Building Dependency Graph

We have two options to build a dependency graph:

1. Manual layer construction— This method uses ZIO's composition operators such as horizontal (++) and vertical (>>>) compositions.
2. Automatic layer construction— It uses metaprogramming to automatically create the dependency graph at compile time.

Assume we have the following dependency graph with two top-level dependencies:

           DocRepo                ++          UserRepo
      ____/   |   \____                       /     \
     /        |        \                     /       \
 Logging  Database  BlobStorage          Logging   Database
                         |                  
                      Logging            

Now, assume that we have written an application that finally needs two services: DocRepo and UserRepo:

val myApp: ZIO[DocRepo with UserRepo, Throwable, Unit] = ZIO.attempt(???)

1. To create the dependency graph for this ZIO application manually, we can use the following code:

val appLayer: URLayer[Any, DocRepo with UserRepo] =
  ((Logging.live ++ Database.live ++ (Logging.live >>> BlobStorage.live)) >>> DocRepo.live) ++
    ((Logging.live ++ Database.live) >>> UserRepo.live)
    
val res: ZIO[Any, Throwable, Unit] = myApp.provideLayer(appLayer)

2. As the development of our application progress, the number of layers will grow, and maintaining the dependency graph manually could be tedious and hard to debug. So, we can automatically construct dependencies with friendly compile-time hints, using ZIO#provide operator:

val res: ZIO[Any, Throwable, Unit] =
  myApp.provide(
    Logging.live,
    Database.live,
    BlobStorage.live,
    DocRepo.live,
    UserRepo.live
  )

The order of dependencies doesn't matter:

val res: ZIO[Any, Throwable, Unit] =
  myApp.provide(
    DocRepo.live,
    BlobStorage.live,
    Logging.live,
    Database.live,
    UserRepo.live
  )

If we miss some dependencies, it doesn't compile, and the compiler gives us the clue:

val app: ZIO[Any, Throwable, Unit] =
  myApp.provide(
    DocRepo.live,
    BlobStorage.live,
//    Logging.live,
    Database.live,
    UserRepo.live
  )

  ZLayer Wiring Error  

❯ missing Logging
❯     for DocRepo.live

❯ missing Logging
❯     for UserRepo.live

note

The ZIO#provide method, together with its variant ZIO#provideSome, is default and easier way of injecting dependencies to the environmental effect. We do not require creating the dependency graph manually, it will be automatically generated.

In contrast, the ZIO#provideLayer, and its variant ZIO#provideSomeLayer, is useful for low-level and custom cases.