Hiding The Lead: Coupling, cohesion and microservices
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The terms coupling and cohesion come from the world of structured programming, but they are also thrown about in the context of microservices. In this session, I look at the applicability of these terms to microservice architecture, and also do a deep dive into the different types of coupling to explore how ideas from the 1970s still have a lot of relevance to the types of systems we build today.
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Hiding The Lead: Coupling, cohesion and microservices
- 1. HIDING THE LEAD COUPLING, COHESION AND MICROSERVICES Sam Newman https://www.flickr.com/photos/spongebabyalwaysfull/8793058944/
- 7. @samnewman Customer Service Accounts v1 Independent deployability requires interface stability Maintaining backwards compatibility is key
- 8. @samnewman Customer Service Accounts v2 Independent deployability requires interface stability Maintaining backwards compatibility is key
- 9. @samnewman Customer Service Accounts v2 Independent deployability requires interface stability Maintaining backwards compatibility is key
- 12. @samnewman MODULES Module A Module D Module B Module C Module E
- 13. @samnewman MODULES Module A Module D Module B Module C Module E Allow for independent working
- 14. @samnewman MODULES Module A Module D Module B Module C Module E Allow for independent working And reuse
- 15. @samnewman MODULES Module A Module D Module B Module C Module E Allow for independent working And reuse Namespaces, packages
- 16. @samnewman MODULES Module A Module D Module B Module C Module E Allow for independent working And reuse Namespaces, packages NPMs, Gems, JARs etc
- 17. @samnewman AND MICROSERVICES? Module A Module DModule B Module C Module E
- 18. @samnewman AND MICROSERVICES? Module A Module DModule B Module C Module E “Modules” run on different computers
- 19. @samnewman AND MICROSERVICES? Module A Module DModule B Module C Module E Communication between modules is done via network calls “Modules” run on different computers
- 20. @samnewman AND MICROSERVICES? Module A Module DModule B Module C Module E Communication between modules is done via network calls “Modules” run on different computers Allows for easier independent deployability
- 21. @samnewman Microservices, when done right, are a form of modular architecture
- 25. @samnewman INFORMATION HIDING http://repository.cmu.edu/cgi/viewcontent.cgi?article=2979&context=compsci Published in 1971 Looked at how best to define module boundaries
- 26. @samnewman INFORMATION HIDING http://repository.cmu.edu/cgi/viewcontent.cgi?article=2979&context=compsci Published in 1971 Looked at how best to define module boundaries Found that “information hiding” worked best
- 27. @samnewman INFORMATION HIDING AND MICROSERVICES https://blog.acolyer.org/2016/09/05/on-the-criteria-to-be-used-in-decomposing-systems-into-modules/
- 30. @samnewman Accounts NOT HIDING? Shipping If an upstream consumer can reach into your internal implementation..
- 31. @samnewman Accounts NOT HIDING? Shipping If an upstream consumer can reach into your internal implementation.. …then you can’t change this implementation without breaking the consumer
- 32. @samnewman Accounts NOT HIDING? Shipping If an upstream consumer can reach into your internal implementation.. …then you can’t change this implementation without breaking the consumer
- 33. @samnewman Accounts NOT HIDING? Shipping If an upstream consumer can reach into your internal implementation.. …then you can’t change this implementation without breaking the consumer
- 36. @samnewman HIDING! Accounts Shipping Hide your secrets! Be explicit about what is shared, and what is hidden Hidden
- 37. @samnewman HIDING! Accounts Shipping Hide your secrets! Be explicit about what is shared, and what is hidden Shared Hidden
- 38. @samnewman HIDING! Accounts Shipping Hide your secrets! Be explicit about what is shared, and what is hidden Shared Hidden
- 39. @samnewman HIDING! Accounts Shipping Hide your secrets! Be explicit about what is shared, and what is hidden Shared Hidden Hidden things can change, shared things can’t
- 40. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code
- 41. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code
- 42. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code { “id” : 123, “name” : “sam”, “age” : 15 } JSON
- 43. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code { “id” : 123, “name” : “sam”, “age” : 15 } JSON
- 44. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code { “id” : 123, “name” : “sam”, “age” : 15 } JSON public class Customer { private int id; private String name; private LocalDate dob; … } Code
- 45. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code { “id” : 123, “name” : “sam”, “age” : 15 } JSON public class Customer { private int id; private String name; private LocalDate dob; … } Code
- 46. @samnewman ACCIDENTAL BREAKAGE public class Customer { private int id; private String name; private int age; … } Code { “id” : 123, “name” : “sam”, “age” : 15 } JSON public class Customer { private int id; private String name; private LocalDate dob; … } Code { “id” : 123, “name” : “sam”, “dob” : 15/02/1974 } JSON
- 47. @samnewman DATA TRANSFER OBJECT https://martinfowler.com/eaaCatalog/dataTransferObject.html Customer
- 48. @samnewman DATA TRANSFER OBJECT https://martinfowler.com/eaaCatalog/dataTransferObject.html Customer public class Customer { private int id; private String name; private LocalDate dob; … }
- 49. @samnewman DATA TRANSFER OBJECT https://martinfowler.com/eaaCatalog/dataTransferObject.html Customer public class Customer { private int id; private String name; private LocalDate dob; … } public class CustomerDTO { private int id; private String name; private int age; … }
- 50. @samnewman DATA TRANSFER OBJECT https://martinfowler.com/eaaCatalog/dataTransferObject.html Customer public class Customer { private int id; private String name; private LocalDate dob; … } public class CustomerDTO { private int id; private String name; private int age; … } { “id” : 123, “name” : “sam”, “age” : 15 } JSON
- 51. @samnewman AND CONNECTIONS BETWEEN THEM?
- 52. @samnewman AND CONNECTIONS BETWEEN THEM? “The connections between modules are the assumptions which the modules make about each other.” - David Parnas
- 53. @samnewman AND CONNECTIONS BETWEEN THEM? “The connections between modules are the assumptions which the modules make about each other.” - David Parnas Information hiding is about reducing the assumptions one module has for another
- 56. @samnewman EXPLICIT CONTRACTS? Module DModule B Need an explicit understanding of the assumptions of consumers
- 57. @samnewman EXPLICIT CONTRACTS? Module DModule B Need an explicit understanding of the assumptions of consumers To help us understand what can change easily, and what cannot
- 58. @samnewman EXPLICIT CONTRACTS? Module DModule B Need an explicit understanding of the assumptions of consumers To help us understand what can change easily, and what cannot Schemas can help!
- 62. @samnewman And of course testing…
- 66. @samnewman COUPLING AND COHESION Coupling Cohesion
- 67. @samnewman COUPLING AND COHESION Coupling Cohesion Low
- 68. @samnewman COUPLING AND COHESION Coupling Cohesion Low Strong
- 69. @samnewman COHESION The code that changes together, stays together
- 72. @samnewman COUPLING The degree to which changing one module requires a change in another
- 73. @samnewman CONSTANTINE’S LAW “A structure is stable if cohesion is strong and coupling is low.” - Albert Endres and Dieter Rombach
- 76. @samnewman TYPES OF COUPLING Domain Tramp
- 77. @samnewman TYPES OF COUPLING Domain CommonTramp
- 78. @samnewman TYPES OF COUPLING Domain Common ContentTramp
- 79. @samnewman TYPES OF COUPLING Domain Common ContentTramp
- 80. @samnewman TYPES OF COUPLING Domain Common Content Increasing coupling Tramp
- 81. @samnewman DOMAIN COUPLING Order Processor Warehouse Reserve Stock Payment Take Payment
- 82. @samnewman DOMAIN COUPLING Order Processor Warehouse Reserve Stock Payment Take Payment Domain Coupling
- 85. @samnewman Order Processor Warehouse TRAMP COUPLING
- 86. @samnewman Order Processor Warehouse Shipping Manifest Send Order Request # Item ID 3 2231 2 134 TRAMP COUPLING
- 87. @samnewman Order Processor Shipping Warehouse Shipping Manifest Send Order Request # Item ID 3 2231 2 134 TRAMP COUPLING
- 88. @samnewman Order Processor Shipping Warehouse Shipping Manifest Send Order Request # Item ID 3 2231 2 134 TRAMP COUPLING Shipping Manifest Dispatch Package Request
- 90. @samnewman AVOIDING TRAMP COUPLING Order Processor
- 91. @samnewman AVOIDING TRAMP COUPLING Order Processor Warehouse
- 92. @samnewman AVOIDING TRAMP COUPLING Order Processor Warehouse Send Order Request # Item ID 3 2231 2 134
- 93. @samnewman AVOIDING TRAMP COUPLING Order Processor Shipping Warehouse Send Order Request # Item ID 3 2231 2 134
- 94. @samnewman AVOIDING TRAMP COUPLING Order Processor Shipping Warehouse Shipping Manifest Send Order Request # Item ID 3 2231 2 134
- 95. @samnewman AVOIDING TRAMP COUPLING (CONT) Order Processor Warehouse Shipping 1342 22313 ItemID# Send Order Request Shipping Manifest Dispatch Package Request 123 The Place, UK Express Shipping
- 99. @samnewman COMMON COUPLING Warehouse Stock Levels Order Processor
- 100. @samnewman COMMON COUPLING Warehouse Stock Levels Order ProcessorForecasting
- 101. @samnewman COMMON COUPLING Warehouse Stock Levels Order ProcessorForecasting Change to the common data impacts multiple microservices
- 102. @samnewman CONTENT COUPLING Order ID Status 123 PLACED 456 SHIPPED
- 103. @samnewman CONTENT COUPLING Order ID Status 123 PLACED 456 SHIPPED Order Processor
- 104. @samnewman CONTENT COUPLING Order ID Status 123 PLACED 456 SHIPPED WarehouseOrder Processor
- 105. @samnewman CONTENT COUPLING Order ID Status 123 PLACED 456 SHIPPED WarehouseOrder Processor Information hiding bypassed
- 106. @samnewman CONTENT COUPLING Order ID Status 123 PLACED 456 SHIPPED WarehouseOrder Processor Information hiding bypassed Unclear what can safely be changed in the Order service
- 107. @samnewman CONTENT COUPLING Order ID Status 123 PLACED 456 SHIPPED WarehouseOrder Processor Information hiding bypassed Unclear what can safely be changed in the Order service Aka Pathological Coupling
- 108. @samnewman TYPES OF COUPLING Domain Common ContentTramp
- 109. @samnewman TYPES OF COUPLING Domain Common Content Increasing coupling Tramp
- 111. @samnewman IN SUMMARY Information hiding is super important
- 112. @samnewman IN SUMMARY Information hiding is super important Some coupling is worse than others
- 113. @samnewman IN SUMMARY Information hiding is super important Some coupling is worse than others Information hiding, low coupling, strong cohesion = Independent Deployability