Event SourcingCameron Fletcher

Agenda

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

What is an Event?

An EventAn event is something that has happened in the past as a result of a change in state of our domain

Fill glassRequest to change state

Glass has been filledEvent

Glass is already full!

Glass is broken!Exception

Events as aStorage Mechanism

Carts

CartId HasCheckedOut1 False

CartItems

CartId ItemId Name1 1 Domain Driven Design1 2 Enterprise Integration

Patterns1 3 Event Centric

Item Added

Item Added

Item Added

Checkout Completed

Shopping Cart Created

True

Item Added

Item Added

Item Added

Checkout Completed

Shopping Cart Created

Domain Object

Item Added

Item Added

Item Added

Item Removed

Shopping Cart Created

Checkout Completed

Versioning

7

6

5

4

3

2

1

snapshot

Cart Domain Object Example// request to mutate statepublic void Add(Item item){

if (this.itemCount > 5)throw new CartFullException(this.id);

ApplyChange(new ItemAdded(this.id, item.Name));

}

// mutate stateprivate void Apply(ItemAdded @event){

this.itemCount++;}

Domain Object Base

protected void ApplyChange(Event @event){

this.ApplyChange(@event, true);}

private void ApplyChange(Event @event, bool isNew){

this.AsDynamic().Apply(@event);

if (isNew)this.uncommittedChanges.Add(@event);

}

Domain Object Reconstitutionpublic void LoadFromHistory(IEnumerable<Event> history){

foreach (var @event in history)this.ApplyChange(@event, false);

}

Domain Object Persistencepublic IEnumerable<Event> GetUncommittedChanges() {

return this.uncommittedChanges;}

public void MarkChangesAsCommitted(){

this.uncommittedChanges.Clear();}

Testing with Events

Overdraw Attempts are RejectedGiven

An account with a balance of 100

WhenA debit is requested for 101

ThenAn InsufficientBalanceException is thrown

Overdraw Attempts are RejectedGiven

An account is createdA deposit was made for 100

WhenA debit is requested for 101

ThenAn InsufficientBalanceException is thrown

Overdraw Attempts are RejectedGiven

A series of events

WhenA command

ThenZero or more events, or an exception is

thrown

Overdraw Attempts are RejectedGiven

An account is createdA deposit was made for 100A debit is requested for 101

WhenA debit is requested for 20

ThenAn InsufficientBalanceException is thrown

Performance and Scalability

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

Demonstration

Summary

Questions?