A/B Testing & Feature Flags

MediatorK makes A/B testing straightforward — define one command per variant, register a handler for each, then let the caller pick which command to send based on a feature flag. The mediator and handlers stay completely unaware of each other.

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How it works

Each variant is its own command that shares the same response type. The flag decision happens at the call site — before send() is called.

// Two variants — same response type, different implementations
data class StripePaymentCommand(val orderId: String, val amount: Double) : Request<PaymentResult>
data class NetworkPaymentCommand(val orderId: String, val amount: Double) : Request<PaymentResult>

Register both handlers at startup:

class PaymentRegistrar : MediatorRegistrar {
    override fun register(registry: HandlerRegistry) {
        registry.scope {
            +StripePaymentHandler()
            +NetworkPaymentHandler()
        }
    }
}

At the call site, fetch the flag and send the matching command:

val useStripe = featureFlags.isEnabled("stripe_checkout")

val result = mediator.send(
    if (useStripe) StripePaymentCommand(orderId, amount)
    else           NetworkPaymentCommand(orderId, amount)
)

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Android ViewModel example

class CheckoutViewModel(
    private val mediator: Mediator,
    private val featureFlags: FeatureFlagService,
) : ViewModel() {

    val paymentResult = MutableStateFlow<PaymentResult?>(null)

    fun pay(orderId: String, amount: Double) {
        viewModelScope.launch {
            val useStripe = featureFlags.isEnabled("stripe_checkout")

            paymentResult.value = mediator.send(
                if (useStripe) StripePaymentCommand(orderId, amount)
                else           NetworkPaymentCommand(orderId, amount)
            )
        }
    }
}

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Why this works well

	Detail
No coupling	Handlers know nothing about flags or each other
Easy to clean up	When the experiment ends, delete one command + one handler
Testable	Test each variant independently by sending its command directly
Pipeline applies to both	LoggingBehavior, AuthBehavior, etc. wrap both variants automatically

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Next

→ Pipeline Behaviors — add cross-cutting concerns like logging and auth that apply to all variants