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Chain of Responsibility and Mediator

Chain of Responsibility​

A request passes through ordered handlers until handled or transformed.

fun interface RequestInterceptor {
    suspend fun intercept(chain: Chain): Response
}

OkHttp interceptors are a practical example. Order changes behavior: authentication before logging may expose headers; retry around cache differs from cache around retry.

Validation chain example:

val validators: List<CheckoutValidator> = listOf(
    InventoryValidator(), AddressValidator(), PaymentValidator()
)
val failures = validators.flatMap { it.validate(draft) }

This collects all errors. A true first-handler chain would stop. Specify stop/continue behavior, ordering, parallelism, and error isolation.

Mediator​

Mediator centralizes interactions among colleagues so they do not reference each other directly.

class CheckoutCoordinator(
    private val cart: CartState,
    private val navigator: CheckoutNavigator,
    private val analytics: Analytics,
) {
    fun onAddressSelected(id: AddressId) { /* coordinate */ }
}

Useful for complex UI flows, but a mediator can become a god coordinator that owns all business logic. Keep it scoped to interaction/orchestration and delegate domain/data rules.

Event buses are broad mediators with weak contracts. Global string events create hidden many-to-many coupling, ordering ambiguity, lifecycle leaks, and refactoring hazards. Prefer typed local events/state owners.

Pipeline versus chain versus decorator​

  • Pipeline: all stages typically transform in sequence.
  • Chain: a handler may stop/pass responsibility.
  • Decorator: nested wrappers preserve one interface.

Code shape can resemble each; intent and contract distinguish them.

Failure design​

For each handler decide:

  • timeout and cancellation
  • may mutate request?
  • may run more than once?
  • exception isolation
  • required order
  • tracing of which handler decided
  • behavior when none handles
Order is architecture

If correctness or security depends on handler order, encode and test order explicitly. DI Set iteration is not a workflow specification.

Exercises​

  1. Order auth, redaction, logging, cache, and retry interceptors.
  2. Refactor a global event bus into scoped typed coordination.
  3. Choose chain, pipeline, or decorator for document processing.