An Introduction to CQRS
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Presented at dotnetkk.de on March 20th 2013 by Dennis Traub
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An Introduction to CQRS
- 1. An Introduction to Command/Query Responsibility Segregation presented by Dennis Traub .NET Stammtisch Konstanz/Kreuzlingen - 20. März 2013 Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 2. Who‘s that guy? Dennis Traub Founder, Owner, CEO, Single Employee of D. Traub Software Development Consultancy Consultant, developer, trainer, speaker 39 years old Married, father of one 20+ years in software development Hire me at mail@dennistraub.de @dtraub Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 3. AGENDA DDD Recap CQRS Break Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 4. AGENDA DDD Recap CQRS Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 5. The „Blue Book“ by Eric Evans Image Placeholder Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 6. Domain-Driven Design Domain Service Value Object Aggregate Specification The well- known Reposit ory Patterns Entity Factory And so on . . Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 7. Domain-Driven Design Ubiquit ous Language Core Domain Domain Expert Bounded Context The important Domain Model Patterns (Generic) Subdomains Context Map And so on . . Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 8. don‘t build one big model that works for everyone Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 9. the tactical Building Blocks we don‘t use DDD when there is no value in formalizing the problem Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 10. we only use DDD in parts where we get a competitive advantage Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 11. we focus our effort and resources on the most important subdomain Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 12. a.k.a. The Core Domain Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 13. AGENDA DDD Recap CQRS Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 14. Presentation Layer Application Services Domain / Business Model / BLL DAL / O/R-Mapper Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 15. Pros of the „Best Practice Architecture“ 1 Easy to build 2 Accepted by management 3 Well-know throughall all seniority levels 4 We know ist limitiations Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 16. Cons of the „Best Practice Architecture“ 1 Limited scalability 2 Loss of intent 3 Lazy loading, locking, race conditions, … 4 Few classes do way too much stuff Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 17. And: If the model is basically CRUD where does Business Logic live? Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 18. Domain Model Responsibilities 1 Business Logic 6 Aggregation 2 Validation 7 Presentation 3 Structure 8 Persistence 4 Projection 9 Processes 5 Associations 10 Lazy / Eager Loading Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 19. the S in SOLID Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 20. SINGLE RESPONSIBILITY PRINCIPLE Just because you can doesn‘t mean you should Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 21. why do we try to build One Model to Rule Them All? Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 22. reading and writing are completely different from eachother Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 23. Sometimes it‘s cheaper to do two things than dealing with the trade-offs Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 24. AGENDA DDD Recap CQRS Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 25. Command Query Separation Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 26. Command Query Separation (CQS) „Every method should either be a command that performs an action or a query that returns data to the caller“ -- Dr. Bertrand Meyer Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 27. Command Query Separation (CQS) in other words: Asking a question should not change the answer Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 28. Command/Query Responsability Segregation Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 29. CQS on an Architectural Level Queries Commands Project Details Project List Project Data Store Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 30. Queries and Projection (Read) Show something Thin Read Layer Data Store Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 31. Thin Read Layer Concerns: 1 Filtering, Scope 2 Data Presentation Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 32. Thin Read Layer Attributes: 1 Data-Oriented 2 Indexable 3 Can be denormalized and distributed for fast access 4 Only Structure Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 33. Behavior and Modification (Write) Do something Command Handler Domain Model O/R-Mapper Data Store Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 34. Domain Model Concerns: 1 Business Logic 2 Validation Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 35. Domain Model Attributes: 1 Normalized 2 Transactional 3 Object-Oriented 4 Persistence Ignorant 5 Only Behavior Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 36. Commands Refactor Application Service Calls to Objects 1 Serializable 2 Can be enveloped 3 Can be intercepted 4 Can be enriched 5 Communicate intent Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 37. Command Handlers Refactored Application Service Methods: 1 Unify interface to all application services 2 Can be wrapped (e.g. Transaction, Authorization, Logging) 3 Can be enveloped (e.g. REST) Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 38. Command Handlers Rule of thumb: One Command Handler per use Case Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 39. This is CQRS Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer Data Store Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 40. BREAK Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 41. AGENDA DDD Recap CQRS Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 42. Domain Event Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 43. Domain Event captures the memory of something interesting affecting the domain Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 44. Domain Event the essence is to capture events that trigger a change to the state Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 45. Domain Event event objects are processed to cause the respective change Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 46. Domain Event and stored to provide an audit log Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 47. Shopping Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 48. we don‘t know: What led to this state? Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 49. Added to Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 50. Added Added to to Cart Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 51. Added Added Added to to to Cart Cart Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 52. Added Added Added Removed to to to from Cart Cart Cart Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 53. Which model contains more information? Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 54. Shopping Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 55. Added Added Added Removed to to to from Cart Cart Cart Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 56. what if we capture every single event? Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 57. … and reproduce state from this stream of events? Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 58. An Event Stream … Added Added Added Removed to to to from Cart Cart Cart Cart Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 59. An Event Stream … Added Added Added Removed to to to from Cart Cart Cart Cart Shopping Cart Can be projected into: Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 60. An Event Stream … Added Added Added Removed to to to from Cart Cart Cart Cart Sales History Shopping Cart … or into this: Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 61. An Event Stream … Added Added Added Removed to to to from Cart Cart Cart Cart Sales History Audit Trail Shopping Cart … or into this: Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 62. An Event Stream … Added Added Added Removed to to to from Cart Cart Cart Cart Sales History Audit Trail Shopping Campaign Effectiveness Cart … or into this: Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 63. AGENDA DDD Recap CQRS Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 64. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer Data Store Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 65. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer SQL Data Store Views Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 66. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer INNER JOIN … SQL LEFT OUTER JOIN … Data Store Views UNION … GROUP BY … Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 67. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer SELECT * FROM … Table Data Store per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 68. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer Table Data Store per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 69. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer Event Table Data Store per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 70. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer Event Event Table Data Store per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 71. Client Commands Queries Command Handlers Domain Model O/R Mapper Thin Read Layer Event Event Event Table Data Store per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 72. Simplified: Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 73. Events Domain Read Model Commands DTOs Client Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 74. Some Code Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 75. Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 76. Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 77. if ISBN exists in ShoppingCartDetails Increase Amount else Add New Row Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 78. if Shopping Cart ID exists in ShoppingCartSummary Increase NumberOfItems Add OfferedPrice to TotalAmount else Add New Row Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 79. Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 80. Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 81. if Amount > 1 ShoppingCartDetails Decrease Amount else Delete Row Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 82. Subtract OfferedPrice to TotalAmount Decrease NumberOfItems Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 83. Testability Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 84. Testing the Write Model Command Handlers Domain Model Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 85. Testing the Write Model Command Handlers Given that certain Events happened Domain Model Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 86. Testing the Write Model When a specific Command is sent Command Handlers Given that certain Events happened Domain Model Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 87. Testing the Write Model When a specific Command is sent Command Handlers Given that certain Events happened Domain Model Event Event Then certain Events (and only those!) should be emitted Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 88. Testing the Read Model Thin Read Layer Table per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 89. Testing the Read Model Thin Read Layer Given that certain Events happened Table per Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 90. Testing the Read Model Thin Read Layer Given that certain Events happened Table per When a specific Event happens Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 91. Testing the Read Model Then each query should return the expected results Thin Read Layer Given that certain Events happened Table per When a specific Event happens Query Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 92. Main Values Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 93. Main Values additive only we don‘t lose information Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 94. Main Values linearly scalable and distributable Read Model Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 95. Main Values every new view can be created from the beginning of time Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 96. Main Values We can come up with new questions at any time Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 97. Main Values built-in integration model Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 98. Proven Technology Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 99. mature business models move away from Update/Delete and become purely transactional Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 100. Human resources Medicine Bookkeeping Banking Finances Government … Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 101. AGENDA DDD Recap CQRS Step by Step the strategic patterns probably simpler than you think! towards CQRS 1 2 3 4 5 6 „Best Practice“ Domain Events End what we call an Architecture are first class citizens Questions & Answers Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 102. ? QUESTIONS Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de
- 103. THANK YOU! Dennis Traub – Software Development Consultancy @dtraub – mail@dennistraub.de