The document discusses moving from a simple CQRS architecture to an event-centric architecture using domain events. It describes how domain events can be used to build denormalized read models from an event stream and log events for future retrieval. Testing strategies are also covered, including validating that read models contain expected data given certain events and that commands emit the proper domain events.

1. CQRS – Eine Einführung
Pt. 2 – Event-Centric Architecture
.Net Online Usergroup – 20.02.2011
Presenter: Dennis Traub
Speaker, Trainer, Author, Software Developer since 1991
Twitter: @DTraub

2. CQRS Revisited
Domain Events
An Event-Centric Architecture
Some Code
What about Testing?
Summary

3. CQRS revisited
Data Storage
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

4. CQRS Revisited
Domain Events
An Event-Centric Architecture
Some Code
What about Testing?
Summary

5. Definition: Domain Event
• Martin Fowler: (PEAA, 2002)
▫ Captures the memory of something interesting
which affects the domain
▫ The essence […] is to capture things that can
trigger a change to the state
▫ These event objects are then processed to cause
changes to the system, and stored to provide an
audit log

6. State
Shopping Cart

7. What we dont know:
What led to this state?

8. Added
to
Cart

9. Added Added
to to
Cart Cart

10. Added Added Added
to to to
Cart Cart Cart

11. Added Added Added Removed
to to to from
Cart Cart Cart Cart

12. Which model contains
more Information?

13. This one?
Shopping Cart

14. Or this one?
Added Added Added Removed
to to to from
Cart Cart Cart Cart

15. What if we capture every single event?

16. And reproduce state from
this stream of events?

17. An Event Stream …
Added Added Added Removed
to to to from
Cart Cart Cart Cart

18. An Event Stream …
Added Added Added Removed
to to to from
Cart Cart Cart Cart
Shopping
Cart
Can be projected into:

19. An Event Stream …
Added Added Added Removed
to to to from
Cart Cart Cart Cart
Sales History
Shopping
Cart
… or into this:

20. An Event Stream …
Added Added Added Removed
to to to from
Cart Cart Cart Cart
Sales History
Audit Trail
Shopping
Cart
… or into this:

21. An Event Stream …
Added Added Added Removed
to to to from
Cart Cart Cart Cart
Sales History
Campaign Effectiveness
Shopping
Campaign Effectiveness
Cart
… or into this:

22. CQRS Revisited
Domain Events
Towards an Event-Centric Architecture
Some Code
What about Testing?
Summary

23. Simple CQRS
Relational Data SQL
Storage Views
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

24. Optimized Write / Complex to Read
INNER JOIN …
LEFT OUTER JOIN …
Relational Data UNION … SQL
Storage GROUP BY … Views
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

25. Optimized Write / Complex to Read
INNER JOIN …
LEFT OUTER JOIN …
Relational Data UNION … SQL
Storage GROUP BY … Views
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

26. Replace Views with Table per Query
SELECT * FROM …
Table
Relational Data per
Storage Query
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

27. Emit Events when they occur
Table
Relational Data per
Storage View
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

28. Denormalize Events into Read Model
Denormalizer
Table
Relational Data per
Storage View
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Queries
Client

29. Add an Event Log for future retrieval
Denormalizer
Table
Relational Data per
Storage View
ORM Thin Read Layer
Domain Model
Command Handlers
Commands Event Log Queries
Client

30. Simplified:
Events
Domain Read Model
Commands DTOs
Client

31. CQRS Revisited
Domain Events
Towards an Event-Centric Architecture
Some Code
What about Testing?
Summary

35. Denormalized Store:
If ISBN exists in ShoppingCartDetails
Increase Amount
Else
Add New Row

36. Denormalized Store:
If Shopping Cart ID exists in ShoppingCartSummary
Increase Number of Items
Add Offered Price to Total Amount
Else
Add New Row

38. Denormalized Store:
If Amount > 1 in ShoppingCartDetails
Decrease Amount
Else
Delete Row

39. Denormalized Store:
Subtract Offered Price from Total Amount
Decrease Number of Items

40. CQRS Revisited
Domain Events
Towards an Event-Centric Architecture
Some Code
What about Testing?
Summary

41. Testing the Read Model
Denormalizer
Table
per
View
Thin Read Layer

42. Testing the Read Model
Given that certain Events have occurred
Denormalizer
Table
per
View
Thin Read Layer

43. Testing the Read Model
Given that certain Events have occurred
Denormalizer
Table
per
When a specific Event has occurred View
Thin Read Layer

44. Testing the Read Model
Given that certain Events have occurred
Denormalizer
Table
per
When a specific Event has occurred View
Thin Read Layer
Then each query should produce the expected results

45. Testing the Read Model
Given that a book with an ISBN of xxx and a
price of USD 24.00 was added to a new shopping cart
When a book with an ISBN of yyy and a price
of USD 19.00 was added to that same shopping cart
Then the Shopping Cart Summary should contain 2 books
and have a total amount of USD 43.00
And the Shopping Cart Details should contain book xxx
And the Shopping Cart Details should contain book yyy

46. Testing the Write Model
Domain Model
Command Handlers

47. Testing the Write Model
Given that certain Events have occurred
Domain Model
Command Handlers

48. Testing the Write Model
Given that certain Events have occurred
Domain Model
Command Handlers
When a specific Commands is sent
Commands

49. Testing the Write Model
Then the expected Events (and only those) should be emitted
Given that certain Events have occurred
Domain Model
Command Handlers
When a specific Commands is sent
Commands

50. Testing the Write Model
Given that an Add Book To Shopping Cart event with
an ISBN of xxx had been sent
When a Remove Book From Shopping Cart command is
sent with an ISBN of zzz
Then a Book Removal Failed event should be emitted
And NO Book Was Removed From Shopping Cart event
should be emitted

51. CQRS Revisited
Domain Events
Towards an Event-Centric Architecture
Some Code
What about Testing?
Summary

52. Main Value:
• Additive only, i.e. we don‘t lose information
• Linearly scalable and distributable Read Model
• Every new view can be created from the
beginning of time
• We can come up with new questions at any time
• Built-In Integration Model

53. Proven Technology:
Mature business models move away from
Update/Delete and become purely transactional:
▫ Human Resources ▫ Banking
▫ Medicine ▫ Financial Trading
▫ Bookkeeping ▫ Government
▫ …

54. What we haven‘t touched yet:
▫ Messaging ▫ Eventual Consistency
▫ Event Sourcing ▫ Integration Scenarios
▫ Sagas ▫…

55. Resources
• http:// cqrs.wordpress.com
• http:// groups.google.com/group/dddcqrs
• http:// distributedpodcast.com
• http:// abdullin.com/cqrs
• http://cqrs.wikidot.com

56. Vielen Dank!
Dennis Traub
@Dtraub
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