Event Sourcing and CQRS Patterns

Overview

Traditional CRUD stores only current state — once you update a record, the history is gone. Event Sourcing inverts this: every state change is recorded as an immutable event appended to an event log. The current state is always derived by replaying events. CQRS (Command Query Responsibility Segregation) separates the write side (commands that emit events) from the read side (projections that build optimized query models).

When to use: Audit trail requirements, financial ledgers, collaborative editing, domain-driven systems with complex business rules, systems needing temporal queries ("what was the balance on date X?"), or microservice architectures requiring reliable async integration.

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