Database.kt

// Database interface
interface Database {
  fun getUserById(id: String): Observable<User>
  fun getAllUsers(): Observable<List<User>>
  fun getUserChangeEvents(): Observable<ChangeEvent<User>>
}

// Describes what happened to what item
data class ChangeEvent<T>(val eventType: ChangeEventType, val t: T)

// Describes what happened to an item
enum class ChangeEventType { ADDED, REMOVED, CHANGED, MOVED }

// Concrete Impl
class DatabaseImpl : Database {
  // Impl...
}

// Database module (optional, you could just add this to AppModule)
@Module
class DatabaseModule {
  @Provides
  @Singleton
  fun provideDatabase(): DatabaseImpl()
}

// Application level module
@Module(includes=arrayOf(DatabaseModule::class))
class AppModule {
  /* ... */
}

// Application level dagger component
@Singleton
@Component(modules=arrayOf(AppModule::class)) 
class AppComponent {
  fun database(): Database
}

class SomeFragment : Fragment {
  @Inject
  lateinit var database: Database
  
  fun onCreate(/*...*/) {
    database.getAllUsers().subscribe {
      // Do something with your users
    }
  }
}