Event-Driven Architecture

The log is the source of truth: producers emit immutable events; consumers react asynchronously. This decouples services at deployment and operational boundaries and lets new consumers replay history — at the cost of eventual consistency and significantly more failure surface area than synchronous calls.

Event Sourcing

Store every state change as an immutable event; derive current state by replaying the log. The aggregate's current state is a projection, not the record of truth.

• Snapshots cap replay cost: persist a projected state checkpoint every N events and replay only from the latest snapshot.
• Projections are disposable — design them to be rebuilt by replaying; avoid hand-rolled caches that drift from the log.
• Event store must be append-only; mutating or deleting events destroys the audit trail and breaks replaying consumers.
• GDPR erasure: store PII in a side-table and tombstone the key rather than mutating event history.

CQRS (Command Query Responsibility Segregation)

Separate the write model (commands that mutate state) from the read model (queries over projections). Often paired with event sourcing but independent of it.