Buses

Command buses and Event buses

1. Buses Command buses and Event buses
2. Magnus Nordlander • PHP consultant specializing in Symfony • Working at Fervo
3. Hybrid Bus RTB by Kecko
 https://www.flickr.com/photos/kecko/8217510887/
4. Foro Romano by Bert Kaufmann
 https://www.flickr.com/photos/22746515@N02/8043630550/
5. The three degrees of doing • Do it yourself • Ask someone else to do it • Command it be done
6. Emperor Trajan at Tower Hill by DncnH
 https://www.flickr.com/photos/duncanh1/9548613224/
7. Commands • A command is a message which is imperative in nature • ChangeUserPassword • SendWelcomeEmail
8. Commands • Always handled by exactly one handler • Task-oriented • Never returns a value • Required for Command/Query Separation
9. Command handlers • Immutable services • Handles exactly one command type each • Only performs the "primary" task related to the event • Remember SRP and OCP
10. Command bus libs • SimpleBus • Tactician • Broadway
11. Demo
12. Why? • Models changes in domain language, capturing intent • ChangePassword vs UpdateUser • Maps very well to a Task oriented UI
13. Why? • Reinforces Command-Query Responsibility Separation (CQRS) • Asking a question should not change the answer • Clearly indicates which code changes system state
14. Restrictive? • Only performs primary task • No return value
15. Events
16. Events • Events are messages which are informational in nature • UserPasswordWasChanged • WelcomeEmailWasSent
17. Events • Handled by zero or more listeners • Always after-the-fact • Cannot change what has already happened
18. Event listener • Immutable service • Acts upon one type of event and performs some action • Should be called every time a pertinent event is handled • No listener should be able to stop propagation
19. Event listener • Example: • WhenNewUserSignedUpSendWelcomeEmail
20. Event bus libs • SimpleBus • Broadway • Symfony EventDispatcher • Laravel EventDispatcher • League EventEmitter
21. Demo
22. Connecting the dots • Command handlers can emit events • Event subscribers can issue commands
23. Learning more • Matthias Noback has a series of excellent blog posts • http://php-and-symfony.matthiasnoback.nl/tags/ command%20bus/ • Also has a Hexagonal Symfony workshop
24. Questions?