Event Sourcing at Scale

Instead of storing current state, store every change that ever happened — then derive any view of the data by replaying the event log.

Key Takeaways

1. Event sourcing stores every state change as an immutable event — the current state is derived by replaying the event log
2. The event log is the source of truth — not the current state, which is just a projection (a materialized view of the events)
3. Projections rebuild read models from events — different views of the same data for different use cases, without affecting the event log
4. Event sourcing provides a complete audit trail — every change is recorded with who, what, and when, which is required in financial and compliance systems
5. Snapshots prevent replay from becoming too slow — periodically save the current state so replay starts from the snapshot, not from the beginning