CQRS:

1. CQRS: Crack for Architecture Addicts… -or- …An Idea Who’s Time Has Come?

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22. CQRS: Crack for Architecture Addicts… -or- …An Idea Who’s Time Has Come?

24. Do I Suck? http://speakerrate.com/talks/9298

25. Who am I? • …and why should you care? • Steve Bohlen • I Read Books + Write Software • vs. “Read Software + Write Books”  • Blog, Screencast, Speak, Share, Learn

26. Nearly 20 years developing software LISP, Delphi, C/C++, VB, VB.NET, C#, Ruby Senior Engineer Springsource/VMware Co-Founder, NYC Alt.Net User Group http://nyalt.net • Co-Organizer, NYC DDD User Group • http://dddnyc.org Contributor: various OSS projects Nhibernatehttp://www.nhforge.org • NDbUnithttp://www.googlecode.com/ndbunit • Spring.NET http://www.springframework.net blog: http://blog.unhandled-exceptions.com e-mail: sbohlen@gmail.com twitter: @sbohlen

28. Beta Presentation

29. Opinions Ahead

31. Agenda • What’s wrong with Traditional Application Architecture? • What is CQRS? • Commands and Events • Persistence • Understanding Event Sourcing • Exploring a simple CQRS sample app • Exploring a comprehensive CQRS sample app (?) • Resources to Learn more • Q+A

32. What’s wrong withTraditional Application Architecture?

33. Traditional Architecture

34. Traditional Architecture Client Return DTO ACK Returned LAN/WAN Request DTO Command Remote Services Application Services Domain Object Domain Object Domain Object Domain Object RDBMS

35. Application Design Pain Points • Scalability of System is by larger and larger RDBMS • Surprise, surprise!  • Persistence means ‘save the Current State’ of the domain model • Query concerns are at odds with Transactional concerns • 1NF vs. 3NF • more…

36. Domain Modeling Pain Points • Business Rules only exist in user’s heads • Converting Domain Objects to DTOs (and back again!) • Domain Object Persistence • Lazy vs. Eager Fetching • Optimizing Domain for efficient Queries • Setter/Getter Anti-Pattern • Tests are (often) brittle • more…

37. Business Pain Points • Must know what business questions we want to ask up front • Optimizing for Queries and Optimzing for Transactions are tied together (and expensive) • Evolving Requirements is hard/expensive • more…

38. Self-Delusion • One model can be made to efficiently serve two (or more!) different masters • Competing optimization needs of Queries and the Transactions can be efficiently balanced • Getters/Setters are a necessary evil • Persisting State is the only way to think about the problem

41. What is CQRS?

42. Command-Query Separation Command-Query Separation (CQS) states that every method should either be a command that performs an action, or a query that returns data to the caller, but not both. In other words, asking a question should not change the answer. More formally, methods should return a value only if they are referentially transparent and hence possess no side effects. -Bertrand Meyer, designing the Eiffel language

43. Interface Segregation Principle

44. Command-QueryResponsibility Segregation • The CQS principles applied to your domain modeling and system architecture • Enable each element of your architecture to be optimized for a single responsibility • Decomposition for your architecture rather than just for your objects

45. Slicing and Dicing our Architecture Client Return DTO ACK Returned LAN/WAN Request DTO Command Client Remote Services Server Application Services Domain Object Domain Object Domain Object Domain Object RDBMS Read Write

46. Refactoring our Architecture Client Return DTO ACK Returned LAN/WAN Request DTO Command Remote Services Remote Services Remote Services Application Services Application Services Domain Object Domain Object Domain Object Domain Object Light-Weight Query Services Persistence RDBMS Persistence Eventually Read Write

47. Scaling our Systems Write Read C A P CAP Theorem: onsistency, vailability, artition-Tolerance; pick any two! A P C

48. About “Eventual Consistency” • Domain Experts deal in ‘stale’ data all the time • Edit screens, Reports, Analysis, OLAP • ‘Staleness’ is often already implicitly in our systems • Caching, Optimistic Concurrency, and more • Eventual Consistency is making that ‘staleness’ an explicit (and acknowledged) fact in the system • Language matters: say staleand oldernot out-of-dateor inconsistent • Opportunity to actually discuss ‘how stale is OK?’ • Often we make data stale without this discussion at all

49. Commands and Events in CQRS

50. Refresher: the Aggregate Root Pattern Aggregate Boundary Aggregate Boundary Entity1 Entity1 Entity2 Entity2 ValueObject1 Entity3 Aggregate Boundary ValueObject1 Entity1 ValueObject2 ValueObject1 ValueObject2