Builder pattern in LLD - System Design Exercise

Design: Builder Pattern Implementation

Design and implement the builder pattern for creating complex objects in a low-level design context. Out of scope: integration with UI or persistence layers.

Functional Requirements

FR1: Create complex objects step-by-step with different configurations

FR2: Separate construction logic from the final object representation

FR3: Allow reuse of the construction process for different object types

FR4: Support immutability of the final constructed object

FR5: Enable chaining of builder methods for easy readability

Non-Functional Requirements

NFR1: Construction process should be flexible and extensible

NFR2: Final object creation should be thread-safe if used concurrently

NFR3: Minimize code duplication in object creation

NFR4: Keep the builder interface simple and intuitive

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Builder interface or abstract class defining construction steps

Concrete builder classes implementing the steps

Director class to control the building process (optional)

Product class representing the complex object

Client code that uses the builder to create objects

Design Patterns

Fluent interface for chaining builder methods

Immutable object pattern for the final product

Factory pattern for creating builders

Separation of concerns between construction and representation

Reference Architecture

Client --> Builder Interface --> Concrete Builder --> Product
          \                                   /
           --------> Director (optional) <---

Components

Builder Interface

Abstract class or interface in chosen language

Defines methods for building parts of the product

Concrete Builder

Class implementing Builder Interface

Implements construction steps and assembles the product

Product

Immutable class or data structure

Represents the complex object being built

Director (optional)

Class controlling the building sequence

Encapsulates the order of building steps

Client

User code

Uses builder to create product instances

Request Flow

1. Client creates a Concrete Builder instance

2. Client optionally creates a Director and passes the builder

3. Director calls builder methods in a specific order to build the product

4. Builder assembles parts internally during method calls

5. Client calls builder's getResult() to retrieve the final Product

6. Product is returned as an immutable object

Database Schema

Not applicable for Builder pattern as it is a design pattern for object creation, not data storage.

Scaling Discussion

Bottlenecks

Complexity grows if many product variations require many builder subclasses

Thread safety issues if builder instances are shared across threads

Excessive chaining can reduce readability if not designed well

Solutions

Use parameter objects or configuration classes to reduce number of builder subclasses

Ensure each thread uses its own builder instance or synchronize builder methods

Design fluent interfaces carefully to maintain clarity and avoid long chains

Interview Tips

Time: 10 minutes to explain the pattern and components, 15 minutes to design and draw the architecture, 10 minutes to discuss scaling and trade-offs, 10 minutes for Q&A

Explain separation of construction and representation

Highlight benefits of immutability and method chaining

Discuss optional Director role and when to use it

Show understanding of thread safety and extensibility

Relate pattern usage to real-world examples like building complex UI components or configuration objects