1) Event sourcing stores the history of an application as a sequence of events, rather than storing the current application state. This allows testing by replaying events and integrating systems by subscribing to the stream of events. 2) In event sourcing, an event represents something that happened in the past, such as a customer moving or an order being placed. Applications can be tested by replaying events to ensure the correct outcome occurs. 3) Event sourcing avoids the impedance mismatch between objects and databases since events are part of the domain model. Applications can be rebuilt by replaying all events to reconstruct the state.

1. STATE OR INTENT?
An Introduction to Event Sourcing
Oslo/NDC
Tomas Jansson
14/06/2013

2. THIS IS ME
Tomas Jansson
Manager & Group Lead .NET
BEKK Oslo
@TomasJansson
tomas.jansson@bekk.no
github.com/mastoj
blog.tomasjansson.com

3. WHY USE NOSQL IF WE ARE STORING THE SAME THING?
State
Tables
ORM
Anemic domain model
State
Document
Mapping
Anemic domain model
RDBMS
NoSQL
(Document store)

4. WHAT DO WE REALLY GAIN WITH A DOCUMENT STORE?
?

5. STORE THE INTENT INSTEAD OF THE STATE
Event
Sourcing
Understand
the
business
Testing
No
impedance
mismatch
Integration
Error fixing
Additional
value from
history

6. WHAT IS AN EVENT?
An event is something that has happened in the past
Examples:
CustomerMoved
TableReserved
OrderPlaced
Compare to actions:
MoveCustomer
ReserveTable
PlaceOrder

7. UNDERSTAND THE BUSINESS
What data is stored
What was the
intention (the event)
Focus

8. TESTING 1: SOMEONE CORRECTS THE ADDRESS OF A CUSTOMER
[TestFeature]
public class CustomerTests : BaseTestSetup
{
[Test]
public void When_The_Changing_The_Address_The_Address_Should_Change()
{
// Given
AddEvent(new CustomerCreated(1, "Tomas"));
AddEvent(
new AddressAdded(1, new Address("Street 1", "Oslol", "4040", "Norway")));
var customer = _domainRepository.Get<Customer>(1);
// When
customer.CorrectAddress(new Address("Street 1", "Oslo", "4040", "Norway"));
var events = _domainRepository.Commit();
// Then
Assert.AreEqual(1, events.Count());
Assert.Contains(
new AddressChanged("Street 1", "Oslo", "4040", "Norway"), events);
}
}

9. TESTING 2: A CUSTOMER MOVES
[TestFeature]
public class CustomerTests : BaseTestSetup
{
[Test]
public void When_The_Customer_Moves_The_Address_Should_Change_And_Notify()
{
// Given
AddEvent(new CustomerCreated(1, "Tomas"));
AddEvent(
new AddressAdded(1, new Address("Street 1", "Oslo", "4040", "Norway")));
var customer = _domainRepository.Get<Customer>(1);
// When
customer.Move(new Address("New Street 9", "Oslo", "6666", "Norway"));
var events = _domainRepository.Commit();
// Then
Assert.AreEqual(2, events.Count());
Assert.Contains(new CustomerMoved(1), events);
Assert.Contains(
new AddressChanged("Street 1", "Oslo", "4040", "Norway"), events);
}
}
Testing what we really expect to happen, and we also tests the
absence of side effects!

10. NO IMPEDANCE MISMATCH
public class Customer
{
private Address _address;
public Customer(IEnumerable<IEvent> events)
{ ... }
private void Handle(AddressChanged @event)
{
_address = @event.Address;
}
public void Move(Address address)
{ ... }
public void ChangeAddress(Address address)
{ ... }
.
.
.
}
Build instance from events
AddressChanged (an event)
is part of the domain

11. THE LAST THREE
Easier to make integration
async and explicit
Parties that are intrested can
just tail the event source
Focus is on behavior  fixing
the root cause instead of
changing the data
Complete audit log
Just replay the events
Integration Error fixing
Additional value
from history

12. THE NEGATIVE SIDE OF EVENT SOURCING
Defining events is hard!
No tooling available
Lack of information regarding Event Sourcing
Projection of the events might be considered complex

13. RESOURCES
Rinat Abduling about why event sourcing: http://bliki.abdullin.com/event-sourcing/why
Events as a storage mechanism: http://cqrs.wordpress.com/documents/events-as-storage-mechanism/
My post about the subject: http://tinyurl.com/StateOrIntent

14. Thank you!
@TomasJansson