Free AOP

Aspect-Oriented Programming (AOP) separates cross-cutting concerns (logging, auth, caching, metrics) from business logic without modifying the core code.

MediatorK gives you AOP for free through pipeline behaviors. Every behavior you register applies to all handlers automatically; your handler code stays completely untouched. The handler is pure business logic; the pipeline is pure infrastructure.

---

The idea

Without AOP, you reach for logging inside every handler:

class GetUserHandler(private val db: UserRepository) : RequestHandler<GetUserQuery, User> {
    override suspend fun handle(
        mediator: Mediator,
        requestContext: RequestContext,
        request: GetUserQuery
    ): User {
        println("→ GetUserQuery(id=${request.id})")   // ← you added this
        val user = db.findById(request.id) ?: error("not found")
        println("← GetUserQuery result=$user")        // ← and this
        return user
    }
}

With AOP, the handler stays pure and logging lives in one place:

// Handler — zero logging code
class GetUserHandler(private val db: UserRepository) : RequestHandler<GetUserQuery, User> {
    override suspend fun handle(
        mediator: Mediator,
        requestContext: RequestContext,
        request: GetUserQuery
    ): User = db.findById(request.id) ?: error("not found")
}

// Wiring — one line enables logging for every request
val mediator = MediatorFactory.create(
    registrars = listOf(AppRegistrar()),
    pipelineBehaviors = listOf(
        LoggingPipelineBehavior(logger = ::println),
    ),
)

Output for every request dispatched:

→ GetUserQuery
← GetUserQuery result=User(id=user-1, name=Alice)

---

Stack multiple concerns

Each behavior is an independent aspect. Combine them freely:

val mediator = MediatorFactory.create(
    registrars = listOf(AppRegistrar()),
    pipelineBehaviors = listOf(
        LoggingPipelineBehavior(logger = ::println,           order = -100), // outermost
        TimingPipelineBehavior(onTiming = { name, ms ->
            println("$name took ${ms}ms")
        }),
        ErrorTrackingPipelineBehavior { request, error ->
            Sentry.captureException(error)
        },
    ),
)

None of these touch a single handler.

---

Next

→ Built-in Behaviors