Design Example: Social Network

A Social Network (like Facebook or LinkedIn) manages user profiles, friendships, and a dynamic feed of user-generated content and notifications.

Step 1: Requirements Gathering

Core Use Cases:

• Profile Management: Users can create profiles and update information.
• Connections: Users can follow or send friend requests to others.
• Posts: Users can share text, images, or links.
• Feed: A dynamic view showing posts from a user's connections.
• Notifications: Alerts for new likes, comments, or connection requests.
• Search: Find users or content by keywords or tags.

Relationship Types:

• One-Way (Follower): Like Twitter or Instagram.
• Two-Way (Friendship): Like Facebook (requires acceptance).

Step 2: Identify Core Objects

• User: The central entity with profile data and connections.
• Profile: Contains bio, photos, and settings.
• Post: Content created by a user (encapsulated in a FeedItem).
• Connection: Represents the link between two users.
• Notification: Alerts for specific actions.
• FeedService: Logic for generating a user's personalized feed.

Step 3: Design Class Diagram

Step 4: Implementation in TypeScript

class User {
  private friends: Set<User> = new Set();
  private posts: Post[] = [];
 
  constructor(
    public id: string,
    public name: string,
    public profile: Profile
  ) {}
 
  public addFriend(friend: User) {
    this.friends.add(friend);
    friend.friends.add(this); // Assuming two-way friendship
    // Notify through NotificationService
  }
 
  public createPost(content: string) {
    const post = new Post(content, this);
    this.posts.push(post);
    return post;
  }
}
 
class Post {
  private comments: Comment[] = [];
  private likes: User[] = [];
 
  constructor(
    public content: string,
    public author: User
  ) {}
 
  public addLike(user: User) {
    this.likes.push(user);
    // Notify author of the like
  }
}

Deep Dive: Observer Pattern for Feed & Notifications

The Observer Pattern is critical for social networks. When a user creates a post, their friends (the observers) should have their feeds updated (or receive notifications).

interface FeedObserver {
  onNewPost(post: Post): void;
}
 
class FeedService implements FeedObserver {
  onNewPost(post: Post) {
    console.log(`Updating feed for followers of ${post.author.name}`);
    // Add logic to persist in feed DB or Cache
  }
}

Wrap Up

Designing a Social Network involves complex Graph Relationships and high-frequency Status Updates. By decoupling the persistence of posts from the generation of feeds, you ensure the system remains responsive even with millions of concurrent users.

[!TIP] Use the Composite Pattern to handle different types of content (text, image, video) within a single Post object.