Event Sourcing

Cameron Fletcher

Agenda

What is an Event?

Events as a Storage Mechanism

Versioning

Testing with Events

Performance and Scalability

Demonstration

What is an Event?

An Event

An 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 HasCheckedOut

1 False

CartItems

CartId ItemId Name

1 1 Domain Driven Design

1 2 Enterprise Integration Patterns

1 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 Rejected

Given

An account with a balance of 100

When

A debit is requested for 101

Then

An InsufficientBalanceException is thrown

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

Overdraw Attempts are Rejected

Given

A series of events

When

A command

Then

Zero or more events, or an exception is thrown

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

Performance and Scalability

Performance and Scalability Benefits

Append-only model

Partitioning (Sharding)

Persisting Objects

Reconstituting Objects

Demonstration

Summary

Questions?