Event sourcing is a pattern for modelling your application’s business logic. It states that all changes to application state should be defined and stored as a sequence of events. The idea of recording events for information storage is nothing new. It has been used for decades in finance, healthcare, and other fields. A few years ago it was rediscovered in software design and its advantages are many: - Suitable for building scalable, highly concurrent, distributed systems. - The stored events give you the true history of a system. This audit is required by law in some industries. - The system’s state can be reversed to any point in the past for retroactive debugging and data analysis. - Gives freedom to refactor your business logic, thus allows much better response to new requirements. - The required infrastructure is simple - no monstrous databases are involved. The focus of my talk will be the Event Sourcing pattern, but I’ll also briefly describe CQRS - an architecture that goes hand in hand with Event Sourcing.

1. Introduction to Event Sourcing
… and CQRS

2. Vladik Khononov
Solutions Architect at Plexop
vladikk http://vladikk.com vladikk
http://il.linkedin.com/in/vladikkhononov/

3. Introduction to Event Sourcing
… and CQRS

4. Domain Driven Design?

6. Introduction
How we are used to do things

7. Business
Domain
Model
Logic
PresentationDAL

8. Model: A simplified representation of a system
or phenomenon.
– www.dictionary.com
מודל: ייצוג תאורטי של מערכת מורכבת, שמטרתו
לחקות את המערכת בהיבטים מהותיים. המודל
אינו מתאר כל תופעה במערכת, אלא מתייחס
להיבטים מוגדרים ומצומצמים שלה. המודל
מבוסס על קירוב של המציאות בדרך של הפשטה,
איחוד ישויות והתעלמות מגורמים שהשפעתם
אינה מהותית.
– www.wikipedia.org

9. User
Interface
Storage
Domain
Model

10. Domain Model

11. Good Domain Model
• Not too much
• Not too less
• Sweet spot
• The information we need
• The information we might need

12. Why domain models fail
• We absolutely suck at predicting the future
• No single model can suit all the use cases
(e.g.: transactional processing, business intelligence, search)
• Model transformations are painful

13. Case Study:
Customers Management

14. • A customer has customer id number and a name
• A customer has contact information: email, phone
number
• A customer can be in on of the following states:
New, CallLater, Converted, NotInterested
• A seller has seller id number, name and password
• The selling home page contains a list of the
customers in the following statuses:
• New
• CallLater (when scheduled call date is met)

15. • Seller chooses a customer to call from the home
page
• Seller can change the customer’s status to
“Converted”, “Not Interested”
• Seller can schedule a future call by setting the
future call date. The Customer’s status will change
to “CallLater”
• Seller can change name, email, and phone number

16. enum Status {
New, CallLater, Converted,
NotInterested
}
class Customer {
int Id;
string Name;
Status Status;
DateTime? ScheduledCall;
string Email;
string PhoneNumber;
}
CREATE TABLE Customers (
ID INTEGER,
Name CHAR(40),
Email CHAR(40),
PhoneNumber CHAR(40),
Status INTEGER,
ScheduledCall DATETIME,
PRIMARY KEY (ID)
)
class Seller {
int Id;
string Name;
string Password;
}
CREATE TABLE Sellers (
ID INTEGER,
Name CHAR(40),
Password CHAR(40)
PRIMARY KEY (ID)
)

17. • Seller can find customers by name, email, and/or
phone number in the search page
• The customers should be found by the current and
the past values

18. • If no new customers are available on the home
page, it should display customers in the
NotInterested status, if it was set more than a 30
days ago

19. • Analysts should be able to review status changes
history for each customer - change date and the
new status

20. class Seller {
int Id;
string Name;
string Password;
}
CREATE TABLE Sellers (
ID INTEGER,
Name CHAR(40),
Password CHAR(40)
PRIMARY KEY (ID)
)
enum Status {
New, CallLater, Converted,
NotInterested
}
class Customer {
int Id;
string Name;
Status Status;
DateTime? ScheduledCall;
string Email;
string PhoneNumber;
}
CREATE TABLE Customers (
ID INTEGER,
Name CHAR(40),
Email CHAR(40),
PhoneNumber CHAR(40),
Status INTEGER,
ScheduledCall DATETIME,
PRIMARY KEY (ID)
)

21. Id Name Email Phone number Status
Scheduled
Call
10 John john@gmail.com 04-2342343 New

22. Id Name Email Phone number Status
Scheduled
Call
10 John john@gmail.com 04-2342343 CallLater 27/10/2014

23. Id Name Email Phone number Status
Scheduled
Call
10 John john@gmail.com 08-9876653 CallLater 27/10/2014

24. Id Name Email Phone number Status
Scheduled
Call
10 John john@gmail.com 08-9876653 Converted

25. Id Name Email Phone number Status
Scheduled
Call
10 John john@gmail.com 08-9876653 Converted

26. Event Sourcing
Capture all changes to an application
state as a sequence of events
~ Martin Fowler

28. class StatusChanged : IEvent {
Status NewStatus;
DateTime CreatedOn; int CreatedBy;
}
class FutureCallScheduled : IEvent {
DateTime ScheduledCallTime;
DateTime CreatedOn; int CreatedBy;
}
class ContactDetailsWereUpdated : IEvent {
string NewName;
string NewEmail;
string NewPhone;
DateTime CreatedOn; int CreatedBy;
}

29. class Customer {
int Id;
string Name;
string Email;
string PhoneNumber;
Status Status;
DateTime? ScheduledCall;
List<IEvent> Events;
…
void Apply(StatusChanged e) {
Status = e.NewStatus;
Events.Add(e);
}
….
….
}

30. class Customer {
int Id;
string Name;
string Email;
string PhoneNumber;
Status Status;
DateTime? ScheduledCall;
List<IEvent> Events;
…
…
void Apply(FutureCallScheduled e) {
ScheduledCall = e.ScheduledCallTime;
Events.Add(e);
}
…
}

31. class Customer {
int Id;
string Name;
string Email;
string PhoneNumber;
Status Status;
DateTime? ScheduledCall;
List<IEvent> Events;
…
…
…
void Apply(ContactDetailsWereUpdated e) {
Name = e.NewName;
Email = e.NewEmail;
PhoneNumber = e.NewPhoneNumber;
Events.Add(e);
}
}

32. Id Name Email Phone number Status
Scheduled
Call
10 John john@gmail.com 08-9876653 Converted

33. 1. new StatusChanged(Status.CallLater)
2. new FutureCallScheduled(’27/10/2014’)
3. new ContactDetailsWereUpdated(
NewPhoneNumber=’08-9876653’
)
4. new StatusChanged(Status.Converted)

34. Event Storage
Entity Id + New events
Event1, Entity Id
Event2,
Event3,
….

35. class CustomerSearchModel {
int Id;
List<string> Names;
List<string> Emails;
List<string> PhoneNumbers;
Status Status;
DateTime? ScheduledCall;
…
…
…
void Apply(ContactDetailsWereUpdated e) {
Names.Add(e.NewName);
Emails.Add(e.NewEmail);
PhoneNumbers.Add(e.NewPhoneNumber);
}
}

36. class CustomerAnalysisModel {
int Id;
string Name;
string Email;
string PhoneNumber;
List<StatusChange> StatusChanges;
DateTime? ScheduledCall;
…
void Apply(StatusChanged e) {
StatusChanges.Add(
new StatusChange(e.CreatedOn, e.NewStatus)
);
}
….
….
}

37. class Customer {
int Id;
string Name;
string Email;
string PhoneNumber;
Status Status;
DateTime? ScheduledCall;
List<IEvent> Events;
}

38. class CustomerSearchModel {
int Id;
List<string> Names;
List<string> Emails;
List<string> PhoneNumbers;
Status Status;
DateTime? ScheduledCall;
}

39. class CustomerAnalysisModel {
int Id;
string Name;
string Email;
string PhoneNumber;
List<StatusChange> StatusChanges;
DateTime? ScheduledCall;
}

40. Good Domain Model
✓ Not too much
✓ Not too less
✓ Sweet spot
✓ The information we need
✓ The information we might need

41. Why domain models fail
✓ We absolutely suck at predicting the future
✓ No single model can suit all the use cases
(e.g.: transactional processing, business intelligence, search)
✓ Model transformations are painful

42. Event Storage
Entity Id + New events
Event1, Entity Id
Event2,
Event3,
….

43. CQRS
Command Query Responsibility Segregation

44. Layered Architecture

45. Step 1

46. Step 2

47. Step 3

48. CQRS

49. Conclusions
• Event based models
• Effective modeling
• No future predicting required
• Time machine
• Retroactive debugging
• Infrastructural freedom
• Infinite scalability
• Easy to scale writes
• Easy to scale reads

50. Before you try this at home
• Read “Implementing Domain Driven Design” by
Vaughn Vernon
• Read “Domain Driven Design” by Eric Evans
• Follow Greg Young
• Read DDD/CQRS on Google Groups

51. Questions?

52. http://vladikk.com
http://twitter.com/vladikk
http://facebook.com/vladikk
http://il.linkedin.com/in/vladikkhononov