Stop Building the Whole App

When an engineer is working on a single feature, they only care about changing code, running it, and validating fast.

At Qonto, our app has 1k+ modules and 300+ features, yet the default workflow still forces engineers to build and install more than they need, so they spend minutes waiting and clicking around to reach the right screen.

That’s why we changed our approach: instead of building the full app, we generate DevApps, small per-feature apps that build fast, open the right screen, and let engineers validate changes with the same confidence as the main app.

In this talk, you’ll learn how we:
- Designed an architecture that builds only one feature and its dependencies
- Solved key challenges (DI, navigation, login, data)
- Prevented DevApps from breaking as the codebase evolves
- Built tooling engineers love (skip login, auto-fill forms, mock services)
- Measured and increased adoption and performance

Come learn how to build mini apps and get your feedback loop back.

Transcript

1. Stop Building the Whole App Florian Mierzejewski
2. Qonto Making day-to-day banking easier for SMEs and freelancers • An all-in-one business finance platform combining a business account with invoicing, spend management, and accounting tools. • Built for entrepreneurs and SMEs across Europe simplifying everyday finances so they can focus on growing their business. • Supporting businesses from day one from company creation to daily payments and compliant e-invoicing.
3. Context Our team as of last week 60 + Android 60 + iOS 30 + Squads Split in 5 domains
4. Context Our codebase as of last week 1.4 k Modules 1.7 M LoC 350 PRs/week
5. The Problem
6. The Problem Everything’s going up ⬆ Features I A ⬆⬆ Modules ⬆⬆⬆ Code
7. The Problem What else is going up? ⬆ ⬆ Build Time ⬆ ⬆
8. The Problem Full Build (no local cache)
9. The Problem Incremental Build - Feature
10. The Problem Engineers are waiting • Syncing/Configuring the whole codebase • Building locally and fetching from the remote cache • Packaging and installing the whole app →
11. The Problem We are out of tricks Latest Hardware Decent Modularisation Remote Cache
12. Change of Strategy
13. Change of Strategy Always more features, not less AI is making things worse, faster
14. What do you do on the day to day as part of a feature squad?
15. Standard App App Feature Feature Feature Feature Dependencies Feature Feature Feature
16. Qonto Dependencies
17. DevApp Feature 1 Feature 2 Dependencies Feature 3
18. We just build and launch what we need
19. “As an Engineer I can generate quickly & easily a devapp for one or multiple Our feature modules. It just work Vision and I can always build & run it without any error. I have access to all the tools I need to test & build my feature.”
20. “As an Engineer I can generate quickly & easily a devapp for one or multiple feature modules. It just work and I can always build & run it without any error. I have access to all the tools I need to test & build my feature.” Our Vision
21. “As an Engineer I can generate quickly & easily a devapp for one or multiple feature modules. It just work and I can always build & run it without any error. I have access to all the tools I need to test & build my feature.” Our Vision
22. Figuring out the Plan
23. Figuring out the Plan Kaizen • Get motivated engineers in the same room regularly • Validate the direction • Plan the execution →
24. Figuring out the Plan Proof of Concept DevApp d r a h o o T Actual Feature Dependencies
25. Figuring out the Plan Proof of Concept DevApp d r a h o o T Simple Feature Dependencies
26. Figuring out the Plan Proof of Concept s s e c c u S DevApp Dummy Feature
27. Figuring out the Plan Proof of Concept DevApp Dummy Feature One Dependency ✓
28. Figuring out the Plan Proof of Concept DevApp Dummy Feature One Dependency DevApp Simple Feature Dependencies ✓ ✓
29. Figuring out the Plan Proof of Concept DevApp Dummy Feature One Dependency DevApp Simple Feature Dependencies DevApp Actual Feature Dependencies ✓ ✓ ✓
30. Figuring out the Plan Proof of Concept DevApp Dummy Feature One Dependency DevApp Simple Feature Dependencies DevApp Actual Feature Dependencies ✓ ✓ ✓
31. Figuring out the Plan Validating the Direction 2-3x Faster Build Times on Average
32. Figuring out the Plan Bottlenecks Identified Build System Dependency Injection Navigation
33. Figuring out the Plan Bottlenecks Identified Database Login
34. Execution
35. Build System
36. Execution Build System features/ItemsCatalog/DevApp/build.gradle plugins { id("android-app-module") } plugins { id("qonto-devapp-module") } android { namespace = "..." defaultConfig { applicationId = "..." versionCode = 1 versionName = "1.0" buildConfigField("...") } signingConfigs { ... } buildFeatures { ... } packaging { ... } } Before After
37. Execution Build System features/ItemsCatalog/DevApp/build.gradle dependencies { implementation(project(":features:ItemsCatalog")) devApp { features = [ ":features:ItemsCatalog", ] } // dependencies on the common module for ItemsCatalog implementation(project(":common:ItemsCatalog")) // Needed for AuthenticationHelper implementation(project(":common:User")) implementation(project(":common:Login")) implementation(project(":common:CommonBuildVariant")) // Needed for ItemCatalogInitializer implementation(project(":common:Startup")) // Needed for UserScopedCacheModule implementation(project(":common:Session")) // Needed for UnsupportedNavigationModule implementation(project(":common:Navigation")) // Daos for ItemsCatalog implementation(project(":common:Membership")) implementation(project(":common:Organization")) implementation(project(":common:Attachment")) implementation(project(":common:Money")) implementation(project(":common:Address")) // Navigation for ItemsCatalog implementation(project(":common:InternationalTransfer")) implementation(project(“:features:UnitSelector”)) // A lot more dependencies after that as well […] // Needed for ItemCatalogInitializer implementation(libs.androidxStartup) // Needed for fake interceptors implementation(libs.okhttp) // Needed for Dagger implementation(libs.daggerAndroidSupport) // Needed for DB implementation(libs.room) // A lot more dependencies after that as well […] } Before After
38. Execution Single Feature DevApp features/ItemsCatalog/DevApp/build.gradle plugins { id("qonto-devapp-module") } devApp { features = [ ":features:ItemsCatalog", ] }
39. Execution Gradle Plugin QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { val devApp: QontoDevAppExtension = extensions.create<QontoDevAppExtension>() val declaredFeaturePaths = devApp.features.get().toList() validateFeatures(declaredFeaturePaths) declaredFeaturePaths.forEach { implementation(project(it)) } dependencies.add("debugCompileOnly", runtimeJars()) pluginManager.apply("android-app-module") configureAndroidBlock() configureCommonDependencies() // etc... } }
40. Execution Gradle Plugin QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { val devApp: QontoDevAppExtension = extensions.create<QontoDevAppExtension>() val declaredFeaturePaths = devApp.features.get().toList() validateFeatures(declaredFeaturePaths) declaredFeaturePaths.forEach { implementation(project(it)) } dependencies.add("debugCompileOnly", runtimeJars()) pluginManager.apply("android-app-module") configureAndroidBlock() configureCommonDependencies() // etc... } }
41. Execution Gradle Plugin QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { val devApp: QontoDevAppExtension = extensions.create<QontoDevAppExtension>() val declaredFeaturePaths = devApp.features.get().toList() validateFeatures(declaredFeaturePaths) declaredFeaturePaths.forEach { implementation(project(it)) } dependencies.add("debugCompileOnly", runtimeJars()) pluginManager.apply("android-app-module") configureAndroidBlock() configureCommonDependencies() // etc... } }
42. Execution Gradle Plugin QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { val devApp: QontoDevAppExtension = extensions.create<QontoDevAppExtension>() val declaredFeaturePaths = devApp.features.get().toList() validateFeatures(declaredFeaturePaths) declaredFeaturePaths.forEach { implementation(project(it)) } dependencies.add("debugCompileOnly", runtimeJars()) pluginManager.apply("android-app-module") configureAndroidBlock() configureCommonDependencies() // etc... } }
43. Execution Gradle Plugin QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { val devApp: QontoDevAppExtension = extensions.create<QontoDevAppExtension>() val declaredFeaturePaths = devApp.features.get().toList() validateFeatures(declaredFeaturePaths) declaredFeaturePaths.forEach { implementation(project(it)) } dependencies.add("debugCompileOnly", runtimeJars()) pluginManager.apply("android-app-module") configureAndroidBlock() configureCommonDependencies() // etc... } ✨ l ea ✨ } H o t u A
44. Execution Gradle Plugin QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { val devApp: QontoDevAppExtension = extensions.create<QontoDevAppExtension>() val declaredFeaturePaths = devApp.features.get().toList() validateFeatures(declaredFeaturePaths) declaredFeaturePaths.forEach { implementation(project(it)) } dependencies.add("debugCompileOnly", runtimeJars()) pluginManager.apply("android-app-module") configureAndroidBlock() configureCommonDependencies() // etc... } }
45. Dependency Injection
46. Execution Dependency Injection AppComponent.kt @Module( includes = [ // Features Feature1Module::class, Feature2Module::class, … // Common Common1Module::class, Common2Module::class, … // 600+ more lines ], ) interface AppComponent @Module( includes = [ // Features Feature1Module::class, Feature2Module::class, … // Common Common1Module::class, Common2Module::class, … // X+ more lines ], ) interface DevAppComponent Main App Dev App
47. One Side Quest...
48. And 20% Build Times Improvement Later…
49. Execution Metro features/ItemsCatalog/DevApp/AppGraph.kt @SingleIn(AppScope::class) @DependencyGraph(scope = AppScope::class) interface ItemsCatalogGraph : AppGraphContract { @DependencyGraph.Factory fun interface Factory { fun create(@Provides application: Application): ItemsCatalogAppGraph } } @BindingContainer @ContributesTo(AppScope::class) object ItemsCatalogDevAppModule
50. Execution Metro features/ItemsCatalog/DevApp/AppGraph.kt @SingleIn(AppScope::class) @DependencyGraph(scope = AppScope::class) interface ItemsCatalogGraph : AppGraphContract { @DependencyGraph.Factory fun interface Factory { fun create(@Provides application: Application): ItemsCatalogAppGraph } } @BindingContainer @ContributesTo(AppScope::class) object ItemsCatalogDevAppModule
51. Navigation
52. Execution Navigation DevApp Feature 1 Feature 2 Dependencies Feature 3 Feature 4
53. Execution Navigation features/ItemsCatalog/DevApp/ItemsCatalogDevAppModule.kt @BindingContainer @ContributesTo(AppScope::class) object ItemsCatalogDevAppModule { @Provides fun provideFeature4Navigation(): Feature4Navigation = object : Feature4Navigation { override fun createContract() { return error("Feature4 is not included in this DevApp”) } } // Repeat X times for each navigation interface not provided }
54. Execution Multi Features DevApp QontoDevAppPlugin.kt open class QontoDevAppExtension @Inject constructor(objects: ObjectFactory) { val features: SetProperty<String> = objects.setProperty<String>().convention() } class QontoDevAppPlugin : Plugin<Project> { override fun apply(project: Project) = project.run { // … val generateTask = DevAppSourceGenerationTask.register(project = this) } }
55. Execution Navigation DevAppSourceGenerationTask.kt scanner.scan(jarFiles).use { scan -> val devAppInfo = infoExtractor.extractInfo(scan) unsupportedNavigationDiFileGenerator.generateUnsupportedNavigationDi() }
56. Execution Navigation DevAppSourceGenerationTask.kt scanner.scan(jarFiles).use { scan -> val devAppInfo = infoExtractor.extractInfo(scan) unsupportedNavigationDiFileGenerator.generateUnsupportedNavigationDi() } ✨ H o t u A ✨ l a e
57. Get all dependencies of the feature(s) recursively For each dependency, check which navigation interfaces are included For each navigation interface included, check which ones have no implementations available For each implementation not available, provide a fake one
58. Database
59. Execution Database DevApp Feature 1 Feature 2 Database Feature 3
60. Execution Common A Database Feature 1 Database Common B … Common Y Common Z
61. Execution Common A Database Feature 1 Database Common … Common Common Z
62. Execution Database QontoDabase.kt / FeatureDatabase.kt @Database( entities = [ LocalCard::class, LocalBankAccount::class, // +100 more ], ) @TypeConverters( value = [ // 30+ converters ], ) abstract class QontoDatabase : RoomDatabase() { @Database( entities = [ LocalEntityA1::class, LocalEntityA2::class, LocalEntityZ1::class, ], ) @TypeConverters( value = [ DateRoomConverter::class, DoubleListRoomConverter::class, IntListRoomConverter::class, ], ) abstract class FeatureDatabase : RoomDatabase() { abstract fun daoA1(): DaoA1 abstract fun daoA2(): DaoA2 abstract fun daoZ1(): DaoZ1 } abstract fun cardDao(): CardDao abstract fun bankAccountDao(): BankAccountDao // + 100 more } Before After
63. Execution Database DevAppSourceGenerationTask.kt scanner.scan(jarFiles).use { scan -> val devAppInfo = infoExtractor.extractInfo(scan) // Navigation generation before… databaseFileGenerator.generateDatabase(outputDirectory, devAppInfo) databaseDiFileGenerator.generateDatabaseDi(outputDirectory, devAppInfo) } ✨ H o t u A ✨ l ea
64. Get all dependencies of the feature(s) recursively For each dependency, check which Dao are included For each Dao included, add it to our new generated DB
65. Login
66. Execution Login Build 🔄
67. Execution Login Build 🔄
68. Execution Login Build 🔄
69. Execution Demo 00:17 VS 00:05
70. Adoption & Improvements
71. Adoption Communication • Show rather than tell • Incentivize without forcing • Find champions that are excited about what you built • Invest time for improvements that encourage a switch →
72. Improvements Dev Shortcuts
73. Improvements UI & UX
74. Improvements Scenarios EsimDevAppScenario.kt DevApp/Esim/build.gradle data object{ HomeScreenUltraPerk : EsimDevAppScenario { plugins override val title = "Home screen - Included with Ultra” id("qonto-devapp-module") override val setup = mockSetup { } services { searchEsimDataPackagesService { searchEsimDataPackages { fakeSuccess { ultraSearchResponse() } } devApp { { fakeSuccess { ultraGetResponse() } } features getEsimDataPackageById = [ } } ":features:Esim", }] override fun start(context: Context) { mockServices = [ context.startActivity(EsimHomePageActivity.createIntent(context)) "SearchEsimDataPackagesApiService", } ] } }
75. Conclusion
76. Results 60% of all local builds are devapps builds 70% of squads using devapps weekly 98% of engineers have at least tried dev-apps once
77. Results When using dev-apps… 2.4min saved per build on average 30s p50 build time
78. What’s Next? Improve Performance Better IDE Integration Prevent Breaking DevApps
79. Food for thoughts • Always challenge the status-quo • Try hard but don’t hesitate to go simpler first • How should your team be working in 6 months? 1 year? →
80. Thank You We’re hiring! Florian Mierzejewski