An Introduction to CQRS
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The document provides an introduction to the Command/Query Responsibility Segregation (CQRS) pattern. It outlines an agenda for the presentation that includes recapping domain-driven design principles, explaining CQRS, and providing a step-by-step approach towards implementing CQRS. The presentation argues that separating reading from writing allows for simpler and more scalable system designs compared to traditional architectures that try to support both with a single model.
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