Chain of Responsibility (Behavioral)

Intent / problem it solves

Pass a request along a chain of handlers until one handles it (or it drops). Decouples sender from receivers and lets you reorder or add handlers without changing client code.

When to use / when NOT

Use for middleware pipelines, approval workflows, filters, or validation chains.

Avoid when a fixed orchestrator or explicit routing is clearer; watch for hard-to-debug chains.

Structure

Handler interface defines successor link or abstract processing; concrete handlers either handle or forward.

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Go example

package main

import "fmt"

type Request struct {
	Token string
}

type Handler interface {
	Handle(request Request) string
}

type AuthHandler struct{ Next Handler }

func (handler AuthHandler) Handle(request Request) string {
	if request.Token == "" {
		return "missing"
	}
	if handler.Next != nil {
		return handler.Next.Handle(request)
	}
	return "ok"
}

type RateHandler struct{ Next Handler }

func (handler RateHandler) Handle(request Request) string {
	if request.Token == "throttle" {
		return "rate"
	}
	if handler.Next != nil {
		return handler.Next.Handle(request)
	}
	return "ok"
}

func main() {
	chain := AuthHandler{Next: RateHandler{Next: nil}}
	fmt.Println(chain.Handle(Request{Token: "throttle"}))
}

JavaScript example

class TokenPipeline {
  constructor() {
    this.handlers = [];
  }

  use(handler) {
    this.handlers.push(handler);
    return this;
  }

  run(context) {
    for (const handler of this.handlers) {
      const outcome = handler(context);
      if (outcome) {
        return outcome;
      }
    }
    return 'pass';
  }
}

const pipeline = new TokenPipeline()
  .use((context) => (context.token ? null : 'missing'))
  .use((context) => (context.token === 'bad' ? 'rejected' : null));

console.log(pipeline.run({ token: 'bad' }));

Interview phrase

“Chain of responsibility models middleware: each link can handle or pass along, so I can add cross-cutting steps without entangling the client with every rule.”

Map to LLD for request filters, approval flows, or plugin chains in LLD case studies.

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