append-only event store, creating a complete historical record. Each command generates an event—like OrderCreated—that is stored permanently, enabling full audit trails, temporal queries, and the ability to replay events to reconstruct any past system state.

CQRS (Command Query Responsibility Segregation) separates state-changing commands from read operations, allowing independent optimization. Commands modify state through an event store; queries run against specialized read models built via projections. This separation enables high-scale systems with eventual consistency and multiple denormalized views for fast analytics.

Event sourcing fits systems requiring complete audit trails, temporal queries, event replay capabilities, or multiple read models. Use it for domains needing regulatory compliance, financial transactions, complex domain logic with bounded contexts, or high-scale read/write separation where eventual consistency is acceptable.

Projections transform events into specialized read models by subscribing to the event store and updating denormalized views. Each projection listens for relevant events—like OrderCreated—and updates a query-optimized model. Multiple projections can run independently, providing different views for analytics, reporting, or fast lookups.

Event sourcing requires managing eventual consistency between command and read sides, adds storage overhead from storing all events, and increases complexity in debugging and operational understanding. Read model consistency depends on projection handling; failures risk stale views requiring rebuilding from the event store.