Event Sourcing & CQRS — Notes

Where you find this in the wild

• Banking ledgers, trading systems (regulatory replay).
• Kafka Streams / Materialize.
• Axon Framework (JVM), EventStoreDB.
• Most CQRS systems in production are only CQRS — not event sourced.

Event schema rules

• Events are immutable facts in past tense (AccountOpened, OrderShipped).
• Never delete; "delete" is a new event (AccountClosed).
• Add fields only; don't remove or rename. Use upcasters for forward migration.
• Encode in a stable format (Avro, Protobuf with schema registry).

Read model lag SLO

• Define one — typical 1-5 s in production.
• Surface lag on dashboards; alert when > threshold.

Anti-patterns

• Putting commands in the event log ("UserClickedSubmit" — commands are wishes, events are facts).
• Treating events as DTOs (they're domain facts; downstream views should not couple to them).
• Replaying with code that has business-logic changes that depend on time-of-replay.

Refs

• Greg Young: "CQRS Documents" (free).
• Vaughn Vernon: Implementing Domain-Driven Design.
• Martin Kleppmann: Designing Data-Intensive Applications, ch. 11.
• EventStoreDB docs.
• "Event Sourcing You Are Doing It Wrong" — Martin Kleppmann talk.