Micro Modular Architecture with Bazel Ryo Aoyama iOSDC Japan 2020

Ryo Aoyama Mercari Software Engineer Architect iOS Tech Lead @ra1028 @ra1028fe5

Agenda • Drawbacks of Monolith • Micro Modular Architecture • What is Bazel? • The power of Bazel with micro modules

Drawbacks of Monolith

Disclaimer: Monolithic application is NOT an absolute bad thing.

FeatureA FeatureB FeatureC FeatureD FeatureZ … Networking DesignSystem Analytics Database Architecture etc… Entity Monolithic Application All features are concentrated in one or a few modules.

Monolithic Application Build time increases as the product grows: Build time can increase linearly or exponentially.

Coupling complexity between features: Monolithic Application Coupling complexity between features gradually increases and there is no going back.

Barriers to evolutionary potential: Monolithic Application The introduction of new technology affects the entire app and is pretty costly. And also, it does not have the ability to evolve progressively.

So we need to tackle more advanced architecture.

Micro Modular Architecture

Micro Modular Architecture Our coined term showing a highly modularized application structure. =

The idea is to split our application into hundreds of smaller modules.

Concepts One module has only one screen (aka view controller). All the layers needed for the screen are also in the same module. • Module by feature

Concepts The screens do not depend on each other and work independently. • Independent between features

Concepts Create one module for just one functionality. Larger functionalities consist of a composition of these smaller modules. • Fine-grained shared functionality modules

Monolith Application Feature Feature Feature Feature Feature Common Common Common Common Concepts

Looks great, but will the modules depend on each other complexly?

Modularization Group Applications Features Libraries Domain Grouping modules to give order to the way they depend on each other.

Application Feature Feature Feature Feature Feature Common Common Common Common Modularization Group The modules in this group contain an App Delegate, and that's it. It might also contain DI to feature modules. Applications

Application Feature Feature Feature Feature Feature Common Common Common Common Modularization Group Features Each vertical feature module is independent and can only depend on modules in the Domain and Libraries groups.

Application Feature Feature Feature Feature Feature Common Common Common Common Modularization Group Domain (Library) Contains modules of functionality shared by several feature modules. It also allows modules to have service-specific information.

Application Feature Feature Feature Feature Feature Common Common Common Common Modularization Group Libraries Contains modules of generalized functionality. It cannot have service-specific information.

Modularization Group Applications Features Libraries Domain Grouping modules and make them unidirectional dependencies. Circular dependency Complexity

Next, let's stop into Bazel for more advanced micro modular architecture.

What is Bazel?

Bazel • Fast + Reliable + Scalable + Extensible • Multiple language support • Starlark language for build scripts • Remote Caching • Open Source https://github.com/bazelbuild/bazel

Example of Benefits My product completes clean build, unit test, and land to testers in around 1m 30s in the shortest. But it's still too early stage for a sample.

Example of Benefits Pinterest https://medium.com/pinterest-engineering/developing-fast-reliable-ios-builds-at-pinterest-part-one-cb1810407b92 Local Build: CI Build: Beta Distribution: Build Success Rate: 4m 38s -> 3m 38s, 21% improved 10m 24s -> 7m 34s, 27% improved 14m 32s -> 7m 52s, 45% improved 80% -> 97%-100%

Tokopedia Example of Benefits https://medium.com/tokopedia-engineering/how-tokopedia-achieved-1000-faster-ios-build-time-7664b2d8ae5 Local Clean Build: 56m 31s -> 4m 56s, 91% improved

LINE Example of Benefits https://engineering.linecorp.com/en/blog/improving-build-performance-line-ios-bazel Minimum: Maximum: Average: 28m 24s -> 4m 24s, 85% improved 35m 25s -> 26m 32s, 25% improved 30m 58s -> 14m 32s, 53% improved

Example of Benefits Amazingly, these improvements have been brought to each individual developer by remote caching.

Build Scripts WORKSPACE Defines external dependencies. BUILD Contains build scripts for 1~N modules. .bazelrc Configuration file for project-specific options. xxx.bzl Defines utility extension for build scripts which will be loaded from the BUILD.

Bazel for iOS rules_apple rules_swift apple_support https://github.com/bazelbuild/rules_apple https://github.com/bazelbuild/rules_swift https://github.com/bazelbuild/apple_support

Bazel for iOS for more complicated modules rules_apple_line https://github.com/line/rules_apple_line

How to build load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive") http_archive( name = "build_bazel_rules_apple", sha256 = “….", url = “https://github.com/bazelbuild/rules_apple/releases/download/x.x.x/rules_apple.x.x.x.tar.gz”, ) load( "@build_bazel_rules_apple//apple:repositories.bzl", "apple_rules_dependencies", ) apple_rules_dependencies() load( "@build_bazel_rules_swift//swift:repositories.bzl", "swift_rules_dependencies", ) swift_rules_dependencies() load( "@build_bazel_apple_support//lib:repositories.bzl", "apple_support_dependencies", ) apple_support_dependencies() WORKSPACE

Library How to build load("@build_bazel_rules_swift//swift:swift.bzl", "swift_library") swift_library( name = "Sample", srcs = glob(["Sources/**/*.swift"]), data = glob(["Resources/**/*"]), visibility = ["//visibility:public"], ) BUILD

Test How to build load("@build_bazel_rules_swift//swift:swift.bzl", "swift_library") load("@build_bazel_rules_apple//apple:ios.bzl", "ios_unit_test") swift_library( name = "TestLib", srcs = glob(["Tests/**/*.swift"]), ) ios_unit_test( name = "Test", deps = [":TestLib"], ) BUILD

Application How to build load("@build_bazel_rules_apple//apple:ios.bzl", "ios_application") load("@build_bazel_rules_swift//swift:swift.bzl", "swift_library") swift_library( name = "AppLib", srcs = glob(["App/**/*.swift"]), ) ios_application( name = "App", app_icons = glob(["Assets.xcassets/AppIcon.appiconset/**"]), bundle_id = "com.example.app", families = [ "iphone", "ipad", ], infoplists = ["Info.plist"], launch_storyboard = "Launch.storyboard", minimum_os_version = "14.0", deps = [":AppLib"], ) BUILD

How to build $ bazel build //… Build all targets $ bazel build //package-name:target-name Build specified target

Carthage CocoaPods • pinterest/PodToBUILD • bazel-ios/cocoapods-bazel • with build scripts: swift_library, objc_library • with build scripts: apple_dynamic_framework_import Our Choice Third-party Library

Bazel + Xcode BUILD xcodeproj

Tulsi by Google https://github.com/bazelbuild/tulsi XCHammer by Pinterest https://github.com/pinterest/xchammer Bazel + Xcode

index-import by Lyft https://github.com/lyft/index-import Bazel + Xcode Replaces the paths in the index file created by Bazel so that it can be used for Xcode.

Downside • High adoption cost • High learning cost • iOS is not officially supported • Some Xcode features have been disabled

Downside Wrong setting will rather increase the build time.

Alternatives Buck by Facebook https://github.com/facebook/buck Gradle https://github.com/gradle/gradle

The power of Bazel with micro modules

Aggressive Caching Once a module is built, it won't be rebuilt until the code in that module is changed.

Aggressive Caching Caches are created per module. Micro modular architecture maximizes cache efficiency. =

Test Results Caching Bazel will not run already succeeded tests.

Test Results Caching That’s can be ignored by bazel test //… --nocache_test_results

Remote Caching Once it's built somewhere, all developers don't have to rebuild it. Build once, reuse everywhere:

Remote Caching Remote Cache Client CI Client Client Cache Cache Push

Remote Caching Caches can be minimized by bazel build //… ——remote_download_minimal

Remote Caching Now, all we need is ample network bandwidth instead of Mac Pro.

Static Linking Too many dynamic frameworks can have a negative impact on the launch speed of your app.

Static Linking Bazel compiles all modules into static libraries, so micro modules has no negative impacts.

Dependency Diagrams Bazel can track all dependency graphs and parse them with the bazel query command. It can be output as a graphical diagram with Graphviz.

Dependency Diagrams https://github.com/kastiglione/bazel-xcode-demo-swift-driver

Conclusion

• Micro Modular Architecture maximizes the synergy with Bazel, and vice versa. Conclusion • Build speed has room for improvement in software architecture as well as hardware performance. • Bazel will dramatically improve our developer experience.

References • Bazel Document • Faster Together: Uber Engineering’s iOS Monorepo • How Tokopedia Achieved 1000% Faster iOS Build Time • Improving Build Performance of LINE for iOS with Bazel • Developing fast & reliable iOS builds at Pinterest (Part one) • kastiglione/bazel-xcode-demo-swift-driver https://eng.uber.com/ios-monorepo https://engineering.linecorp.com/en/blog/improving-build-performance-line-ios-bazel https://medium.com/tokopedia-engineering/how-tokopedia-achieved-1000-faster-ios-build-time-7664b2d8ae5 https://medium.com/pinterest-engineering/developing-fast-reliable-ios-builds-at-pinterest-part-one-cb1810407b92 https://bazel.build https://github.com/kastiglione/bazel-xcode-demo-swift-driver

Thank you Ryo Aoyama iOSDC Japan 2020