When to use which behavioral pattern in LLD - System Design Exercise

Design: Behavioral Design Patterns Usage Guide

In scope: behavioral design patterns explanation and usage scenarios. Out of scope: code implementation details, structural or creational patterns.

Functional Requirements

FR1: Explain common behavioral design patterns

FR2: Describe scenarios where each pattern is most useful

FR3: Provide guidance on choosing the right pattern based on problem context

FR4: Include examples of real-life situations for each pattern

Non-Functional Requirements

NFR1: Focus on clarity and simplicity for beginners

NFR2: Avoid technical jargon

NFR3: Patterns must be relevant to common software design problems

NFR4: Guide should be applicable to low-level design (LLD)

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Objects representing roles in patterns (e.g., Observer, Command, State)

Interfaces or abstract classes defining behavior contracts

Context objects holding state or managing interactions

Event or message dispatching mechanisms

Design Patterns

Strategy - for interchangeable algorithms

Observer - for event subscription and notification

Command - for encapsulating requests

State - for managing object states and transitions

Chain of Responsibility - for passing requests along a chain

Mediator - for centralizing complex communications

Iterator - for sequential access to collections

Template Method - for defining skeleton of an algorithm

Visitor - for operations on object structures

Reference Architecture

Behavioral Patterns Usage Guide

+-------------------+       +-------------------+
| Problem Scenario  |-----> | Identify Pattern  |
+-------------------+       +-------------------+
          |                            |
          v                            v
+-------------------+       +-------------------+
| Pattern Selection  |-----> | Apply Pattern     |
+-------------------+       +-------------------+
          |                            |
          v                            v
+-------------------+       +-------------------+
| Example Scenario   |-----> | Benefits & Tradeoffs|
+-------------------+       +-------------------+

Components

Strategy Pattern

Conceptual

Choose an algorithm at runtime from a family of algorithms.

Observer Pattern

Conceptual

Notify multiple objects about state changes in another object.

Command Pattern

Conceptual

Encapsulate a request as an object to parameterize clients.

State Pattern

Conceptual

Allow an object to alter its behavior when its internal state changes.

Chain of Responsibility Pattern

Conceptual

Pass a request along a chain of handlers until one handles it.

Mediator Pattern

Conceptual

Centralize complex communication between objects.

Iterator Pattern

Conceptual

Provide a way to access elements of a collection sequentially.

Template Method Pattern

Conceptual

Define the skeleton of an algorithm, deferring steps to subclasses.

Visitor Pattern

Conceptual

Separate an algorithm from the object structure it operates on.

Request Flow

1. 1. Identify the problem scenario requiring behavioral design.

2. 2. Analyze if behavior needs to change dynamically or if communication is complex.

3. 3. Match the scenario to a behavioral pattern based on intent and problem.

4. 4. Apply the chosen pattern to structure object interactions.

5. 5. Observe benefits like flexibility, decoupling, and easier maintenance.

Database Schema

Not applicable for this conceptual guide.

Scaling Discussion

Bottlenecks

Overusing patterns can lead to unnecessary complexity.

Choosing wrong pattern may cause rigid or inefficient design.

Complex communication patterns can become hard to maintain at scale.

Solutions

Use patterns only when they clearly solve a problem.

Evaluate tradeoffs before selecting a pattern.

Refactor and simplify communication paths as system grows.

Combine patterns thoughtfully to balance flexibility and simplicity.

Interview Tips

Time: Spend 10 minutes understanding the problem, 20 minutes discussing pattern options and their fit, 10 minutes explaining tradeoffs and examples, 5 minutes for questions.

Explain the problem context clearly before choosing a pattern.

Describe why a particular behavioral pattern fits the scenario.

Discuss benefits like decoupling, flexibility, and maintainability.

Mention tradeoffs and when not to use a pattern.

Use simple real-life analogies to illustrate pattern usage.