Event Sourcing in practice

Transcript

1. History Defines Us... Event Sourcing in practice Adam Kosiński, 4Developers 2013
2. About what is Event Sourcing? Imagine a bank account: Transaction Amount Balance after transaction Payment Deposit + $3000 $3000 Bill Payments - $1800 $1200 CC Charge - $900 $300 Speed Fine - $300 $0
3. Save state approach $0 WTF!? Why no money? Event sourcing Payment Deposit + $3000 Bill Payments - $1800 CC Charge - $900 Speed Fine - $300 $0 Hmm… need cheaper room Retrieving our data:
4. Seriously, what is ES? Entity (State 0) Entity (State 1) Domain Event #1
5. Entity (State 1) Entity (State n) Domain Event #1 Domain Event #2 Domain Event #n ...
6. Domain Event #1 ... Domain Event #2 Domain Event #3 Domain Event #4 Domain Event #5 Domain Event #6 Entity (Version 6) We only need events Persisted!
7. What is Domain Event • Something important • Something foreseen in domain model • Something happened in the past
8. Sample Domain: Room Access Control AC Software Hardware Controlers Access Control Zone Lock Valid Permission None or invalid Permission
9. Access Control Model
10. Demo #0 Initial, save state model
11. Demo #1 What We need for Event Sourcing: • Domain Events • Event Store • Event handling methods
12. What can be used as event store: • SQL Relational DB (slight overkill) • NoSql (ie. RavenDB) • EventStore by Greg Young • Even simple file storage :)
13. Event Sourcing pros: • No ambiguity between events and data model • Complete historical data • No DB maintenance when modyfying • Very fast data update
14. Event Sourcing Limitations
15. Performance when events grow • Can be troublesome to replay incerasing number of events every time • Solution: Serialized, captured state of entity - Snapshot
16. Snapshot Event #1 Snapshot (Version3) Entity (Version 6) Event #2 Event #3 Event #4 Event #5 Event #6 Event #4 Event #5 Event #6
17. Demo #2 • Creating and restoring snapshots • Snapshot store
18. Yet another issue - Queries • How should we filter queries? • Almost always a good idea: CQRS select * from eventStore.Entities where … (? ? ?)
19. What's with Command-Query Responsibility Segregation ? User UI DB Query (non-domain views) Domain Logic Read Side view-only data Lift up domain entities Update domain Entities Command (non-domain parameters)
20. Why CQRS helps ES User UI Read Store Query (non-domain views) Domain Logic Read Side view-only data Add Events Command (non-domain parameters) Event Store Synchronization
21. Demo #3 • How does Read Store look like • Synchronization – Read Side Update
22. ES – to do or not to do • Yes when: – Event Driven Architecture – History data need – Concurency – Scalable, modifiable systems • Maybe no when: – Simple CRUD apps – No events needed – Initial cost too high – Legacy software
23. What next: • Greg Young blog, Event Store website: – http://geteventstore.com/blog/ • Udi Dahan – http://www.udidahan.com/ • Project CQRS Journey – http://cqrsjourney.github.io/ • Szymon Pobiega – http://simon-says-architecture.com/
24. Thanks for watching! meet me at: https://twitter.com/a_kosinski