Visitor pattern in LLD - System Design Exercise

Design: Visitor Pattern Implementation

Design the core Visitor pattern structure including element interfaces, visitor interfaces, concrete elements, and concrete visitors. Out of scope are specific domain operations or UI integration.

Functional Requirements

FR1: Allow adding new operations to existing object structures without modifying their classes

FR2: Support multiple types of elements in the object structure

FR3: Enable different visitor implementations to perform distinct operations on elements

FR4: Maintain separation of concerns between data structures and operations

Non-Functional Requirements

NFR1: The system should handle up to 1000 elements in the object structure efficiently

NFR2: Operations should execute with minimal overhead (target under 10ms per visit)

NFR3: Design should be extensible to add new visitors without changing element classes

NFR4: Maintain code clarity and simplicity for easy maintenance

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Element interface with accept(visitor) method

Concrete element classes implementing Element interface

Visitor interface with visit methods for each element type

Concrete visitor classes implementing Visitor interface

Object structure holding elements and managing traversal

Design Patterns

Visitor pattern for operation separation

Composite pattern if object structure is hierarchical

Double dispatch to resolve method calls

Open/Closed principle for extensibility

Reference Architecture

ObjectStructure
   |
   |-- Element (interface)
   |      |-- ConcreteElementA
   |      |-- ConcreteElementB
   |
   |-- Visitor (interface)
          |-- ConcreteVisitor1
          |-- ConcreteVisitor2

Components

Element Interface

Interface in chosen language

Defines accept(visitor) method for elements to accept visitors

Concrete Elements

Classes implementing Element interface

Represent different types of elements in the object structure

Visitor Interface

Interface with visit methods

Declares visit methods for each concrete element type

Concrete Visitors

Classes implementing Visitor interface

Implement specific operations to perform on elements

Object Structure

Collection or composite of elements

Holds elements and manages traversal for visitor application

Request Flow

1. Client creates object structure with multiple elements

2. Client creates a concrete visitor instance

3. Client calls accept(visitor) on each element in the structure

4. Each element calls visitor.visit(this) passing itself

5. Visitor executes operation specific to element type

6. Client collects or uses results from visitor operations

Database Schema

Not applicable for Visitor pattern as it is a behavioral design pattern focusing on object interactions rather than data storage.

Scaling Discussion

Bottlenecks

Large object structures causing slow traversal

Adding many element types increasing visitor interface complexity

Multiple visitors increasing processing time

Tight coupling if visitor interface changes frequently

Solutions

Use efficient data structures for object structure to speed traversal

Group similar element types or use default visit methods to reduce interface size

Apply visitors selectively or asynchronously to manage load

Design visitor interface carefully to minimize changes and use adapter patterns if needed

Interview Tips

Time: Spend 10 minutes explaining the problem and requirements, 15 minutes designing the interfaces and classes with diagrams, 10 minutes discussing scaling and trade-offs, and 10 minutes answering questions.

Explain the problem of adding operations without changing element classes

Describe how double dispatch works in Visitor pattern

Show clear separation between elements and operations

Discuss extensibility and maintenance benefits

Mention potential drawbacks like interface explosion and how to mitigate them