The document discusses decoupling applications using Symfony Messenger and events based on domain-driven design principles. It covers domain-driven design concepts like bounded contexts, aggregates, and domain events. It then explains how to implement domain events in Symfony using UUIDs, an outbox pattern for publishing events, and consuming events from other services. Symfony Messenger is presented as a way to decouple applications using a command bus and middleware while publishing events to transports like Redis.

1. Decoupling your application using
Symfony Messenger and events

2. Hugo Monteiro
• Software Architect & Developer
• Co-founder of Spotahome
Twitter: @hmmonteiro
Github: @monteiro

3. Why?
“Why do we need to decouple our
application with Symfony Messenger
and Events”

5. Domain Driven Design

6. Domain Driven Design
“Domain-driven design (DDD) is
a software design
philosophy[1] that focuses on
understanding domain experts
and building the software as a
model of its domain.[2]"

7. Domain Driven Design
•Domain Experts

8. Domain Driven Design
•Ubiquitous Language

9. Domain Driven Design
• Bounded context
https://martinfowler.com/bliki/BoundedContext.html

10. Domain Driven Design
• Aggregate (root)
Example: Order + Line items is an aggregate root

11. Domain Driven Design
• Invariants are business rules
Example: Create user only if age >= 18

12. Domain Driven Design
Invariant example: admin users cannot be disabled

13. Domain Driven Design
• Domain Events
Example: UserWasCreated, OrderWasStarted, OrderWasShipped,
UserWasDisabled, BookingWasPaid, BookingWasCancelled, etc.

14. Domain events - How it works
Concepts Real world example

15. Domain driven design
• Challenge: How are entities saved in the database, if we
normally need an ID returned from the database?

16. Domain events - UUIDs
• Every time an aggregate is created, we need to know its id, so we can
publish the domain event with the aggregate root id (e.g. entity id).
• Libraries to generate UUIDs: Symfony UID component, Ramsey UUID

17. Domain events - UUIDs

18. Domain events - Publishing
• Domain events are published to the messaging systems
• What happens when there is a failure publishing domain events to the
messaging systems (e.g. Redis, Rabbitmq, SQS, etc).

19. Domain events publishing
Outbox pattern

20. Symfony Messenger
• How to decouple our application with Symfony Messenger
and domain events

21. Symfony Messenger

22. Symfony Messenger
CommandBus

23. Symfony Messenger
• Middleware

24. Symfony Messenger

25. Symfony Messenger

26. Symfony Messenger - Transports
• Redis, AWS SQS, RabbitMQ, Doctrine
• More transports:
• messenger-enqueue-transport

27. Symfony Messenger
• Now that we understand all the main concepts, let’s build something!

28. Implementation
• Let’s build a rent a car software

29. Implementation

32. Implementation

35. Implementation

39. Event Store (outbox)

40. Publish to the Transport

41. Publish to the Transport

42. Consuming from another service

43. Consuming from another service

44. Testing
Demo:
monteiro/symfonycon-winter-2021

45. Conclusions

46. Thank you
Twitter: @hmmonteiro
Github: @monteiro

47. References
• Photos:
• Photo by Alex Green from Pexels: https://www.pexels.com/photo/crop-faceless-female-counselor-talking-to-client-
5699486/
• Photo by Greta Hoffman from Pexels: https://www.pexels.com/photo/mature-gardeners-talking-in-greenhouse-7728658/
• Photo by Brett Jordan from Pexels: https://www.pexels.com/photo/brown-wooden-blocks-on-white-table-8953965/
• Microsoft Docs: Domain Events Design implementation
• Symfony Messenger Documentation
• Mathias Noback: Advanced Web Development Architecture
• Command Bus: https://matthiasnoback.nl/2015/01/a-wave-of-command-buses/
• Vaughn Vernon Implementing Domain Driven Design (red & blue)
• Neil Brooks: The absolute beginner's guide to Domain Driven Design with Symfony