Custom Feature Store

FeatureStore has five suspend functions:

interface FeatureStore {
    suspend fun get(id: String): Feature?
    suspend fun getAll(): List<Feature>
    suspend fun put(feature: Feature)                                  // insert or replace
    suspend fun update(id: String, transform: (Feature) -> Feature): Feature
    suspend fun delete(id: String)                                     // no-op when missing
}

Rules every implementation must follow (the contract tests check them):

• get returns null for unknown ids.
• update is atomic against concurrent updates, throws FeatureNotFoundException for unknown ids, and rejects a transform that changes the id with IllegalArgumentException.
• enable, disable, group, enableGroup and disableGroup are extension functions built on update and getAll; you do not implement them.

Serialize a Feature with ff4kJson (or a Json whose module extends ff4kSerializersModule) when your backend stores text:

class RedisFeatureStore(private val redis: RedisCommands) : FeatureStore {
    override suspend fun get(id: String): Feature? =
        redis.get("ff4k:feature:$id")?.let { ff4kJson.decodeFromString(it) }

    override suspend fun getAll(): List<Feature> =
        redis.keys("ff4k:feature:*").mapNotNull { redis.get(it) }.map { ff4kJson.decodeFromString(it) }

    override suspend fun put(feature: Feature) {
        redis.set("ff4k:feature:${feature.id}", ff4kJson.encodeToString(feature))
    }

    override suspend fun update(id: String, transform: (Feature) -> Feature): Feature {
        // use WATCH/MULTI or a version field so concurrent updates do not lose writes
        val updated = transform(get(id) ?: throw FeatureNotFoundException(id))
        require(updated.id == id)
        put(updated)
        return updated
    }

    override suspend fun delete(id: String) {
        redis.del("ff4k:feature:$id")
    }
}