Event Sourcing

1. Event Sourcing Cameron Fletcher

2. Agenda • What is an Event? • Events as a Storage Mechanism • Versioning • Testing with Events • Performance and Scalability • Demonstration

3. What is an Event?

4. An Event An event is something that has happened in the past as a result of a change in state of our domain Glass has been filled Fill glass Event Request to change state Glass is already full! Glass is broken! Exception

5. Events as aStorage Mechanism

8. Shopping Cart Created Item Added Carts CartId HasCheckedOut Item Added True 1 False Item Added CartItems Name CartId ItemId Checkout Completed Domain Driven Design 1 1 Enterprise Integration Patterns 1 2 Event Centric 1 3

9. Shopping Cart Created Item Added Item Added Item Added Domain Object Checkout Completed

10. Shopping Cart Created Item Added Item Added Item Added Item Removed Checkout Completed

11. Versioning

12. 7 6 5 snapshot 4 3 2 1

13. Cart Domain Object Example // request to mutate state publicvoidAdd(Item item) { if (this.itemCount > 5) thrownewCartFullException(this.id); ApplyChange( newItemAdded(this.id, item.Name)); } // mutate state privatevoidApply(ItemAdded @event) { this.itemCount++; }

14. Domain Object Base protectedvoid ApplyChange(Event @event) { this.ApplyChange(@event, true); } privatevoid ApplyChange(Event@event, bool isNew) { this.AsDynamic().Apply(@event); if (isNew) this.uncommittedChanges.Add(@event); }

15. Domain Object Reconstitution publicvoidLoadFromHistory(IEnumerable<Event> history) { foreach (var @event in history) this.ApplyChange(@event, false); }

16. Domain Object Persistence publicIEnumerable<Event> GetUncommittedChanges() { returnthis.uncommittedChanges; } publicvoid MarkChangesAsCommitted() { this.uncommittedChanges.Clear(); }

17. Testing with Events

18. Overdraw Attempts are Rejected Given An account with a balance of 100 When A debit is requested for 101 Then An InsufficientBalanceException is thrown

19. Overdraw Attempts are Rejected Given An account is created A deposit was made for 100 When A debit is requested for 101 Then An InsufficientBalanceException is thrown

20. Overdraw Attempts are Rejected Given A series of events When A command Then Zero or more events, or an exception is thrown

21. Overdraw Attempts are Rejected Given An account is created A deposit was made for 100 A debit is requested for 101 When A debit is requested for 20 Then An InsufficientBalanceException is thrown

22. Performance and Scalability

23. Performance and Scalability Benefits • Append-only model • Partitioning (Sharding) • Persisting Objects • Reconstituting Objects

24. Demonstration

25. Summary

26. Questions?