Reference: Ops: OpenFeature

Installation (Gradle)#

dependencies {
    
    implementation(platform("org.http4k:http4k-bom:6.53.0.0"))

    implementation("org.http4k:http4k-ops-openfeature")
}

About#

This module integrates OpenFeature feature flag evaluation into the http4k filter chain. A ServerFilter evaluates all flags for the incoming request once and stores the snapshot in the Request, which typed OpenFeatureFlag Lenses can then read in a type-safe manner. An Http4kOpenFeatureProvider is also supplied for those wishing to plug http4k into the official OpenFeature SDK as a FeatureProvider.

OpenFeature#

The ServerFilters.PopulateOpenFeatureContext filter accepts any OpenFeature client (typically from http4k-connect-openfeature, real or fake) and a function deriving an EvaluationContext from the request. Downstream handlers extract values via OpenFeatureFlag Lenses — each supporting required, optional, and defaulted variants.

Kotlin example.kt
package content.ecosystem.http4k.reference.openfeature

import dev.openfeature.sdk.ImmutableContext
import org.http4k.connect.openfeature.FakeOpenFeature
import org.http4k.connect.openfeature.model.FlagKey
import org.http4k.core.Method.GET
import org.http4k.core.Request
import org.http4k.core.Response
import org.http4k.core.Status.Companion.OK
import org.http4k.core.then
import org.http4k.filter.PopulateOpenFeatureContext
import org.http4k.filter.ServerFilters
import org.http4k.lens.OpenFeatureFlag

fun main() {

    // a fake OpenFeature provider - substitute the real OpenFeature.Http client
    val fake = FakeOpenFeature().apply {
        this[FlagKey.of("dark-mode")] = true
        rule(FlagKey.of("greeting")) { ctx -> ctx.context["targetingKey"] == "alice" } returns "hello, alice"
    }

    // typed Lenses for reading flags out of a Request
    val darkMode = OpenFeatureFlag.boolean().defaulted("dark-mode", false)
    val greeting = OpenFeatureFlag.string().optional("greeting")

    // the filter evaluates all flags once per request from the supplied EvaluationContext
    val app = ServerFilters.PopulateOpenFeatureContext(fake.client()) { ImmutableContext("alice") }
        .then { req -> Response(OK).body("darkMode=${darkMode(req)}, greeting=${greeting(req)}") }

    println(app(Request(GET, "/")).bodyString())
}
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