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ڊେͳΞϓϦ։ൃΛࢧ͑Δϑϥά؅ཧज़
 Flag-based feature management for a large scale application Abstract in three lines: 1. Branches are hard to manage in a large application 2. Enables/disables features in a property file 3. Implemented as a Gradle plugin Slides: in English
 Talk: in Japanese

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Flag-based feature management for a large scale application Munetoshi Ishikawa

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What I’d Like to Say For a large scale application ‣ Hard to manage lots of feature branches ‣ Hard to conduct “large” refactoring

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What I’d Like to Say For a large scale application ‣ Hard to manage lots of feature branches ‣ Hard to conduct “large” refactoring Go with feature flags, no more feature branches

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Topics ‣ Pain of feature branches ‣ Overview of feature flags ‣ Flag implementation ‣ Limitations and considerations

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Topics ‣ Pain of feature branches ‣ Overview of feature flags ‣ Flag implementation ‣ Limitations and considerations

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Feature Branch Example: git-flow, GitHub flow main branch (trunk, master, develop…)

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Feature Branch Example: git-flow, GitHub flow feature branch main branch (trunk, master, develop…)

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Feature Branch Example: git-flow, GitHub flow feature branch pull request main branch (trunk, master, develop…)

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Feature Branch Example: git-flow, GitHub flow feature branch pull request main branch (trunk, master, develop…)

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Case of Large Scale Application Lots of branches for a large scale application main branch feature branch

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What’s the Problem? Conflicts, conflicts, conflicts… - Commonly used code updated, like an API/library - Project wide refactoring, like package renaming feature development main branch …

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What’s the Problem? Conflicts, conflicts, conflicts… - Commonly used code updated, like an API/library - Project wide refactoring, like package renaming feature development main branch … refactoring

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What’s the Problem? Conflicts, conflicts, conflicts… - Commonly used code updated, like an API/library - Project wide refactoring, like package renaming feature development main branch … refactoring Conflict!

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How to Resolve Conflicts 1/2 Merge refactoring first feature development main branch …

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How to Resolve Conflicts 1/2 Merge refactoring first feature development main branch … refactoring

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How to Resolve Conflicts 1/2 Merge refactoring first feature development main branch … refactoring resolve

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How to Resolve Conflicts 1/2 Merge refactoring first Many branches are required to resolve conflicts feature development main branch … refactoring resolve

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How to Resolve Conflicts 2/2 Merge feature development first feature development main branch …

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How to Resolve Conflicts 2/2 Merge feature development first feature development main branch … refactoring

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How to Resolve Conflicts 2/2 Merge feature development first feature development main branch … refactoring resolve

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How to Resolve Conflicts 2/2 Merge feature development first feature development main branch … refactoring resolve 1. The conflict becomes large

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How to Resolve Conflicts 2/2 Merge feature development first feature development main branch … refactoring 1. The conflict becomes large 2. No chance to resolve with “all” branches resolve

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How to Resolve Conflicts 2/2 Merge feature development first feature development main branch … refactoring 1. The conflict becomes large 2. No chance to resolve with “all” branches resolve Conflict!

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How to Minimize the Conflicts Apply each “halfway” update to each other - Merge before finishing the implementation - Merge before fixing the release plan

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How to Minimize the Conflicts Apply each “halfway” update to each other - Merge before finishing the implementation - Merge before fixing the release plan Guard a “halfway” feature by a boolean flag

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Topics ‣ Pain of feature branches ‣ Overview of feature flags ‣ Flag implementation ‣ Limitations and considerations

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Feature Flag Concept ‣ Merge any pull request to the main branch main branch pull request

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Feature Flag Concept ‣ Merge any pull request to the main branch ‣ Enable/disable a feature with a property file main branch pull request flag property file FEATURE_STICKER=ENABLED FEATURE_THEME=DISABLED

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Feature Flag Usage Example Case1: Replace presentation logic presenter = if (BuildConfig.FEATURE_STICKER) NewPresenter() else OldPresenter()


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Feature Flag Usage Example Case1: Replace presentation logic presenter = if (BuildConfig.FEATURE_STICKER) NewPresenter() else OldPresenter()
 Case2: Change view visibility button.isVisible = BuildConfig.FEATURE_THEME

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What a Feature Flag Realizes ‣ Minimizes conflicts ‣ Separates release schedule from the development progress ‣ Disables features if critical issues found on the release branch ‣ Shares the halfway features to other members

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What a Feature Flag Realizes ‣ Minimizes conflicts ‣ Separates release schedule from the development progress ‣ Disables features if critical issues found on the release branch ‣ Shares the halfway features to other members

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Conflict Minimization Apply a project wide update to halfway features main branch feature PR

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Conflict Minimization Apply a project wide update to halfway features main branch feature PR refactoring PR

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Conflict Minimization Apply a project wide update to halfway features main branch feature PR refactoring PR Conflict

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Conflict Minimization Apply a project wide update to halfway features - Forget about ongoing refactoring once a pull request is merged main branch feature PR refactoring PR Conflict saved PRs

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What a Feature Flag Realizes ‣ Minimizes conflicts ‣ Separates release schedule from the development progress ‣ Disables features if critical issues found on the release branch ‣ Shares the halfway features to other members

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Release schedule Feature PRs can be merged before creating target release branch main branch release branch 1.0 2.0

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Release schedule Feature PRs can be merged before creating target release branch main branch release branch 1.0 2.0 feature “STICKER” PRs (for 2.0)

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Release schedule Feature PRs can be merged before creating target release branch main branch release branch 1.0 2.0 feature “STICKER” PRs (for 2.0) STICKER=DISABLED STICKER=ENABLED

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Release schedule Feature PRs can be merged before creating target release branch main branch release branch 1.0 2.0 feature “STICKER” PRs (for 2.0) STICKER=DISABLED STICKER=ENABLED

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What a Feature Flag Realizes ‣ Minimizes conflicts ‣ Separates release schedule from the development progress ‣ Disables features if critical issues found on the release branch ‣ Shares the halfway features to other members

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Feature Disabling 1/2 Hard to revert after resolving conflict main branch release branch feature branch

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Feature Disabling 1/2 Hard to revert after resolving conflict main branch release branch problem found feature branch

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Feature Disabling 1/2 Hard to revert after resolving conflict main branch release branch refactoring PR problem found feature branch

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Feature Disabling 2/2 Just turn off problematic features problem found main branch release branch feature PR refactoring PR

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Feature Disabling 2/2 Just turn off problematic features STICKER=ENABLED DISABLED → problem found main branch release branch feature PR refactoring PR

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What a Feature Flag Realizes ‣ Minimizes conflicts ‣ Separates release schedule from the development progress ‣ Disables features if critical issues found on the release branch ‣ Shares the halfway features to other members

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Halfway Feature Sharing ‣ Code is visible for other developers - For a related or dependent feature development ‣ A feature is deployable for a demo and testing - For project owners, testers, and designers

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Topics ‣ Pain of feature branches ‣ Overview of feature flag ‣ Flag implementation ‣ Limitations and considerations

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Implementation Options ‣ Gradle plugin and BuildConfig ‣ Build variants ‣ Annotation processing ‣ Reflection

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Implementation Options ‣ Gradle plugin and BuildConfig ‣ Build variants ‣ Annotation processing ‣ Reflection

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Key Idea of Implementation 1. Create a Java property file 2. Write the property content to BuildConfig FEATURE_STICKER=true
 FEATURE_THEME=false feature_flag.properties

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Key Idea of Implementation 1. Create a Java property file 2. Write the property content to BuildConfig Properties()... .forEach { (key, value) -> variant .buildConfigField("boolean", "$key", "$value") } FeatureFlagPlugin.kt (Gradle plugin)

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Key Idea of Implementation 1. Create a Java property file 2. Write the property content to BuildConfig public static final boolean FEATURE_STICKER = true; public static final boolean FEATURE_THEME = false; BuildConfig.java (output)

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Requirements ‣ IDE warnings workaround ‣ Flag control by build variants ‣ Flag control by release version ‣ Runtime flag modification

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Requirements ‣ IDE warnings workaround ‣ Flag control by build variants ‣ Flag control by release version ‣ Runtime flag modification

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IDE Warnings Warning for a conditional branch with a boolean constant if (BuildConfig.FEATURE_STICKER) {

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IDE Warning Workaround Call `Boolean.valueOf(...)` for a debug build 
 val valueString = "Boolean.valueOf($value)" FeatureFlagPlugin.kt (Gradle plugin)

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IDE Warning Workaround Call `Boolean.valueOf(...)` for a debug build (Use a literal for release to help ProGuard) val isRelease = variant.buildType.name == "release"
 val valueString = if (isRelease) "$value" else "Boolean.valueOf($value)" FeatureFlagPlugin.kt (Gradle plugin)

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Requirements ‣ IDE warnings workaround ‣ Flag control by build variants ‣ Flag control by release version ‣ Runtime flag modification

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Flag Control by Build Variants Enable with debug while disable with release # Enabled only for debug FEATURE_STICKER=debug # Enabled for both of debug and release FEATURE_THEME=release feature_flag.properties

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Implementation of Control by Variants 1/2 const val DEBUG_NAME = "debug" const val RELEASE_NAME = "release" val BUILD_TYPE_TO_FLAG_LIST_MAP = mapOf( DEBUG_NAME to arrayOf(DEBUG_NAME, RELE...), RELEASE_NAME to arrayOf(RELEASE_NAME) ) FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Control by Variants 2/2 val buildTypeName = variant.buildType.name FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Control by Variants 2/2 val buildTypeName = variant.buildType.name val availableFlagValues = BUILD_TYPE_TO_FLAG_LIST_MAP[buildTypeName] .orEmpty() FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Control by Variants 2/2 val buildTypeName = variant.buildType.name val availableFlagValues = BUILD_TYPE_TO_FLAG_LIST_MAP[buildTypeName] .orEmpty() val isEnabled = availableFlagValues.contains(flagValue) // WHERE, flagValue is "debug" or "release" FeatureFlagPlugin.kt (Gradle plugin)

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Requirements ‣ IDE warnings workaround ‣ Flag control by build variants ‣ Flag control by release version ‣ Runtime flag modification

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Flag Control by Release Version 1/2 May forget to enable before creating the target release branch main branch release branch 1.0 2.0 feature “STICKER” PRs (for 2.0) STICKER=debug STICKER=release

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Flag Control by Release Version 1/2 May forget to enable before creating the target release branch main branch release branch 1.0 2.0 feature “STICKER” PRs (for 2.0) STICKER=debug STICKER=release

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Flag Control by Release Version 2/2 Reserve target release version as soon as release schedule fixed main branch release branch 1.0 2.0 feature “STICKER” PRs (for 2.0) STICKER=debug,release:since_2.0

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Implementation of Control by Version val tokenizedFlag = flag.split(":since_")
 ... FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Control by Version val tokenizedFlag = flag.split(":since_")
 ... val sinceVersion = VersionNumber .parse(tokenizedFlag.getOrNull(1).orEmpty()) FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Control by Version val tokenizedFlag = flag.split(":since_")
 ... val sinceVersion = VersionNumber .parse(tokenizedFlag.getOrNull(1).orEmpty()) availableFlagValues.contains(buildTypeName) && sinceVersion <= project.applicationVersion FeatureFlagPlugin.kt (Gradle plugin)

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Bonus Points for Control by Version Easy to know when we can clean up the flag STICKER=...:since_2.0 main branch 2.0

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Requirements ‣ IDE warnings workaround ‣ Flag control by build variants ‣ Flag control by release version ‣ Runtime flag modification

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Runtime Flag Modification Make flags overridable in a debug menu for - Demoing - Debugging - Side-by-side testing

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Implementation of Flag Modification 1/2 `buildConfigField` outputs `static final` (I don’t like Java’s reflection. Especially, sun.misc.Unsafe) variant.buildConfigField( "boolean", "$key_DUMMY", "$value " ) FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Flag Modification 1/2 `buildConfigField` outputs `static final` (I don’t like Java’s reflection. Especially, sun.misc.Unsafe) variant.buildConfigField( "boolean", "$key_DUMMY", "$value; " ) FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Flag Modification 1/2 `buildConfigField` outputs `static final` (I don’t like Java’s reflection. Especially, sun.misc.Unsafe) variant.buildConfigField( "boolean", "$key_DUMMY", "$value; public static boolean $key = $value" ) FeatureFlagPlugin.kt (Gradle plugin)

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Implementation of Flag Modification 1/2 `buildConfigField` outputs `static final` (I don’t like Java’s reflection. Especially, sun.misc.Unsafe) public static final boolean FEATURE_STICKER_DUMMY = true; public static boolean FEATURE_STICKER = true; BuildConfig.java (output)

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Implementation of Flag Modification 2/2 Make overridable flag accessible without reflection - Create a flag list with supplier/consumer pair

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Implementation of Flag Modification 2/2 Make overridable flag accessible without reflection - Create a flag list with supplier/consumer pair val codeBuilder = StringJoiner("\n", "new HashMap<>(){{", "}}") flags.forEach { key -> codeBuilder .add("""put("$key", new Pair<>(() -> $key, (b) -> $key = b));""") } variant.buildConfigField( "Map>", "FEATURE_FLAG_ACCESSOR_MAP", codeBuilder.toString() ) FeatureFlagPlugin.kt (Gradle plugin, pseudocode)

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Implementation of Flag Modification 2/2 Make overridable flag accessible without reflection - Create a flag list with supplier/consumer pair public static final Map<...> FEATURE_FLAG_ACCESSOR_MAP = new HashMap<>() {{ put( "FEATURE_STICKER", ) ... BuildConfig.java (output, pseudocode)

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Implementation of Flag Modification 2/2 Make overridable flag accessible without reflection - Create a flag list with supplier/consumer pair public static final Map<...> FEATURE_FLAG_ACCESSOR_MAP = new HashMap<>() {{ put( "FEATURE_STICKER", new Pair<>( () -> FEATURE_STICKER, (b) -> FEATURE_STICKER = b) ) ... BuildConfig.java (output, pseudocode)

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Topics ‣ Pain of feature branches ‣ Overview of feature flag ‣ Flag implementation ‣ Limitations and considerations

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Appropriate Cases of Feature Flags ‣ Easy to apply a feature flag - Replace a class, layout file, or value ‣ Hard to apply a feature flag - Update API, library, or SDK version ‣ Enough to use a branch - Make a prototype or trial implementation

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Considerations for Pull Requests ‣ Keep each pull request small ‣ Think about the order/structure of pull requests eg., create a skeleton class → implement the logic

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Considerations for Release Testing ‣ Regression tests required for halfway feature ‣ Hard to guarantee the flag completeness - Might be revealed partially

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Summary ‣ Pain of feature branches ‣ Overview of feature flag ‣ Flag implementation ‣ Limitations and considerations

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Summary ‣ Pain of feature branches ‣ Overview of feature flag ‣ Flag implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Overview of feature flag ‣ Flag implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Overview of feature flag ‣ Flag implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Good for release process / information sharing ‣ Flag implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Good for release process / information sharing ‣ Flag implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Good for release process / information sharing ‣ Extendible Gradle plugin implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Good for release process / information sharing ‣ Extendible Gradle plugin implementation ‣ Limitations and considerations

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Summary ‣ Unmanageable conflicts ‣ Good for release process / information sharing ‣ Extendible Gradle plugin implementation ‣ Feature flags are not silver bullets

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References ‣ GitLab flow - https://docs.gitlab.com/ee/workflow/gitlab_flow.html ‣ Trunk based development - https://trunkbaseddevelopment.com/

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Thank you for listening