Architecting a Collaborative Whiteboard Application: Low-Level Design
Low Level Design

Architecting a Collaborative Whiteboard Application: Low-Level Design

S

Shivam Chauhan

14 days ago

Ever wondered how those real-time collaborative whiteboards work? You know, the ones where everyone can draw and brainstorm together? I have too. It's pretty interesting.

I remember the first time I used one. It felt like magic, seeing everyone's ideas pop up instantly. But behind that seamless experience lies a well-thought-out low-level design. I am gonna break that down in this blog.

Why This Matters

Building a collaborative whiteboard isn't just about drawing lines on a screen. It's about:

  • Handling concurrent updates from multiple users.
  • Ensuring data consistency across all clients.
  • Optimizing performance for a smooth, real-time experience.

If you mess up the low-level design, you'll end up with a laggy, buggy mess. Trust me, no one wants that.

Key Components

Let's break down the core components you'll need:

  • Canvas: The visual area where users draw and interact.
  • Shape Objects: Representations of lines, circles, rectangles, and other drawing elements.
  • User Sessions: Management of connected users and their actions.
  • Real-Time Communication: Handling the exchange of drawing data between clients and the server.
  • Data Storage: Persisting the whiteboard state for future access.

Canvas

The canvas is the heart of your application. You can implement it using:

  • HTML5 Canvas: Great for basic drawing and fast rendering.
  • SVG (Scalable Vector Graphics): Ideal for complex shapes and scalability.

For simplicity, let's assume we're using HTML5 Canvas. You'll need to handle:

  • Mouse events (mousedown, mousemove, mouseup) to capture drawing actions.
  • Touch events for mobile devices.
  • Redrawing the canvas whenever the data changes.

Shape Objects

Each drawing element should be represented as an object. For example:

java
interface Shape {
    String getType();
    void draw(Canvas canvas);
    JSONObject toJSON();
}

class Line implements Shape {
    private int x1, y1, x2, y2;
    private String color;

    public Line(int x1, int y1, int x2, int y2, String color) {
        this.x1 = x1;
        this.y1 = y1;
        this.x2 = x2;
        this.y2 = y2;
        this.color = color;
    }

    @Override
    public String getType() {
        return "line";
    }

    @Override
    public void draw(Canvas canvas) {
        // Drawing logic using canvas API
    }

    @Override
    public JSONObject toJSON() {
        // Convert object to JSON for data transfer
    }
}

You'll need similar classes for circles, rectangles, text, etc. Each class should:

  • Implement a common Shape interface.
  • Define attributes like position, size, color, and thickness.
  • Provide a draw() method to render the shape on the canvas.
  • Implement a toJSON() method to serialize the object for data transfer.

User Sessions

Managing user sessions is crucial for:

  • Identifying each user.
  • Tracking their actions.
  • Broadcasting updates to the correct clients.

You can use WebSockets to maintain persistent connections between clients and the server. Each connection represents a user session.

When a user joins the whiteboard, you'll need to:

  • Assign them a unique ID.
  • Store their session information (e.g., username, connection status).
  • Send them the current state of the whiteboard.

Real-Time Communication

This is where the magic happens. You need a robust mechanism to:

  • Broadcast drawing actions to all connected clients.
  • Handle concurrent updates without conflicts.
  • Minimize latency for a smooth experience.

WebSockets are a great choice for real-time communication. When a user draws something, the client sends a message to the server. The server then broadcasts that message to all other connected clients.

To handle concurrent updates, you can use techniques like:

  • Operational Transformation (OT): Allows clients to transform operations based on previous operations, preventing conflicts.
  • Conflict-Free Replicated Data Types (CRDTs): Data structures that can be updated concurrently without requiring central coordination.

OT is more complex to implement but offers better performance for text-based collaboration. CRDTs are simpler but may have limitations for complex drawing operations.

Data Storage

You'll need to persist the whiteboard state so users can access it later. You can use:

  • JSON Files: Simple and easy for small whiteboards.
  • Databases (e.g., MongoDB, PostgreSQL): More scalable and robust for large whiteboards with complex data.

When a user saves the whiteboard, you'll need to:

  • Serialize all the shape objects to JSON.
  • Store the JSON data in your chosen storage mechanism.

When a user loads the whiteboard, you'll need to:

  • Retrieve the JSON data from storage.
  • Deserialize the JSON data into shape objects.
  • Redraw the canvas.

Concurrency Strategies

Handling concurrency is the biggest challenge in a collaborative whiteboard application. Here are some strategies to consider:

  • Centralized Server: The server acts as the single source of truth, resolving conflicts and broadcasting updates.
  • Optimistic Locking: Each client optimistically assumes that their updates won't conflict with others. If a conflict occurs, the client retries the update.
  • Last Write Wins: The last update received by the server overwrites any previous updates. This is the simplest approach but can lead to data loss.

Code Example: Broadcasting Updates

Here's a simplified example of how to broadcast updates using WebSockets:

java
// Server-side code
WebSocketServer server = new WebSocketServer(new InetSocketAddress(8887));
server.start();

// WebSocket handler
public class WhiteboardSocket extends WebSocketAdapter {
    @Override
    public void onWebSocketConnect(Session session) {
        // Add session to a list of connected sessions
    }

    @Override
    public void onWebSocketText(String message) {
        // Parse the message (e.g., drawing action)
        // Broadcast the message to all other connected sessions
        for (Session session : connectedSessions) {
            if (session != this.getSession()) {
                session.getRemote().sendString(message);
            }
        }
    }
}

// Client-side code
WebSocket ws = new WebSocketFactory().createSocket("ws://localhost:8887");
ws.addListener(new WebSocketAdapter() {
    @Override
    public void onTextMessage(String message) {
        // Parse the message and update the canvas
    }
});
ws.connect();

Where Coudo AI Can Help

Want to put your low-level design skills to the test? Check out Coudo AI's machine coding problems. They give you a 1-2 hour window to code real-world features, providing a more authentic experience than typical interview questions.

I especially recommend trying out the Movie Ticket Booking System. It's a great way to see how different components interact in a complex system.

FAQs

Q: What's the best data structure for storing shapes on the canvas?

A: It depends on the complexity of your application. For simple whiteboards, an array or list may be sufficient. For more complex whiteboards with grouping and layering, a tree structure may be more appropriate.

Q: How do I handle undo/redo functionality?

A: You can maintain a stack of operations. Each time a user performs an action, you push the operation onto the stack. To undo, you pop the last operation from the stack and revert the canvas. To redo, you push the operation back onto the stack and reapply it to the canvas.

Q: How do I optimize performance for large whiteboards?

A: Consider using techniques like:

  • Virtualization: Only render the visible portion of the canvas.
  • Caching: Cache frequently used shapes and patterns.
  • Web Workers: Offload drawing operations to a separate thread.

Wrapping Up

Building a collaborative whiteboard application is a challenging but rewarding project. By carefully considering the low-level design, you can create a smooth, responsive, and reliable experience for your users.

Remember, it's all about breaking down the problem into smaller, manageable components. Start with the basics, like handling shapes and broadcasting updates. Then, gradually add more advanced features like concurrency control and data persistence. Good luck, and happy coding!

If you’re looking to sharpen your low-level design skills, check out more practice problems and guides on Coudo AI. Remember, continuous improvement is the key to mastering LLD! This is how you can architect a collaborative whiteboard application.\n\n

About the Author

S

Shivam Chauhan

Sharing insights about system design and coding practices.