Command & [e]Mission Control: Using Command and Event Buses to create a CQRS-friendly application

My first talk at the PHP UK Conference 2018 on Command buses and Event Dispatchers.

1. Command AND [e]Mission Control Barney Hanlon [Massive Battlestar Galactica Fan]
2. Firstl y…
3. Celebrating ten years at PHP UK Conference!
4. Thanks to PHP SW for the warm up! @phpsw
5. What do I do?
7. Foreign currency and payment specialists 60+ IT (developers, DevOps, infrastructure) working on our products Guess what?
8. We’re hiring!
10. The Problem(s) Huge growth led to a huge monolith Finance, especially FinTech, is a rapidly changing market New threats mean changes in compliance and security
12. [Some] Solutions Domain-Driven Design approach Move to micro(ish) services TDD and BDD everywhere
13. Probably nothing new…
14. Domain logic is changing constantly Spotify “model” is improving agility but frameworks/ libraries diverging across squads We have a zillion internal endpoints on our APIs (Still Some) Problems
15. ???
16. All right stop Collaborate and listen
17. V anilla Ice says… We’re not going to refactor all these working APIs just to use the same framework. We do need a common way to handle all these changing things though. What lets us plug in new functionality really easily?
18. (Some) B etter Solutions Command buses to tell our domain that we want to do something. Event buses to inform our domain that something has been done. Action-Domain-Responder pattern to control the interaction between the framework and the command bus.
19. B eing Commanding
20. What’s a “Command”? Gang of Four talk about the Command Pattern. DDD refers to Commands within business processes. CQRS
21. CQRS
22. Command Query Responsibility Segregation
23. –Martin Fowler “At its heart is the notion that you can use a different model to update information than the model you use to read information.”
24. Command Pattern
25. – Wikipedia “In object-oriented programming, the command pattern is a behavioral design pattern in which an object is used to encapsulate all information needed to perform an action or trigger an event at a later time.”
26. – Wikipedia “This information includes the method name, the object that owns the method and values for the method parameters.”
27. In other words… A (very) simple Data Transfer Object (DTO) Knows nothing about how it will be used Only knows enough to complete the command Operate as a Value Object in your Domain.
30. Command Buses
31. – The League of Extraordinary Packages “The term is mostly used when we combine the Command pattern with a service layer.”
32. – The League of Extraordinary Packages “Its job is to take a Command object (which describes what the user wants to do) and match it to a Handler (which executes it). This can help structure your code neatly.”
33. Tactician league/tactician Broadway broadway/broadway Command Bus Libraries
36. Event Buses [Emission]
37. Drupal started with “hooks” - simple named pub/sub Zend Framework (0.x - 1.x) had the “Dispatch cycle” SPL Observer/Subject for implementing Observer pattern Symfony Event Dispatcher (et al) Bounce (coming soon) The Evolution of Event buses in PHP
41. Comparing Commands and Events
42. Commands and Events are both simple DTOs Beautifully dumb Should be self-validating Commands know (or infer) the one to one relationship with a single handler Comparing Commands and Events
43. Know nothing about the mechanism Only know about their Commands/Events Extremely easy to test Command bus abstracts the Command-Handler relationship Event bus manages the many-to-many Event/Listener relationship Command Handlers AND Event Listeners
44. Hexagonal Architecture “Ports & Adapters”
45. – Alistair Cockburn “Allow an application to equally be driven by users, programs, automated test or batch scripts, and to be developed and tested in isolation from its eventual run-time devices and databases.”
46. Credit: Fabricio Epaminondas Hexagonal architecture for Java applications
47. –Alistair Cockburn “The application is blissfully ignorant of the nature of the input device.”
48. What is an adapter? HTTP Command Line Queue Listener Testing framework
49. Using the Command Pattern
50. Web HTTP Controller HTTP Adapter Domain Command Bus Using the Command Pattern
51. Why bother with another level of abstraction?
52. “Take as long as you need to understand your domain logic before building a working product. Your product can change, your logic shouldn’t.” – No CEO ever
53. Apps without buses
56. Downsides Code duplication is exponential with the number of services involved. HTTP Controllers and console are inherently knowing more about your domain through endpoint names. Infers you know your Domain upfront.
57. EXAMPLE:Domain relativel y unexplored
59. Adv ant ages The Controller’s single responsibility is turning a Request into a Command Command Bus easily mocked to test HTTP adapter. Interactions between services within the Domain abstracted away. Can write the HTTP side of the app without knowing any domain services
60. Application Flow
61. Do/Done Cycle Console/Controller as Adapter Create Command - “Do Something” Command Bus Command Handler Create Event - “Something was Done” Event Bus
65. Command Event Command Command Command Event Event Event Command Command Command Event Event Event
66. Action Domain Responder
67. Action-Domain-Responder First heard of it via the Slim framework documentation “Web-specific refinement of the MVC pattern” More suited to the closure-based micro frameworks https://github.com/pmjones/adr
68. T aking ADR Further Command/Event buses to perform the interactions
69. Request
70. Request Action
72. Request Command Action
73. Command Bus Request Command Action
75. Command Bus Request Command Handler Action
76. Command Bus Request Command Service Handler Action
77. Command Bus Request Command Service Handler Service Service Action
79. Command Bus Request Command Service Handler Service Service Event Action
81. Command Bus Request Command Service Handler Service Service Event Event Bus Action
82. Command Bus Request Command Service Handler Service Service Event Listeners Event Bus Action
84. Command Bus Request Command Service Handler Service Service Event Listeners Event Bus Action Responder
85. Command Bus Request Command Service Handler Service Service Event Listeners Response Event Bus Action Responder
87. Command Bus Request Command Service Handler Service Service Event Listeners Response Event Bus Action Responder
89. Switch out the action/responder pair for any adapter Responder can be as smart or dumb as you like Multiple events can populate the Responder CQRS and Command/Events fit well together Self-validating DTOs allow you to “read your own writes” “Succeed or Throw” makes application logic simpler.
90. @shrikeh https://github.com/shrikeh Thank you!
91. Questions
92. @shrikeh https://github.com/shrikeh SO SA Y WE ALL