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 glass
Request to change state
Glass has been filled
Event
Glass is already full!
Glass is broken!
Exception
Events as a
Storage Mechanism
Shopping Cart Created
Item Added
Carts
Item Added
CartId
HasCheckedOut
1
True
False
Item Added
CartItems
Checkout Completed
CartId
ItemId
Name
1
1
Domain Driven Design
1
2
Enterprise Integration Patterns
1
3
Event Centric
Shopping Cart Created
Item Added
Item Added
Item Added
Checkout Completed
Domain Object
Shopping Cart Created
Item Added
Item Added
Item Added
Item Removed
Checkout Completed
Versioning
7
6
5
4
3
2
1
snapshot
Cart Domain Object Example
// request to mutate state
public void Add(Item item)
{
if (this.itemCount > 5)
throw new CartFullException(this.id);
ApplyChange(
new ItemAdded(this.id, item.Name));
}
// mutate state
private 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 Reconstitution
public void LoadFromHistory(IEnumerable<Event> history)
{
foreach (var @event in history)
this.ApplyChange(@event, false);
}
Domain Object Persistence
public 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?