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Monarch 🦋

Monarch is a small, flexible, type safe, and multiplatform abstraction for feature flags.

In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state.

— Wikipedia

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[versions]
monarch = "0.2.2"

[libraries]
monarch-compose = { module = "io.github.kevincianfarini.monarch:compose", version.ref = "monarch" }
monarch-core = { module = "io.github.kevincianfarini.monarch:core", version.ref = "monarch" }
monarch-integration-environment = { module = "io.github.kevincianfarini.monarch:environment-integration", version.ref = "monarch" }
monarch-integration-launchdarkly = { module = "io.github.kevincianfarini.monarch:launch-darkly-integration", version.ref = "monarch" }
monarch-mixin-kotlinxjson = { module = "io.github.kevincianfarini.monarch:kotlinx-serialization-mixin", version.ref = "monarch" }
monarch-test = { module = "io.github.kevincianfarini.monarch:test", version.ref = "monarch" }

Usage

Defining flags

Monarch provides compile time safety for the feature flags you define and consume. Flag keys are bound to a type and a default value.

object FancyFeatureEnabled : BooleanFeatureFlag(
    key = "fancy_feature_enabled",
    default = false,
)

The FancyFeatureEnabled flag can later be referenced in code and checked by the compiler.

Monarch provides several feature flag types in the 'core' artifact.

  • BooleanFeatureFlag
  • LongFeatureFlag
  • DoubleFeatureFlag
  • StringFeatureFlag

Obtaining values

Values can be obtained from a FeatureFlagManager for a given feature flag.

fun showFeature(manager: FeatureFlagManager) {
    if (manager.currentValueOf(FancyFeatureEnabled)) {
        showFancyFeature()
    } else {
        showBoringFeature()
    }
}

Observing value changes as a Flow

Some third party SDKs, like LaunchDarkly, provide callbacks for flags when their values change. Monarch provides an additional abstraction for this, called ObservableFeatureFlagManager, which exposes these changes as a Flow.

suspend fun showFeature(manager: ObservableFeatureFlagManager) {
    manager.valuesOf(FancyFeatureEnabled).collect { enabled -> 
        if (enabled) {
            showFancyFeature()
        } else {
            showBoringFeature()
        }
    }
}

Observing value changes as a Compose State

Monarch offers a companion artifact that makes observing flag values as Compose State simple.

@Composable 
fun ShowFeature(manager: ObservableFeatureFlagManager) {
    val enabled by manager.stateOf(FancyFeatureEnabled)
    if (enabled) {
        FancyFeature()
    } else {
        BoringFeature()
    }
}

Testing

Monarch provides an out-of-the-box test implementation of an ObservableFeatureFlagManager called InMemoryFeatureFlagManager. It can be mutated under test to exercise specific branches of code dictated by your flags.

@Test 
fun test_flag_changes() = runTest {
   val manager = InMemoryFeatureFlagManager()
   manager.valuesOf(FancyFeatureEnabled).test {
       assertFalse(awaitItem())
       manager.setCurrentValueOf(FancyFeatureEnabled, true)
       assertTrue(awaitItem())
   }
}

Supplying a FeatureFlagManager

Monarch's built-in implementations of FeatureFlagManager take lists of FeatureFlagManagerMixin and a FeatureFlagDataStore.

A FeatureFlagDataStore is the entity closely tied to the underlying feature flagging SDK. It's unlikely you will implement this unless you're integrating with a feature flagging platform that Monarch doesn't currently support.

Typical usage of FeatureFlagManager implementations expects that the FeatureFlagDataStore, all FeatureFlagManagerMixin instances, and the FeatureFlagManager itself will be provided as part of your dependency graph. Below is a sample with Dagger.

@Module object FeatureFlaggingModule {
    
    @Provides 
    fun providesDataStore(): FeatureFlagDataStore { /* omitted */ }
    
    @Provides @IntoSet 
    fun providesJsonMixin(
        json: Json
    ): FeatureFlagManagerMixin = JsonFeatureFlagManagerMixin(json)
    
    @Provides 
    fun providesManager(
        dataStore: FeatureFlagDataStore, 
        mixins: Set<FeatureFlagManagerMixin>,
    ): FeatureFlagManager = MixinFeatureFlagManager(
        store = dataStore, 
        mixins = mixins.toList(),
    )
}