Chain of Responsibility pattern in LLD - System Design Exercise

Design: Chain of Responsibility Pattern Implementation

Design focuses on the pattern implementation and request flow. Out of scope are UI details, persistence, and distributed system concerns.

Functional Requirements

FR1: Design a system where multiple handlers can process a request in a sequence.

FR2: Each handler decides either to process the request or pass it to the next handler.

FR3: The system should allow dynamic addition or removal of handlers.

FR4: Requests should be processed by the first capable handler only.

FR5: Support different types of requests with different handlers.

Non-Functional Requirements

NFR1: The system should handle up to 1000 requests per second.

NFR2: Latency for processing each request should be under 100ms.

NFR3: The design should be extensible to add new handlers without modifying existing code.

NFR4: Ensure thread safety if handlers are used in concurrent environments.

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Handler interface or abstract class

Concrete handler implementations

Request object abstraction

Chain manager or builder to link handlers

Client that sends requests

Design Patterns

Chain of Responsibility pattern

Command pattern for encapsulating requests

Decorator pattern if handlers add behavior

Observer pattern if handlers notify others

Reference Architecture

Client
  |
  v
[Chain Manager] --> [Handler 1] --> [Handler 2] --> [Handler 3] --> ... --> [Handler N]
Each handler decides to process or pass the request to the next handler.

Components

Handler Interface

Abstract class or interface in chosen language

Defines method to process request and link to next handler

Concrete Handlers

Classes implementing Handler interface

Implement specific logic to handle certain requests or pass along

Request Object

Class or data structure

Encapsulates request data and type

Chain Manager

Class or builder pattern

Creates and links handlers into a chain

Client

Any caller code

Sends requests to the chain for processing

Request Flow

1. Client creates a request object.

2. Client sends the request to the Chain Manager.

3. Chain Manager passes the request to the first handler in the chain.

4. Handler checks if it can process the request.

5. If yes, handler processes and returns response.

6. If no, handler passes request to the next handler.

7. This continues until a handler processes the request or chain ends.

8. If no handler processes, return a default response or error.

Database Schema

Not applicable as this pattern focuses on in-memory request handling and does not require persistent storage.

Scaling Discussion

Bottlenecks

Long chains can increase latency as requests pass through many handlers.

Single-threaded chains may limit throughput under high load.

Adding/removing handlers dynamically may cause inconsistent chain states.

Handlers with heavy processing can slow down the chain.

Solutions

Limit chain length or group handlers by request type to reduce unnecessary passes.

Use concurrent or asynchronous processing where handlers can run in parallel if independent.

Implement thread-safe chain management with locks or atomic operations.

Optimize handler logic or offload heavy processing to background jobs.

Interview Tips

Time: Spend 10 minutes understanding requirements and clarifying questions, 20 minutes designing the pattern and explaining components, 10 minutes discussing scaling and trade-offs, 5 minutes for questions.

Explain the purpose of the Chain of Responsibility pattern clearly.

Describe how handlers are linked and how requests flow through the chain.

Discuss extensibility and how new handlers can be added without changing existing code.

Mention thread safety and concurrency considerations.

Talk about potential bottlenecks and how to scale the design.

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