Monorepo at Scale: How We Slashed Build Times by 75%

As Android teams grow, monorepos often become a bottleneck—leading to tangled modules, bloated dependency graphs, and slow build times. Optimizing developer efficiency at scale requires fundamental structural changes.

This talk is about how we maintain a high-performance Android monorepo for a team of 20+ developers. We will break down the architectural and tooling strategies required to keep compilation fast and developers productive, focusing on two core pillars:

• Module Structure: Leverage JVM module type/ Practice Dependency Inversion/ Custom IJ plugin to create new modules/ Optimizing incremental compilation by strictly enforcing module boundaries.
• Dependency Injection: Aggregation style of DI wiring/ DI's build tools comparison/ Moving away from KAPT and KSP toward Kotlin Compiler Plugins

This talk is designed for Android developers looking to optimize their projects. Whether you are heavily modularizing your codebase or have just started, you will find valuable insights here. For platform engineers, Android leads, or DI enthusiasts, you will leave with concrete, actionable takeaways to implement in your own projects!

Transcript

1. Monorepo at Scale How We Slashed Build Times by 75% Kevin Chiu Android Team Lead @BandLab
2. Outline 1 Module Structure The Skeleton 2 Dependency Injection The Bloodstream
4. Project Scale • Monorepo of 1,332 modules • 350 JVM modules (26%) • Android module for the rest • Local build time last 7 days (:app:assembleDebug) • Median 20s, Mean 48s (821 samples) : • P25-75 14-48s
5. Module Structure
6. JVM vs Android Module
7. JVM vs Android Module • Assemble a JVM module: 7 tasks • Assemble an Android module: 30 tasks plugins { id("org.jetbrains.kotlin.jvm") } plugins { id("com.android.library") }
8. JVM vs Android Module Partial JVM Delta 📉 Clean build (Conf guration) 66.35s 47.75s -28.03% Clean build (Execution) 286s 284s -0.7% Tasks count 34,461 28,210 -18.14% i All Android
9. JVM vs Android Module JVM Android :post i - :feed :user prof le
10. JVM vs Android Module JVM Android :post i - :feed :user :user prof le :chat
11. Dependency Inversion
12. Dependency Inversion :user prof le :band prof le :post i - i - - - :following feed :trending feed
13. Dependency Inversion :post interface PostApiService { @GET("posts/{id}") suspend fun getPost(@Path("id") postId: String) } : : class PostRepository( private val postApiService: PostApiService, ) { suspend fun getPost(postId: String) Post { return postApiService.getPost(postId) } } Post
14. Dependency Inversion :post interface PostApiService { @GET("posts/{id}") suspend fun getPost(@Path("id") postId: String) } : : class PostRepository( private val postApiService: PostApiService, private val tracker: Tracker, ) { suspend fun getPost(postId: String) Post { tracker.track("post_requested") return postApiService.getPost(postId) } } Post
15. Dependency Inversion :user prof le :band prof le :post - i i - - - :following feed :trending feed
16. Dependency Inversion :post interface PostApiService { @GET("posts/{id}") suspend fun getPost(@Path("id") postId: String) } : : class PostRepository( private val postApiService: PostApiService, ) { suspend fun getPost(postId: String) Post { return postApiService.getPost(postId) } }n Post
17. Dependency Inversion :post:api :post:impl interface PostApiService { @GET("posts/{id}") suspend fun getPost(@Path("id") postId: String) } : : : interface PostRepository { suspend fun getPost(postId: String) } Post class PostRepositoryImpl( private val postApiService: PostApiService, ) : PostRepository { override suspend fun getPost(postId: String) return postApiService.getPost(postId) } }n Post Post {
18. Dependency Inversion :post:api :post:impl interface PostApiService { @GET("posts/{id}") suspend fun getPost(@Path("id") postId: String) } : : : interface PostRepository { suspend fun getPost(postId: String) } Post class PostRepositoryImpl( private val postApiService: PostApiService, private val tracker: Tracker, ) : PostRepository { override suspend fun getPost(postId: String) tracker.track("post_requested") return postApiService.getPost(postId) } } Post Post {
19. Dependency Inversion :user prof le :band prof le :post:api :trending feed :following feed - i i - - - :post:impl
20. Module Creation Plugin
22. Module Creation Plugin • Limit module to :api, :impl, :screen and :ui • Apply convention plugins • Screen templates • Wire the module to top level :app - - github.com/bandlab/bandlab android intellij plugin - / / • https:
23. Module Creation Plugin :screen :impl Business logic Screen container Domain impl Shared VM impl :api :ui Domain model Abstraction Composable UI states
24. Module Creation Plugin :screen :impl Business logic Screen container Domain impl Shared VM impl :api :ui Domain model Abstraction JVM Composable UI states
25. Project Doctor
26. Project Doctor projectDoctor { errorOn(":api should be a JVM module") { projectWith { nameEndsWith(":api") } .thatHasPluginWith { id("com.android.library") } } errorOn(":api cannot depends on :impl") { projectWith { nameEndsWith(":api") } .thatHasDependencyWith { nameEndsWith(":impl") } } }n
27. Project Doctor • Gradle DSL to enforce the desired module structure • Isolated project compatible - github.com/bandlab/gradle project doctor - / / • https:
28. Dependency Injection
29. Graph Aggregation (a.k.a. Anvil)
30. Manual Wiring (Dagger) :user:api api :user:impl UserRepo UserRepoImpl : : : / / @Module abstract class UserRepoModule { @Binds abstract fun bindUserRepo(impl: UserRepoImpl) } UserRepo implementation :app AppGraph @Component( modules = [ tons of modules… UserRepoModule class, ], ) interface AppGraph
31. Aggregation (Anvil) :user:api UserRepo api :user:impl UserRepoImpl : : : @ContributesBinding(AppScope class) class UserRepoImpl : UserRepo : implementation @MergeComponent(AppScope interface AppGraph :app AppGraph class)
32. Aggregation (Anvil) :user:api UserRepo api :user:impl UserRepoImpl @ContributesBinding(AppScope class) class UserRepoImpl : UserRepo : : fun com_example_AppScope(contributed: UserRepoImpl) { return error("Never called") } : :app AppGraph @MergeComponent(AppScope interface AppGraph class) classpath scanning package hints : implementation
33. DI’s Build Tool
34. DI’s Build Tool • Dagger, Hilt: KAPT/ KSP • Kotlin inject: KSP • Koin: KSP/ Kotlin Compiler Plugin • Metro: Kotlin Compiler Plugin Note - : : • KAPT Kotlin Annotation Processing Tool • KSP Kotlin Symbol Processing
35. 🚇 Metro • Compile time DI framework • Kotlin Ergonomic, Coroutine support • Anvil style Aggregation - / / - • https: zacsweers.github.io/metro/
36. Link to the chart
37. Kotlin Compiler Plugin < KSP < KAPT Link to the chart
38. Build Pipeline *.kt :compileKotlin *.class
39. KSP Build Pipeline :kspKotlin *.kt :compileKotlin *.class
40. KAPT Build Pipeline *.kt :kaptGenerateStubsKotlin *.java :compileKotlin :kaptKotlin *.class
41. Dagger KAPT + KSP Build Pipeline :kaptGenerateStubsKotlin *.java *.java :compileJava :kaptKotlin *.class :kspKotlin *.kt :compileKotlin
42. BandLab’s DI Journey
43. Dagger android class MyActivity: AppCompatActivity() - 2017 Aug
44. Dagger android class MyActivity: AppCompatActivity() @Modulen interface MyActivityInjector { @ContributesAndroidInjector fun myActivity() MyActivity }n - : 2017 Aug
45. Dagger android class MyActivity: AppCompatActivity() @Modulen interface MyActivityInjector { @ContributesAndroidInjector(modules = [MyActivityModule fun myActivity() MyActivity class]) }n @Module object MyActivityModule { @Provides fun provideLifecycle(activity: MyActivity) Lifecycle = activity.lifecycle @Provides fun provideCoroutineScope(activity: MyActivity) CoroutineScope = activity.lifecycleScope } : : : : - : 2017 Aug
46. Dagger + Anvil (K1) @ContributesInjector class MyActivity: AppCompatActivity() 2023 Dec
47. @ContributesInjector class MyActivity: AppCompatActivity() We generate this with Anvil Compiler API @MergeSubcomponent interface MyActivitySubcomponent { @Subcomponent.Factory interface Factory { fun create() MyActivitySubcomponent } } : : 2023 Dec / / Dagger + Anvil (K1)
48. @ContributesInjector class MyActivity: AppCompatActivity() We generate this with Anvil Compiler API @MergeSubcomponent( modules = [DefaultActivityDependencies class] ) interface MyActivitySubcomponent { @Subcomponent.Factory interface Factory { fun create() MyActivitySubcomponent } @Module object DefaultActivityDependencies { @Provides fun provideLifecycle() Lifecycle @Provides fun provideCoroutineScope() CoroutineScope More default deps } } : : : : : : / / 2023 Dec / / Dagger + Anvil (K1)
49. @ContributesInjector class MyActivity: AppCompatActivity() We generate this with KSP @MergeSubcomponent( modules = [DefaultActivityDependencies ) interface MyActivitySubcomponent { class] @Subcomponent.Factory interface Factory { fun create() MyActivitySubcomponent } } : : : : 2024 Sep / / Dagger + Anvil-KSP (K2)
50. @ContributesInjector class MyActivity: AppCompatActivity() We generate this with KSP @GraphExtension( bindingContainers = [DefaultActivityDependencies ) interface MyActivityGraphExtension { class] @ContributesTo(AppScope class)n @GraphExtension.Factory interface Factory { fun create() MyActivityGraphExtension } } : : : : : : 2025 Jul / / Metro + KSP
51. Link to the chart
52. Metro + Metro Station @StationEntry class MyActivity: AppCompatActivity() 2026 Jun
53. Metro + Metro Station @StationEntry class MyActivity: AppCompatActivity() { We generate this with custom compiler plugin @GraphExtension( bindingContainers = [DefaultActivityDependencies ) interface FeatureExtension { class] @ContributesTo(AppScope class) @GraphExtension.Factory interface Factory { fun create() FeatureExtension } } } : : : : : / / 2026 Jun
54. Metro Station • Built with Metro Compiler Extension API • Generate code for Metro to process • One :compileKotlin task, no KSP, no KAPT github.com/bandlab/metro station - / / • https:
55. Metro Station @StationEntry class ExampleService : Service() { @Inject lateinit var exampleClass: ExampleClass override fun onCreate() { super.onCreate() exampleClass.doSomething() } }n
56. Metro Station @StationEntry class ExampleService : Service() { @Inject lateinit var exampleClass: ExampleClass override fun onCreate() { super.onCreate() exampleClass.doSomething() } Generated @GraphExtension interface FeatureExtension { @ContributesTo(AppScope class) @GraphExtension.Factory interface Factory { fun create() FeatureExtension } } : : : / / }n
57. Metro Station @StationEntry class ExampleService : Service() { @Inject lateinit var exampleClass: ExampleClass override fun onCreate() { val factory = this.resolveFactory<FeatureExtension.Factory>() factory.create().inject(this) super.onCreate() exampleClass.doSomething() } Generated @GraphExtension interface FeatureExtension { fun inject(service: ExampleService) @ContributesTo(AppScope class) @GraphExtension.Factory interface Factory { fun create() FeatureExtension } } : : : / / }n Note: resolveFactory casts the AppGraph via Context to the contributed factory type
58. Metro Station @StationEntry class ExampleBroadcastReceiver : BroadcastReceiver() { @Inject lateinit var exampleClass: ExampleClass override fun onReceive(context: Context, intent: Intent) { exampleClass.doSomething() } } Note: resolveFactory casts the AppGraph via Context to the contributed factory type
59. Metro Station @StationEntry class ExampleBroadcastReceiver : BroadcastReceiver() { @Inject lateinit var exampleClass: ExampleClass override fun onReceive(context: Context, intent: Intent) { val factory = context.resolveFactory<FeatureExtension.Factory>() factory.create().inject(this) exampleClass.doSomething() } } Note: resolveFactory casts the AppGraph via Context to the contributed factory type
60. Metro Station @StationEntry class ExampleWorker( context: Context, params: WorkerParameters, ) : CoroutineWorker(context, params) { @Inject lateinit var exampleClass: ExampleClass override suspend fun doWork() Result { exampleClass.doSomething() return Result.success() } } Note: resolveFactory casts the AppGraph via : Context to the contributed factory type
61. Metro Station @StationEntry class ExampleWorker( context: Context, params: WorkerParameters, ) : CoroutineWorker(context, params) { @Inject lateinit var exampleClass: ExampleClass override suspend fun doWork() Result { val factory = context.resolveFactory<FeatureExtension.Factory>() factory.create().inject(this) exampleClass.doSomething() return Result.success() } } Note: resolveFactory casts the AppGraph via : Context to the contributed factory type
62. Compiler Pipeline
63. FIR vs IR • FIR represents the structure of Kotlin code • IR represents the semantics of Kotlin code
64. FIR vs IR • FIR represents the structure of Kotlin code • IR represents the semantics of Kotlin code : class HelloDroidcon { fun greet() Boolean { println("Hello, Lisbon!") return true } }
65. :compileKotlin FIR Metro Extensions
66. :compileKotlin FIR Metro Extensions BandLab Extensions
67. :compileKotlin FIR Metro Extensions BandLab Extensions
68. :compileKotlin FIR Metro Compiler Extension API BandLab Extensions Metro Extensions
69. :compileKotlin IR Metro Extensions
70. :compileKotlin IR BandLab Extensions Metro Extensions -Xcompiler plugin order - - (Since Kotlin 2.3)
71. - Mutation Metro + KSP Metro + Metro Station Root Abi 54.79s 28.72s -47.57% Root Non abi 40.26s 17.14s -57.43% Delta 📉
72. - Mutation Dagger + Anvil KSP Metro + Metro Station Delta 📉 Root Abi 115.02s 28.72s -75.03% Root Non abi 77.70s 17.14s -77.94% 2025 Jun 2026 Jun
73. Key Takeaways • Convert Android modules to JVM • Practice dependency inversion • Enforce module structure that scales well • Avoid KAPT and KSP if you can
74. Obrigado! Contacts, Repos and Slides