Command pattern in LLD - Architecture Diagram

System Overview - Command pattern

The Command pattern is a design approach that turns requests or simple operations into objects. This allows for flexible command execution, queuing, logging, and undo functionality. It helps separate the object that issues a request from the one that knows how to perform it.

Architecture Diagram

User
  |
  v
Invoker
  |
  v
Command Interface <-----> Concrete Command
  |                         |
  v                         v
Receiver <------------------

Components

User

actor

Initiates commands by interacting with the Invoker

Invoker

service

Holds and triggers commands without knowing their implementation

Command Interface

interface

Defines the execute method for commands

Concrete Command

service

Implements the command interface and calls actions on the Receiver

Receiver

service

Performs the actual work when a command is executed

Request Flow - 5 Hops

User → Invoker

Invoker → Concrete Command

Concrete Command → Receiver

Receiver → Invoker

Invoker → User

Failure Scenario

Component Fails:Concrete Command

Impact:Command execution fails, so the requested action is not performed

Mitigation:Invoker can catch errors and notify User or retry; commands can be logged for replay

Architecture Quiz - 3 Questions

Which component is responsible for performing the actual work in the Command pattern?

A Receiver

B Invoker

C Concrete Command

D User

Design Principle

The Command pattern cleanly separates the requester of an action from the object that performs it. This allows commands to be stored, queued, logged, or undone, improving flexibility and decoupling in software design.

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