Command pattern for undo in LLD - System Design Exercise

Design: Undo functionality using Command Pattern

Design the command pattern structure to support undo functionality in a local application. Out of scope: redo functionality, distributed undo, or persistent undo history.

Functional Requirements

FR1: Support executing commands that change system state

FR2: Allow undoing the last executed command

FR3: Support multiple undo operations in sequence

FR4: Commands should be encapsulated as objects

FR5: Maintain a history of executed commands for undo

FR6: Ensure commands can be extended easily for new operations

Non-Functional Requirements

NFR1: Undo operations must be efficient with minimal latency

NFR2: Support up to 1000 commands in undo history

NFR3: System should be modular and easy to maintain

NFR4: Memory usage should be optimized for command history

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Command interface with execute and undo methods

Concrete command classes implementing specific actions

Invoker to execute commands and manage history

Receiver which performs the actual operations

Command history stack to track executed commands

Design Patterns

Command pattern for encapsulating requests

Stack data structure for undo history

Memento pattern for saving object state (optional)

Composite pattern if commands can be grouped

Reference Architecture

Client
  |
Invoker (Command Manager)
  |
Command Interface <---- Concrete Commands ----> Receiver
  |
Command History Stack (for undo)

Components

Command Interface

Abstract class or interface

Defines execute() and undo() methods for commands

Concrete Commands

Classes implementing Command Interface

Encapsulate specific actions and their undo logic

Invoker (Command Manager)

Class with command execution and history stack

Executes commands and manages undo stack

Receiver

Domain-specific classes

Performs actual operations requested by commands

Command History Stack

Stack data structure

Stores executed commands for undo operations

Request Flow

1. Client creates a Concrete Command with a Receiver and parameters

2. Client passes command to Invoker to execute

3. Invoker calls command.execute(), which triggers Receiver action

4. Invoker pushes command onto Command History Stack

5. When undo is requested, Invoker pops last command from stack

6. Invoker calls command.undo(), which reverses Receiver action

Database Schema

Not applicable for local command pattern design; focus is on in-memory command objects and history stack.

Scaling Discussion

Bottlenecks

Memory usage grows with large command history

Undo latency if commands have complex undo logic

Difficulty managing commands that affect shared state

Extending commands for new operations may increase complexity

Solutions

Limit command history size with a fixed max stack length

Optimize undo logic in commands for performance

Use careful synchronization if commands modify shared state

Use inheritance and interfaces to keep command extensions clean

Interview Tips

Time: Spend 10 minutes explaining the command pattern basics and undo requirements, 15 minutes designing the components and data flow, 10 minutes discussing scaling and edge cases, and 10 minutes answering questions.

Explain how command pattern encapsulates actions and undo logic

Describe the role of Invoker and command history stack

Discuss how undo reverses the last executed command

Mention handling of commands that cannot be undone

Talk about memory and performance considerations for undo history