Chain of Responsibility là gì? Ứng dụng trong Express.js middleware?

Chain of Responsibility cho phép pass request qua chain of handlers — mỗi handler quyết định xử lý hoặc pass cho handler tiếp theo.

• Express.js middleware là ví dụ hoàn hảo: app.use(authMiddleware, rateLimitMiddleware, validationMiddleware, routeHandler) — mỗi middleware gọi next() để pass request.

Ví dụ TypeScript:

interface Handler {
  setNext(h: Handler): Handler
  handle(request: number): string | null
}

abstract class AbstractHandler implements Handler {
  protected nextHandler?: Handler

  setNext(h: Handler) {
    this.nextHandler = h
    return h
  }

  handle(r: number): string | null {
    return this.nextHandler?.handle(r) ?? null
  }
}

class AuthHandler extends AbstractHandler {
  handle(r: number) {
    if (r < 0) return 'Unauthorized'
    return super.handle(r)
  }
}

• Dùng khi: nhiều object có thể handle request và muốn decouple sender từ receiver; khi handlers nên được configurable hoặc reorderable tại runtime.
• Khác Observer: Chain of Responsibility chỉ một handler xử lý; Observer notify tất cả.
• Không dùng khi: chain quá dài gây performance issue; khi không rõ handler nào sẽ xử lý gây khó debug.

Chain of Responsibility allows a request to pass through a chain of handlers — each handler decides whether to process it or pass it to the next.

• Express.js middleware is a perfect example: app.use(authMiddleware, rateLimitMiddleware, validationMiddleware, routeHandler) — each middleware calls next() to pass the request along.
• TypeScript example:

interface Handler {
  setNext(h: Handler): Handler
  handle(request: number): string | null
}

abstract class AbstractHandler implements Handler {
  protected nextHandler?: Handler

  setNext(h: Handler) {
    this.nextHandler = h
    return h
  }

  handle(r: number): string | null {
    return this.nextHandler?.handle(r) ?? null
  }
}

class AuthHandler extends AbstractHandler {
  handle(r: number) {
    if (r < 0) return 'Unauthorized'
    return super.handle(r)
  }
}

Use it when multiple objects may handle a request and you want to decouple the sender from the receiver, or when handlers should be configurable and reorderable at runtime.

• Difference from Observer: Chain of Responsibility has only one handler process the request; Observer notifies all observers.
• Avoid it when the chain is too long and causes performance issues, or when it's unclear which handler will act, making debugging difficult.