CQRS and Event Sourcing

CQRS separates command (writes) and query (reads) models for scalability and clarity. Event sourcing stores all changes as an append-only log; the current state is a projection of past events.

When it fits

• Auditability and a complete history are required (finance, compliance)
• Read and write workloads scale differently

Risks

• Operational complexity: projections, replays and schema evolution
• Event design mistakes are costly to undo

Real-world examples

• Trading and payments systems maintain ledgers as event streams for full audit trails
• E-commerce order workflows rebuild customer views from events for speed while keeping authoritative events for consistency

Practical tips

• Version events and script safe migrations
• Keep projections disposable and rebuildable