Why Chain of Responsibility pattern in LLD? - Purpose & Use Cases

The Big Idea

What if your code could pass requests like a relay race, smoothly and without confusion?

The Scenario

Imagine you have a customer support system where every request must be checked by multiple teams one after another. You manually write code that checks each team in order, and if one team can't handle it, you pass it to the next. This quickly becomes messy and hard to manage as teams or rules change.

The Problem

Manually coding each step means lots of repeated if-else checks. It's slow to update because you must change many places. It's easy to make mistakes, like skipping a team or handling requests twice. The code becomes tangled and hard to read, making bugs common and fixing them painful.

The Solution

The Chain of Responsibility pattern lets you link handlers in a chain. Each handler decides if it can process the request or passes it along. This keeps code clean, flexible, and easy to extend. You can add or remove handlers without changing others, making the system scalable and maintainable.

Before vs After

✗ Before

if (teamA.canHandle(request)) {
  teamA.handle(request);
} else if (teamB.canHandle(request)) {
  teamB.handle(request);
} else {
  teamC.handle(request);
}

✓ After

handlerA.setNext(handlerB).setNext(handlerC);
handlerA.handle(request);

What It Enables

This pattern enables building flexible, easy-to-change systems where requests flow smoothly through a chain of handlers without tight coupling.

Real Life Example

In a tech support center, a customer's issue is passed from a basic help desk to specialized teams automatically until someone can solve it, without the customer repeating their problem.

Key Takeaways

Manual checks cause tangled, hard-to-maintain code.

Chain of Responsibility creates a flexible chain of handlers.

It makes adding or changing handlers simple and safe.

Practice

1. What is the main purpose of the Chain of Responsibility pattern in system design?

easy

A. To pass a request along a chain of handlers until one handles it

B. To create multiple copies of an object

C. To ensure only one instance of a class exists

D. To define a family of algorithms and make them interchangeable

Solution

1. Step 1: Understand the pattern's behavior

   The Chain of Responsibility pattern allows a request to be passed along a chain of objects until one can handle it.

2. Step 2: Compare options with the pattern's purpose

   The options describing singleton (one instance), prototype (object copies), and strategy (interchangeable algorithms) refer to other design patterns, not Chain of Responsibility.

3. Final Answer:

   To pass a request along a chain of handlers until one handles it -> Option A

4. Quick Check:

   Chain of Responsibility = pass request along chain [OK]

Hint: Remember: Chain passes request until handled [OK]

Common Mistakes:

• Confusing with Singleton or Strategy patterns
• Thinking it creates object copies
• Assuming it handles all requests at once

2. Which of the following is the correct way to link handlers in a Chain of Responsibility pattern?

easy

A. Handlers are linked using a global static list

B. Handlers are independent and do not reference each other

C. Handlers communicate through a shared database

D. Each handler holds a reference to the next handler in the chain

Solution

1. Step 1: Recall how handlers are connected

   In Chain of Responsibility, each handler has a reference to the next handler to pass the request along.

2. Step 2: Evaluate other options

   Global static lists, shared databases, and independent handlers describe unrelated or incorrect linking methods that contradict the chain concept.

3. Final Answer:

   Each handler holds a reference to the next handler in the chain -> Option D

4. Quick Check:

   Handler links = next handler reference [OK]

Hint: Handlers link by referencing the next handler [OK]

Common Mistakes:

• Using global lists instead of direct references
• Assuming handlers are independent
• Confusing with event broadcasting

3. Consider this simplified handler chain code snippet:

class Handler:
    def __init__(self, successor=None):
        self.successor = successor
    def handle(self, request):
        if self.can_handle(request):
            return f"Handled {request}"
        elif self.successor:
            return self.successor.handle(request)
        else:
            return "Not handled"
    def can_handle(self, request):
        return False

class ConcreteHandlerA(Handler):
    def can_handle(self, request):
        return request == 'A'

class ConcreteHandlerB(Handler):
    def can_handle(self, request):
        return request == 'B'

chain = ConcreteHandlerA(ConcreteHandlerB())
print(chain.handle('B'))

What is the output of this code?

medium

A. Handled B

B. Handled A

C. Not handled

D. Error

Solution

1. Step 1: Trace the request through the chain

   The request 'B' is first checked by ConcreteHandlerA, which returns False for can_handle('B'). It passes the request to ConcreteHandlerB.

2. Step 2: ConcreteHandlerB handles the request

   ConcreteHandlerB's can_handle('B') returns True, so it returns "Handled B".

3. Final Answer:

   Handled B -> Option A

4. Quick Check:

   Request 'B' handled by second handler [OK]

Hint: Request passes chain until a handler returns true [OK]

Common Mistakes:

• Assuming first handler handles all requests
• Confusing return values
• Missing the chain passing logic

4. Given the following code snippet, identify the bug that breaks the Chain of Responsibility pattern:

class Handler:
    def __init__(self, successor=None):
        self.successor = successor
    def handle(self, request):
        if self.can_handle(request):
            return f"Handled {request}"
        else:
            return "Not handled"
    def can_handle(self, request):
        return False

class ConcreteHandler(Handler):
    def can_handle(self, request):
        return request == 'X'

chain = ConcreteHandler()
print(chain.handle('Y'))

What is the main issue here?

medium

A. The successor is not initialized properly

B. The can_handle method always returns True

C. The handler does not pass the request to the successor

D. The handle method has a syntax error

Solution

1. Step 1: Analyze the handle method logic

   The handle method checks can_handle; if False, it returns "Not handled" immediately without passing to successor.

2. Step 2: Identify missing chain passing

   It should call self.successor.handle(request) if successor exists, but this is missing.

3. Final Answer:

   The handler does not pass the request to the successor -> Option C

4. Quick Check:

   Missing successor call breaks chain [OK]

Hint: Always pass request to successor if not handled [OK]

Common Mistakes:

• Forgetting to call successor.handle()
• Assuming can_handle always returns True
• Ignoring successor initialization

5. You are designing a logging system using the Chain of Responsibility pattern. You want to handle logs of different severity: DEBUG, INFO, WARNING, ERROR. Each handler should process logs at its level and pass higher severity logs down the chain. Which design best fits this requirement?

hard

A. Each handler processes only its level and passes all logs to the next handler

B. Each handler processes logs at its level and passes only higher severity logs to the next handler

C. Each handler processes all logs regardless of severity and stops the chain

D. Each handler processes logs randomly without order

Solution

1. Step 1: Understand the logging severity flow

   Logs should be handled at their level, and higher severity logs should continue down the chain for further handling.

2. Step 2: Evaluate options for correct chain behavior

   Each handler processes logs at its level and passes only higher severity logs to the next handler matches this: handlers process their level and pass higher severity logs onward. Each handler processes only its level and passes all logs to the next handler passes all logs regardless, which is inefficient. Each handler processes all logs regardless of severity and stops the chain stops chain prematurely. Each handler processes logs randomly without order is random and incorrect.

3. Final Answer:

   Each handler processes logs at its level and passes only higher severity logs to the next handler -> Option B

4. Quick Check:

   Process level, pass higher severity [OK]

Hint: Process own level, pass higher severity logs down [OK]

Common Mistakes:

• Passing all logs without filtering
• Stopping chain too early
• Ignoring severity order