Demystifying Dependency injection with ZIO 2

Dependency injection allows developers to have a standardized way of wiring up applications in different ways for different scenarios. While enabling modularity, customizability, and testability, dependency injection has also been criticized for being too magical, too easy to go wrong at runtime, and unnecessary.

In this presentation, Jaro Regec presents several options for dependency injection in Scala, highlighting the strengths and weaknesses of each, and then discusses how you can use ZIO 2 for dependency injection in a best practice way, without overusing ZIO Environment.

Learn how to create modular, customizable, testable code, without boilerplate, and with compile-time failures for missing dependencies, using ZIO 2.

Transcript

1. Demystifying Dependency injection with ZIO 2 Jaro Regec
2. AgendA Idiomatic ZIO 2 way 03 Solutions in Scala 02 What problems are we trying to solve? Dependency Injection 01 DOs and DON'Ts for ZLayers How to structure our ZIO 2 application?
3. Dependency Injection What? Why? 01
4. Example Without Dependency injection • Service creates its own dependencies • Tight coupling • Dependencies are hidden to the outside world • Hard to test
5. Dependencies are injected through constructor • Loose coupling of dependencies - service does not care how its dependencies are created • Dependencies are not hidden but explicit • Usually comes together with "coding to interfaces" pattern
6. Wire services at the edge of app • It can soon become quite complex and boilerplate-y • Nice to have: ◦ Parallelizing creation of services? ◦ Retrying construction? ◦ Stateful services with 'REF'? ◦ Closing connection pool? • It's just "passing parameters to functions" • Do we need a library for this?
7. DI Solutions in Scala 02 Ways to structure our ZIO 2 application
8. • No autowiring • No lifecycle management • Dependencies are visible to the caller of the method • Hard to use with ZIO • Environment type needs to be the same inside for comprehension READER MONAD
9. • Inferred type is intersection type • Hard to satisfy • Missing power of ZIO's .provide() method • Need to run the reader in order to avoid nested Reader READER MONAD
10. Context Functions • No autowiring • No lifecycle management • No type inference • More flexible than reader monad • Less syntactic and runtime overhead
11. GUICE • Based on runtime reflection & annotations • Not typesafe • Easy to use • Minimal boilerplate
12. mACWIRE • Simple and minimalistic • Based on macros • Typesafe - e.g. "Cannot find a value of type: AdvertisementService" • Less powerful than ZLayers
13. Pros and Cons Reader Monad Context functions Guice Macwire ZLAYER in ZIO 2 TypeSafe No Boilerplate Auto Wiring Finalization Parallel construction Constructors as first class values
14. DI with ZIO 2 Traits, Constructors, Layers, Environment.... 03
15. I love ZIO but I hate ZLayers — My Colleague
16. Service Pattern 1.0 • It's just weird. • Has 'HAS' (ZIO 1 only)
17. ACCESSOR METHODS • Unnecessary indirection in code navigation. • Useful only in main method. • Confusing - may lead to environment misuse.
18. Misuse of Environment • Dependencies are exposed to the caller of the method • Dependencies are propagated all the way upstream • Discoverability issues • Type inference
19. MANUAL Wiring of dependencies • >>> • ++ • Painful
20. Use Constructor for global dependencies • A.K.A Service pattern 2.0 • Trait + case class implementation • Modular and testable
21. DEscribe construction of services with ZLAYER • Composable recipes for building app • Turns constructor into a value • Equipped with all the operators e.g. map, flatMap, zipPar
22. USE ENVIRONMENT FOR LOCAL ELIMINABLE EFFECTS • e.g. Authenticate user session
23. AUTO Wire up you program • Minimal boilerplate
24. Compile time errors and Warnings • Failure by unsatisfied dependency. • Warning by redundant dependency.
25. Summary • Use constructors ◦ For global (non eliminable) services • Use environments ◦ For local eliminable services ◦ Access context at the edge of your application • Use Layers ◦ To describe constructors • Let ZIO autowire • Avoid ◦ Misuse environment for passing services around ◦ Old ZIO 1 patterns & accessor methods
26. CREDITS: This presentation template was created by Slidesgo, including icons by Flaticon, and infographics & images by Freepik THANKS! Questions? jaroslav.regec1@gmail.com https://github.com/sviezypan/zio_world_di_talk jaro_regec PS: my colleague loves ZLayers now :)