System Design ( Day - 58 )

Chain of Responsibility Pattern

Definition
Allow an object to pass a request along a chain of potential handlers, each handler in the chain decided either to process the request or pass it to the next handler.

🏧 Real-World Example: ATM Cash Dispenser

1. Handler Interface

<<abstract>>  
Handler  
- next: Handler  
+ setNext(handler): void  
+ dispense(amount): void

2. Concrete Handlers

• ThousandHandler dispenses ₹1,000 notes and forwards the remaining amount.

• FiveHundredHandler dispenses ₹500 notes, then forwards.

• HundredHandler dispenses ₹100 notes, then forwards.

Each handler checks if the requested amount is ≥ its denomination. If yes, it calculates how many notes, dispenses them, subtracts from the amount, then calls next.dispense(remaining).

3. Client Setup

Handler h1 = new ThousandHandler();
Handler h2 = new FiveHundredHandler();
Handler h3 = new HundredHandler();

h1.setNext(h2);
h2.setNext(h3);

// Client just calls:
h1.dispense(3_600);  
// Output:
// 3 × ₹1000 notes
// 1 × ₹500 note
// 1 × ₹100 note

The Client doesn’t need to know which handler does the work or how many handlers exist—it just starts the chain.

📐 Standard UML Blueprint

🌟 Why Use Chain of Responsibility?

• Extensibility: Add new handlers without changing client code.

• Separation of Concerns: Each handler focuses on one type of request.

• Dynamic Behavior: Reorder or skip handlers at runtime by changing the chain.

Real-World Example

1. Logger