1. CQRS & Domain Models Udi Dahan – The Software Simplist Enterprise Development Expert & SOA Specialist

2. What most architecture looks like

3. Common Mistakes #1 Assuming that layers == tiers We can deploy the same components both client-side and server-side

4. But where do business rules go? Here? Here? Here? Here? Here? Here?

5. Code Reuse On A Pedestal Understanding existing code takes longer than writing new code

6. Too Much Of A “Good Thing”

7. For Example - CQRS Queries View Model UI Queries Commands Publish WS Input Validation Validation Rules View Model Updater DB

8. Common Mistake #2 Assuming that all rules are equally likely to change

9. Rules We Know How To Handle In the domain model only: When a customer cancels an order, if the order was already shipped, charge the customer $$ unless customer is {some status} DB

10. Rules We Struggle With Where to enforce these rules? - First name less than N chars - Username must be unique Not Domain Logic! DB

11. The Domain Model When To Use It & When Not To “If you have complicated and everchanging business rules…” “If you have simple not-null checks and a couple of sums to calculate, a Transaction Script is a better bet” -- Martin Fowler, p119 Patterns of Enterprise Application Architecture

12. Common Mistake #3 Assuming that race conditions affect the domain

13. Race Conditions Cancel Order Ship Order We think this should fail

14. What Domain Events Are For What else needs to happen? When a customer cancels an order, if the order was already shipped, charge the customer $$ unless customer is {some status} DomainEvents.Raise<InvokeCancellationFee>( f => f.OrderId = msg.OrderId );

15. Consider The Reverse Going to ship a cancelled order Race condition between command & query “One in a million” failure Let it fail asynchronously Email back to the user “Sorry, that order doesn’t exist anymore”

16. Aggregate Mistakes public class Customer { publicvoidMakePreferred() { foreach(Ordero inthis.UnbilledOrders) o.Discount(10.Percent); } }

17. In Closing Avoid the common mistakes: #1 Layers & tiers #2 Rate of rule change #3 Race Conditions Think about aggregates & bounded contexts

18. Thank you Udi Dahan – The Software Simplist Enterprise Development Expert & SOA Specialist www.UdiDahan.com email@UdiDahan.com