What are Event Sourcing and CQRS patterns?

Event Sourcing stores state changes as sequence of events rather than current state - events are immutable, append-only. Rebuild state by replaying events. Benefits: complete audit trail, temporal queries, debugging. CQRS (Command Query Responsibility Segregation) separates read (query) and write (command) models. Often combined: commands produce events, events update read models. Challenges: eventual consistency, event schema evolution, complexity. Use for: audit requirements, complex domains, high-scale reads. Consider Event Store, Kafka for implementation.