Command pattern in LLD - System Design Exercise

Design: Command Pattern Implementation

Design the core command pattern structure including command interface, concrete commands, invoker, and receiver. Out of scope: UI integration and persistence of command history.

Functional Requirements

FR1: Allow encapsulation of requests as objects

FR2: Support undo and redo operations

FR3: Enable queuing and logging of commands

FR4: Decouple the object that invokes the operation from the one that knows how to perform it

Non-Functional Requirements

NFR1: Must support multiple command types with different parameters

NFR2: Commands should be executed asynchronously if needed

NFR3: Undo/redo operations should be efficient

NFR4: System should be extensible to add new commands without modifying existing code

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

Key Components

Command interface or abstract class

Concrete command classes implementing the interface

Invoker that calls commands

Receiver that performs the actual work

Command history stack for undo/redo

Design Patterns

Command pattern

Memento pattern for undo/redo state

Queue or buffer for asynchronous command execution

Composite pattern if commands can be grouped

Reference Architecture

Client
  |
  v
Invoker ---> Command Interface <--- Concrete Commands ---> Receiver
  |
  v
Command History (Stack for Undo/Redo)

Components

Command Interface

Abstract class or interface

Defines execute(), undo() methods for commands

Concrete Commands

Classes implementing Command Interface

Encapsulate specific actions and parameters

Invoker

Class

Calls execute() on commands and manages command history

Receiver

Class

Performs the actual operations requested by commands

Command History

Stack data structure

Stores executed commands for undo/redo

Request Flow

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

2. Client passes the command to the Invoker

3. Invoker calls execute() on the command

4. Command calls appropriate method(s) on the Receiver to perform action

5. Invoker stores the command in Command History stack

6. For undo, Invoker calls undo() on the last command from history

7. For redo, Invoker calls execute() again on the undone command

Database Schema

Not applicable - this is an in-memory design pattern implementation without persistent storage.

Scaling Discussion

Bottlenecks

Large number of commands causing memory pressure in history stack

Complex undo/redo logic for commands with side effects

Synchronous execution blocking invoker if commands take long

Difficulty adding new command types if interface is not flexible

Solutions

Limit history size or use persistent storage for command logs

Use Memento pattern to capture state snapshots for undo

Implement asynchronous command execution with queues and worker threads

Design command interface with extensibility in mind and use polymorphism

Interview Tips

Time: 10 minutes to clarify requirements and scope, 20 minutes to design components and data flow, 15 minutes to discuss scaling and trade-offs

Explain how command pattern decouples invoker and receiver

Describe how undo/redo is supported via command history

Discuss extensibility for adding new commands

Mention asynchronous execution for scalability

Highlight real-world use cases like GUI actions, transaction management

Practice

1. What is the main purpose of the Command pattern in system design?

easy

A. To create multiple instances of a class efficiently

B. To ensure only one instance of a class exists

C. To define a family of algorithms and make them interchangeable

D. To encapsulate a request as an object, allowing parameterization and queuing of requests

Solution

1. Step 1: Understand the Command pattern role

   The Command pattern encapsulates a request as an object, which allows you to parameterize clients with queues, requests, and operations.

2. Step 2: Compare with other patterns

   Creating multiple instances relates to Prototype or Factory, a family of algorithms to Strategy, and a single instance to Singleton; these are not Command.

3. Final Answer:

   To encapsulate a request as an object, allowing parameterization and queuing of requests -> Option D

4. Quick Check:

   Command pattern = encapsulate request [OK]

Hint: Command pattern = wrap action as object for flexibility [OK]

Common Mistakes:

• Confusing Command with Singleton or Factory patterns
• Thinking Command creates instances instead of encapsulating actions
• Mixing Command with Strategy pattern

2. Which of the following is the correct method signature for the execute method in a Command interface?

easy

A. void execute(String[] args);

B. void execute();

C. boolean execute(String commandName);

D. int execute(int commandId);

Solution

1. Step 1: Recall Command interface basics

   The Command interface typically defines a simple execute() method without parameters to perform the action.

2. Step 2: Analyze options

   The options with parameters (String[], int commandId, String commandName) or return types are not standard in Command pattern interfaces; the command object itself holds necessary data.

3. Final Answer:

   void execute(); -> Option B

4. Quick Check:

   Command execute method = void execute() [OK]

Hint: Command execute usually has no parameters [OK]

Common Mistakes:

• Adding parameters to execute method unnecessarily
• Confusing Command with other patterns that require arguments
• Assuming execute returns a value

3. Given the following code snippet implementing the Command pattern, what will be the output?

class Light {
  turnOn() { console.log('Light is ON'); }
  turnOff() { console.log('Light is OFF'); }
}

class TurnOnCommand {
  constructor(light) { this.light = light; }
  execute() { this.light.turnOn(); }
}

class TurnOffCommand {
  constructor(light) { this.light = light; }
  execute() { this.light.turnOff(); }
}

class RemoteControl {
  setCommand(command) { this.command = command; }
  pressButton() { this.command.execute(); }
}

const light = new Light();
const remote = new RemoteControl();
remote.setCommand(new TurnOnCommand(light));
remote.pressButton();
remote.setCommand(new TurnOffCommand(light));
remote.pressButton();

medium

A. Light is ON\nLight is OFF

B. Light is OFF\nLight is ON

C. Light is ON\nLight is ON

D. Light is OFF\nLight is OFF

Solution

1. Step 1: Trace first command execution

   The remote sets the command to TurnOnCommand and calls execute, which calls light.turnOn(), printing 'Light is ON'.

2. Step 2: Trace second command execution

   The remote sets the command to TurnOffCommand and calls execute, which calls light.turnOff(), printing 'Light is OFF'.

3. Final Answer:

   Light is ON\nLight is OFF -> Option A

4. Quick Check:

   TurnOn then TurnOff commands print ON then OFF [OK]

Hint: Follow command set and execute calls step-by-step [OK]

Common Mistakes:

• Mixing order of commands
• Assuming commands execute immediately without setting
• Confusing method names turnOn and turnOff

4. In the following code, what is the main issue that prevents the Command pattern from working correctly?

class Light {
  turnOn() { console.log('Light is ON'); }
}

class TurnOnCommand {
  constructor() { }
  execute() { this.light.turnOn(); }
}

const light = new Light();
const command = new TurnOnCommand();
command.execute();

medium

A. The execute method should return a value

B. The Light class is missing the turnOff method

C. The TurnOnCommand constructor does not receive or store the Light object

D. The command object is not instantiated properly

Solution

1. Step 1: Check TurnOnCommand constructor

   The constructor does not accept or assign the Light object to this.light, so this.light is undefined.

2. Step 2: Analyze execute method call

   Calling this.light.turnOn() fails because this.light is undefined, causing an error.

3. Final Answer:

   The TurnOnCommand constructor does not receive or store the Light object -> Option C

4. Quick Check:

   Missing light reference in command = error [OK]

Hint: Ensure command stores receiver object before execute [OK]

Common Mistakes:

• Ignoring missing receiver object in command
• Thinking missing turnOff method causes error here
• Assuming execute must return a value

5. You are designing a text editor with undo functionality using the Command pattern. Which design choice best supports undo operations efficiently?

hard

A. Store a history stack of Command objects and call an undo() method on the last command

B. Keep a log of all text changes as strings and replay them to undo

C. Use a single Command object that modifies text directly without history

D. Implement undo by reloading the entire document from disk

Solution

1. Step 1: Understand undo with Command pattern

   Each Command object should implement both execute() and undo() methods to reverse its action.

2. Step 2: Evaluate design choices

   Storing a history stack of Command objects allows calling undo() on the last command efficiently. Other options either lack command encapsulation or are inefficient.

3. Final Answer:

   Store a history stack of Command objects and call an undo() method on the last command -> Option A

4. Quick Check:

   Undo = command history stack with undo() [OK]

Hint: Undo needs command history with undo method [OK]

Common Mistakes:

• Using string logs instead of command objects
• Not implementing undo in commands
• Reloading entire document is inefficient