The PSR-15-style contract for a single middleware step:

Implementations decide whether/when to call $next->handle() and can act on the message or result before/after doing so.

MiddlewarePipelineInterface extends both MiddlewareInterface and PipelineHandlerInterface, and adds pipe(MiddlewareInterface $middleware): void for registering middleware.

MiddlewarePipe is the default implementation, backed by an SplQueue. Calling handle() runs the queued middleware against an EmptyPipelineHandler as the terminal handler (see below). Cloning a MiddlewarePipe performs a deep clone of its internal queue so pipelines can be reused safely.

Internal, single-use handler that walks the middleware queue one step at a time. Each call to handle() dequeues the next middleware, clones itself for the remaining queue, and invokes the middleware with that clone as its $next. Once the queue is exhausted it delegates to the configured EmptyPipelineHandler. Calling handle() on the same Next instance twice throws Exception\NextHandlerAlreadyCalledException.

The library's built-in terminal middleware, registered by ConfigProvider with the lowest default priority so it runs last. Its constructor takes a MessageHandlerResolverInterface and a StrategyInterface.

process() resolves the handler, asks the strategy for the method name, and guards it with is_callable(). A handler without that method throws Exception\HandlerMethodNotFoundException. It then invokes the method and forwards the returned ResultInterface to $next->handle(), so middleware registered after it can inspect the result. A handler method that returns a non-ResultInterface value raises a TypeError.

The default terminal handler used when a MiddlewarePipe's queue is exhausted. If the message it receives is already a ResultInterface (a result produced by MessageHandlerMiddleware), it returns it as-is; otherwise it throws Exception\MessageException, since no handler ever produced a result.