What is the Chain of Responsibility pattern?

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creational

Chain of Responsibility

Pass a request along a chain of handlers. Each decides to process or pass to the next.

A request arrives and something must handle it, but which handler depends on the request.

An expense claim might be approved by a manager, a director or the CFO depending on the amount. A web request passes through authentication, rate limiting and logging.

Chain of Responsibility links the handlers together. Each one either deals with the request or passes it to the next. The sender just hands it to the first link.

What the interviewer is probing.

1. "How does the chain work?" *Probing:* the mechanism. *Stalls:* "A loop." *Moves up:* each handler either handles the request or passes it on, so the sender does not know which one will deal with it.

2. "Where have you seen it?" *Probing:* the recognition. *Stalls:* "Nowhere." *Moves up:* servlet filter chains, Spring Security, middleware pipelines and logging level hierarchies.

3. "What is the main risk?" *Probing:* the practical failure. *Stalls:* "None." *Moves up:* a request falling off the end unhandled, and a chain whose order matters but is configured far from the handlers.

4. "How does it differ from Decorator?" *Probing:* the subtle distinction. *Stalls:* "It does not." *Moves up:* decorators all contribute; in a chain one handler typically terminates the request.

Customer support escalation — Level 1 handles simple issues, escalates to Level 2, then engineering.

Key Concepts

Each handler has a reference to the next one in the chain.

When a request arrives, the handler decides whether it can deal with it. If yes, it handles it and usually stops there. If not, it passes it along.

The sender only knows the first handler. It has no idea how long the chain is or who eventually responds.

Two things to decide up front. What happens if nobody handles the request, since silently doing nothing is hard to debug. And be aware the behaviour depends on the order of the chain, which is set in configuration rather than visible in any one handler.

java

public abstract class Handler {
    private Handler next;
    public Handler setNext(Handler n) { this.next = n; return n; }
    public void handle(Request req) {
        if (next != null) next.handle(req);
    }
}
public class AuthHandler extends Handler {
    public void handle(Request req) {
        if (req.getHeader("Auth") == null) throw new UnauthorizedException();
        super.handle(req);
    }
}
// Assemble
Handler chain = new AuthHandler();
chain.setNext(new RateLimitHandler()).setNext(new LoggingHandler());