When to use which behavioral pattern in LLD - Architecture Diagram

System Overview - When to use which behavioral pattern

This system helps developers decide which behavioral design pattern to use based on common software design problems. It guides users through scenarios like managing object communication, controlling algorithm variations, or handling state changes, recommending the best pattern for each case.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Behavioral Pattern Selector Service
  |          |           |
  v          v           v
Strategy Service  Observer Service   State Service
  |          |           |
  v          v           v
Database Cache  Message Queue

Components

User

client

Developer or learner requesting pattern recommendations

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Routes requests to the Behavioral Pattern Selector Service

Behavioral Pattern Selector Service

service

Analyzes user input and selects the appropriate behavioral pattern

Strategy Service

service

Handles requests related to the Strategy pattern use cases

Observer Service

service

Handles requests related to the Observer pattern use cases

State Service

service

Handles requests related to the State pattern use cases

Database

database

Stores pattern definitions, use cases, and user queries

Cache

cache

Caches frequent pattern recommendations for faster response

Message Queue

queue

Manages asynchronous processing of complex pattern analysis

Request Flow - 14 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Cache

Cache → API Gateway

API Gateway → Behavioral Pattern Selector Service

Behavioral Pattern Selector Service → Strategy Service

Behavioral Pattern Selector Service → Observer Service

Behavioral Pattern Selector Service → State Service

Behavioral Pattern Selector Service → Database

Database → Behavioral Pattern Selector Service

Behavioral Pattern Selector Service → Cache

Behavioral Pattern Selector Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User

Failure Scenario

Component Fails:Database

Impact:Cached recommendations still work but new queries fail

Mitigation:System continues working by serving from cache; implement failover or circuit breaker

Architecture Quiz - 3 Questions

Which component first checks if a pattern recommendation is already available to speed up response?

A API Gateway

B Cache

C Database

D Load Balancer

Design Principle

This architecture demonstrates modular design by separating concerns into specialized services for each behavioral pattern. It uses caching to improve performance and a load balancer with API gateway to manage traffic efficiently. Delegation to pattern-specific services allows scalability and easier maintenance.

Practice

1. Which behavioral pattern is best suited when you want multiple objects to be notified automatically when one object changes its state?

easy

A. Observer pattern

B. Strategy pattern

C. Command pattern

D. Chain of Responsibility pattern

Solution

1. Step 1: Understand the need for automatic notifications

   The problem requires multiple objects to be updated when one object changes state, which means a one-to-many dependency.

2. Step 2: Match the pattern to the problem

   The Observer pattern is designed exactly for this: it lets observers subscribe to an object and get notified on changes.

3. Final Answer:

   Observer pattern -> Option A

4. Quick Check:

   Change notification = Observer [OK]

Hint: Notifications to many? Use Observer pattern [OK]

Common Mistakes:

• Confusing Strategy with Observer
• Using Command for notifications
• Choosing Chain of Responsibility for updates

2. Which pattern allows you to change an object's behavior at runtime by switching between different algorithms or strategies?

easy

A. Observer pattern

B. Strategy pattern

C. Command pattern

D. Chain of Responsibility pattern

Solution

1. Step 1: Identify the need for interchangeable behaviors

   The question asks about changing behavior dynamically, which means selecting algorithms or methods at runtime.

2. Step 2: Select the pattern that supports behavior switching

   The Strategy pattern encapsulates algorithms and lets you swap them easily without changing the client code.

3. Final Answer:

   Strategy pattern -> Option B

4. Quick Check:

   Change behavior dynamically = Strategy [OK]

Hint: Switch algorithms easily? Use Strategy pattern [OK]

Common Mistakes:

• Mixing Strategy with Observer
• Using Command for behavior changes
• Choosing Chain of Responsibility incorrectly

3. Consider this scenario: You have a request that can be handled by multiple objects in a chain. Each object decides if it can handle the request or passes it on. Which pattern fits this design?
Request -> Handler1 -> Handler2 -> Handler3

medium

A. Command pattern

B. Strategy pattern

C. Observer pattern

D. Chain of Responsibility pattern

Solution

1. Step 1: Analyze the request handling flow

   The request passes through a chain of handlers, each deciding to handle or forward it.

2. Step 2: Identify the matching behavioral pattern

   The Chain of Responsibility pattern allows multiple objects to handle a request in sequence until one handles it.

3. Final Answer:

   Chain of Responsibility pattern -> Option D

4. Quick Check:

   Request passes chain = Chain of Responsibility [OK]

Hint: Request passes chain? Use Chain of Responsibility [OK]

Common Mistakes:

• Confusing Chain with Command
• Using Observer for request handling
• Choosing Strategy incorrectly

4. You have a system where commands need to be queued, logged, and executed later. Which behavioral pattern should you use? Identify the error in this choice:
Using Observer pattern to queue commands.

medium

A. Incorrect, use Command pattern instead

B. Incorrect, use Chain of Responsibility instead

C. Incorrect, use Strategy pattern instead

D. Correct use of Observer

Solution

1. Step 1: Understand the requirement for queuing and executing commands

   Queuing, logging, and executing commands later requires encapsulating requests as objects.

2. Step 2: Identify the pattern that encapsulates requests

   The Command pattern encapsulates requests as objects, allowing queuing and deferred execution.

3. Step 3: Identify the error in using Observer

   Observer is for notifications, not for command encapsulation or queuing.

4. Final Answer:

   Incorrect, use Command pattern instead -> Option A

5. Quick Check:

   Queue commands = Command pattern [OK]

Hint: Queue commands? Use Command, not Observer [OK]

Common Mistakes:

• Using Observer for command queuing
• Confusing Command with Strategy
• Choosing Chain of Responsibility wrongly

5. You are designing a notification system where users can subscribe to different event types, and the system should allow adding new event types without changing existing code. Which combination of behavioral patterns is best suited?

hard

A. Chain of Responsibility for subscriptions and Command for event handling

B. Command for subscriptions and Chain of Responsibility for event handling

C. Observer for subscriptions and Strategy for event handling

D. Strategy for subscriptions and Observer for event handling

Solution

1. Step 1: Identify the subscription mechanism

   Users subscribing to events fits the Observer pattern, which supports dynamic subscription and notification.

2. Step 2: Identify flexible event handling

   Strategy pattern allows interchangeable algorithms for handling different event types without changing existing code.

3. Step 3: Combine patterns for extensibility

   Using Observer for subscriptions and Strategy for event handling supports adding new event types easily and keeps code maintainable.

4. Final Answer:

   Observer for subscriptions and Strategy for event handling -> Option C

5. Quick Check:

   Subscribe = Observer, flexible handling = Strategy [OK]

Hint: Subscribe = Observer, flexible handling = Strategy [OK]

Common Mistakes:

• Mixing Command with subscriptions
• Using Chain of Responsibility for subscriptions
• Confusing Strategy with Observer roles