Command / Query Responsibility Segregation

2. Command / Query Responsibility Segregation Udi Dahan – The Software Simplist Session Code: ARC 304

3. What will you see here? • Distributed Systems Architecture • UI Design • Domain Model What won’t you see here? • Code

4. Distributed Systems Architecture

5. Common Distributed Architecture DB

6. Let’s deal with the obvious…

7. Collaboration is good – isn’t it ? Get data Get data User is looking at stale data Change data

8. So why go through all the layers? Cache DB

9. Why transform between tiers? Cache UI WS DB Map from DTOs & WS to view model Map from DTOs and WS to domain object Use EF to map from tables to domain objects

10. What was the point of all this again?

11. Queries – showing data to the user

12. Why not be upfront about it? List of customers Data correct as of 10 minutes ago

13. Only data, no behavior: not an object

14. Keep it simple - 2-Tier UI Query only Persistent View Model For each view in the UI, have a view in the DB

15. Incrementally add from there

16. Commands – accepting user input

17. Can we fully trust users decisions?

18. Validation and Business Rules • Validation: Is the input potentially good? Structured correctly? Ranges, lengths, etc • Rules: Should we do this? Based on the current system state What the user saw is irrelevant

19. Command Processing Layers DB

20. Command Processing Tiers Transaction Get current state Command WS Input DB Persist Validation Validation Rules

21. Should we do what the user asks?

22. User Interface Design

23. Traditional User Interfaces Orders Save Cancel

25. Capturing user intent • Why is it important? Differentiating between: Correcting a mistyped shipping address The user moved – reroute shipments underway • Sometimes users accidentally modify fields when tabbing between columns

26. Reservation systems

27. Reservation systems

28. Not capturing user intent • In a traditional UI – what we have is the checkbox • Why do users select multiple seats? Because they’re reserving for a family / friends • But then, concurrency happens Somebody else got in first on one of the seats • Try to find a block of seats somewhere else

29. Capturing user intent • Group reservation: • Small group – sitting together • Large group – several small groups • Enter number of people • Enter preferred seat type – indicates cost • System emails back when reservation can be filled Include waiting list functionality

30. Usability benefits More users get what they want with less clicks

31. Scalability benefits Thousands of seats, hundreds of thousands of requests No need to show actual status

32. Revisiting the command

33. What’s a good command? • The kind you can reply with: “Thank you. Your confirmation email will arrive shortly” • Inherently asynchronous • Not really related to an entity

34. Commands versus Entities • It’s easier to validate the command • Less data • More specific • Is this potentially good • Validating large entities is complex

35. Domain Models

36. What aren’t they for? • Validation Commands are validated before the domain model is called • Queries Entity relationships for reading are unnecessary

37. What are they for? • Answering the question: Should I do what this valid command is asking? • If the answer is yes, change its state

38. Putting it all together

39. Queries View Model Queries Commands WS Input DB Validation Validation Rules

40. For better scalability, add pub/sub Queries View Model Queries Commands Publish WS Input Validation Validation Rules Cache Updater DB

41. Summary

42. Summary • Keep queries simple – 2-Tier if possible • No data transfer objects • Have commands reflect user intent • Adjust UI design to capture intent • Get the benefits of asynchronous programming • Simple validation, focused business rules

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

44. question & answer

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