Why Mediator pattern in LLD? - Purpose & Use Cases

The Big Idea

What if one simple helper could stop all the chaos in your system's communication?

The Scenario

Imagine a busy office where every employee tries to talk directly to every other employee to get work done. The phone lines are always busy, messages get lost, and confusion spreads quickly.

The Problem

When everyone talks directly, communication becomes tangled and hard to manage. It's slow, mistakes happen often, and fixing problems means untangling a big mess of conversations.

The Solution

The Mediator pattern acts like a smart office manager who handles all communication. Employees talk only to the manager, who then passes messages correctly and keeps everything organized and clear.

Before vs After

✗ Before

class Employee {
  sendMessage(to, message) {
    to.receiveMessage(message);
  }
  receiveMessage(message) {
    console.log(message);
  }
}

✓ After

class Mediator {
  send(sender, receiver, message) {
    receiver.receiveMessage(message);
  }
}
class Employee {
  constructor(mediator) {
    this.mediator = mediator;
  }
  sendMessage(to, message) {
    this.mediator.send(this, to, message);
  }
  receiveMessage(message) {
    console.log(message);
  }
}

What It Enables

It enables clear, simple, and scalable communication between many parts without chaos or confusion.

Real Life Example

In a chat app, instead of every user sending messages directly to others, a server (mediator) manages all messages, ensuring they reach the right people smoothly.

Key Takeaways

Direct communication between many parts causes confusion and errors.

The Mediator pattern centralizes communication to simplify interactions.

This leads to easier maintenance and better scalability.

Practice

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

easy

A. To store data persistently in a database

B. To increase direct communication between all components

C. To replace all components with a single monolithic class

D. To centralize communication between components and reduce dependencies

Solution

1. Step 1: Understand the role of Mediator

   The Mediator pattern acts as a central hub to manage communication between components, avoiding direct links between them.

2. Step 2: Compare options with Mediator's purpose

   To centralize communication between components and reduce dependencies correctly states the purpose: centralizing communication and reducing dependencies. Other options describe unrelated or incorrect behaviors.

3. Final Answer:

   To centralize communication between components and reduce dependencies -> Option D

4. Quick Check:

   Mediator centralizes communication = A [OK]

Hint: Mediator centralizes communication, not direct links [OK]

Common Mistakes:

• Thinking Mediator increases direct component communication
• Confusing Mediator with data storage patterns
• Assuming Mediator merges components into one

2. Which of the following is the correct way to define a Mediator interface in a low-level design?

easy

A. interface Mediator { void notify(Component sender, String event); }

B. class Mediator { void notifyAll(); }

C. interface Mediator { void sendMessage(String message); }

D. class Mediator { void receive(Component sender); }

Solution

1. Step 1: Identify typical Mediator method signature

   The Mediator usually has a method to notify it about events from components, often with sender and event details.

2. Step 2: Match method signatures to this pattern

   interface Mediator { void notify(Component sender, String event); } matches this pattern with notify(Component sender, String event). Others lack sender info or use incorrect method names.

3. Final Answer:

   interface Mediator { void notify(Component sender, String event); } -> Option A

4. Quick Check:

   Notify method with sender and event = B [OK]

Hint: Mediator notify method includes sender and event [OK]

Common Mistakes:

• Omitting sender parameter in notify method
• Using generic sendMessage without context
• Naming methods incorrectly for Mediator role

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

class Mediator {
  notify(sender, event) {
    if (event === 'A') return 'Handled A';
    if (event === 'B') return 'Handled B';
    return 'Unknown event';
  }
}

const mediator = new Mediator();
console.log(mediator.notify('Component1', 'B'));

medium

A. Handled A

B. Error: notify method missing

C. Handled B

D. Unknown event

Solution

1. Step 1: Analyze notify method logic

   The method returns 'Handled A' if event is 'A', 'Handled B' if event is 'B', else 'Unknown event'.

2. Step 2: Check the call with event 'B'

   The call is mediator.notify('Component1', 'B'), so it matches the second condition and returns 'Handled B'.

3. Final Answer:

   Handled B -> Option C

4. Quick Check:

   Event 'B' returns 'Handled B' [OK]

Hint: Match event string exactly in notify method [OK]

Common Mistakes:

• Confusing event 'B' with 'A'
• Assuming default case triggers for known events
• Expecting error due to missing parameters

4. In the following Mediator implementation, what is the main issue?

class Mediator {
  notify(sender, event) {
    if (event === 'start') {
      sender.start();
    } else if (event === 'stop') {
      sender.stop();
    }
  }
}

class Component {
  start() { console.log('Started'); }
  stop() { console.log('Stopped'); }
}

const mediator = new Mediator();
const comp = new Component();
mediator.notify(comp, 'start');

medium

A. Component class lacks notify method

B. Mediator calls methods on sender directly, creating tight coupling

C. notify method does not handle unknown events

D. Missing constructor in Mediator class

Solution

1. Step 1: Review Mediator's notify method behavior

   The Mediator calls start() or stop() directly on the sender component.

2. Step 2: Identify design issue

   This direct call creates tight coupling between Mediator and Component, defeating the purpose of loose coupling in Mediator pattern.

3. Final Answer:

   Mediator calls methods on sender directly, creating tight coupling -> Option B

4. Quick Check:

   Tight coupling breaks Mediator pattern goal = A [OK]

Hint: Mediator should avoid calling sender methods directly [OK]

Common Mistakes:

• Ignoring tight coupling caused by direct calls
• Thinking missing notify in Component is an error
• Assuming constructor absence causes failure

5. You are designing a chat application where multiple users send messages to each other. Which design using the Mediator pattern best fits this scenario?

hard

A. A ChatRoom mediator receives messages from users and forwards them to the intended recipients.

B. Users store messages locally and synchronize with a database periodically.

C. Each user sends messages directly to all other users without a central controller.

D. Users communicate only through email notifications handled by a separate service.

Solution

1. Step 1: Understand the chat communication needs

   Users need a central place to send and receive messages without direct dependencies on each other.

2. Step 2: Match with Mediator pattern usage

   A ChatRoom mediator receives messages from users and forwards them to the intended recipients. describes a ChatRoom mediator that manages message routing, fitting the Mediator pattern perfectly.

3. Final Answer:

   A ChatRoom mediator receives messages from users and forwards them to the intended recipients. -> Option A

4. Quick Check:

   Central message routing = C [OK]

Hint: Mediator centralizes message routing in chat apps [OK]

Common Mistakes:

• Choosing direct user-to-user messaging (no mediator)
• Confusing data storage with communication pattern
• Selecting unrelated communication methods