event sourcing cameron fletcher. agenda what is an event? events as a storage mechanism ...
TRANSCRIPT
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?
