Middleware Pipeline

Middleware Protocol

Middleware[RequestType, ResponseType] defines cross-cutting interceptors. Each middleware receives the request and a next_handler callable. It may:
- Transform the request before delegating
- Transform the response after delegating
- Short-circuit by returning without calling next_handler

Middleware does not extend Port — its shape (request, next_handler) → response is a different category.

Pipeline

Pipeline composes middleware into an immutable, right-to-left chain. The first middleware executes first inbound and last outbound:

[A, B, C] → A wraps B wraps C wraps handler
Inbound:   A → B → C → handler
Outbound:  A ← B ← C ← handler

When to use

Implement Middleware for any cross-cutting concern: logging, authentication, metrics, rate limiting. Compose them in a Pipeline and pass it to the PresentationAdapter. Order matters — the first middleware wraps the outermost layer.

The chain is pre-built at construction. execute() is a single delegation.