WebSocket for real-time communication in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Designing a scalable WebSocket architecture

You need to design a system that supports 1 million concurrent WebSocket connections for a chat application. Which architecture component is essential to handle this scale efficiently?

A Implement WebSocket connections directly on the database server.

B Use HTTP long polling instead of WebSocket to reduce server load.

C Use a single WebSocket server with multi-threading to handle all connections.

D Deploy multiple WebSocket servers behind a load balancer with sticky sessions.

💡 Hint

Think about how to distribute connections and maintain session consistency.

Multiple WebSocket servers behind a load balancer with sticky sessions allow distributing connections while ensuring messages reach the correct server maintaining session state.

🧠 Conceptual

intermediate

Understanding WebSocket handshake process

What is the main purpose of the WebSocket handshake between client and server?

A To close the HTTP connection after data transfer.

B To upgrade the HTTP connection to a persistent WebSocket connection.

C To encrypt the data transmitted over the connection.

D To authenticate the user credentials before connection.

💡 Hint

Consider what changes from HTTP to WebSocket during connection setup.

The handshake upgrades the HTTP connection to a persistent, full-duplex WebSocket connection allowing real-time communication.

❓ scaling

advanced

Handling message broadcasting in WebSocket systems

You want to broadcast messages from one client to all other connected clients in a distributed WebSocket server environment. Which approach ensures efficient and consistent message delivery?

A Use a centralized message broker like Redis Pub/Sub to distribute messages to all servers.

B Each WebSocket server broadcasts messages only to its connected clients without coordination.

C Clients send messages directly to all other clients without server involvement.

D Broadcast messages only to clients connected to the same server as the sender.

💡 Hint

Think about how to synchronize messages across multiple servers.

A centralized message broker like Redis Pub/Sub allows all WebSocket servers to receive and forward broadcast messages to their clients, ensuring consistency.

❓ tradeoff

advanced

Choosing between WebSocket and HTTP/2 for real-time updates

For a real-time dashboard showing live data updates, which tradeoff is true when choosing WebSocket over HTTP/2 server push?

A HTTP/2 server push is better for bidirectional communication than WebSocket.

B WebSocket connections are stateless and easier to scale than HTTP/2 streams.

C WebSocket provides full-duplex communication but requires more complex server management.

D HTTP/2 server push supports full-duplex communication natively without extra overhead.

💡 Hint

Consider communication direction and server complexity.

WebSocket supports full-duplex communication allowing both client and server to send messages anytime, but managing many connections is more complex than HTTP/2 streams.

❓ estimation

expert

Estimating server resources for WebSocket connections

You plan to support 500,000 concurrent WebSocket connections. Each connection uses 50 KB of memory on the server. How much total memory (in GB) is required just for maintaining these connections?

A Approximately 25 GB

B Approximately 50 GB

C Approximately 10 GB

D Approximately 5 GB

💡 Hint

Multiply connections by memory per connection and convert bytes to gigabytes.

500,000 connections * 50 KB = 25,000,000 KB = 25,000 MB = 25 GB.

Practice

1. What is the primary advantage of using WebSocket over traditional HTTP for real-time communication?

easy

A. It requires no server-side support to function.

B. It uses less bandwidth by compressing all messages automatically.

C. It keeps a persistent connection open for instant two-way data exchange.

D. It guarantees message delivery order without any additional setup.

Solution

1. Step 1: Understand WebSocket connection behavior

   WebSocket establishes a persistent connection between client and server, unlike HTTP which opens and closes connections per request.

2. Step 2: Identify the key benefit for real-time apps

   This persistent connection allows instant two-way communication, ideal for real-time updates.

3. Final Answer:

   It keeps a persistent connection open for instant two-way data exchange. -> Option C

4. Quick Check:

   Persistent connection = Instant two-way communication [OK]

Hint: WebSocket keeps connection open for fast two-way data [OK]

Common Mistakes:

• Confusing WebSocket with HTTP polling
• Assuming WebSocket compresses all messages automatically
• Thinking WebSocket works without server support

2. Which of the following is the correct syntax to open a WebSocket connection in JavaScript?

easy

A. const ws = new WebSocket('http://example.com/socket');

B. const ws = new WebSocket('ws://example.com/socket');

C. const ws = WebSocket.connect('ws://example.com/socket');

D. const ws = WebSocket.open('http://example.com/socket');

Solution

1. Step 1: Recall WebSocket URL scheme

   WebSocket URLs start with ws:// or wss:// for secure connections, not http://.

2. Step 2: Identify correct constructor usage

   The WebSocket object is created with new WebSocket(url), no .connect() or .open() methods exist.

3. Final Answer:

   const ws = new WebSocket('ws://example.com/socket'); -> Option B

4. Quick Check:

   WebSocket uses ws:// + new WebSocket() [OK]

Hint: WebSocket URLs start with ws:// or wss:// [OK]

Common Mistakes:

• Using http:// instead of ws://
• Calling non-existent connect() or open() methods
• Omitting the new keyword

3. Consider this simplified server-side pseudocode handling WebSocket messages:

onMessage(client, message) {
  broadcastToAllClients(message);
}

What will happen if 1000 clients send messages simultaneously?

medium

A. Server may become overwhelmed, causing message delays or dropped connections.

B. All messages will be broadcasted instantly to all clients without delay.

C. Only the first client's message will be broadcasted; others ignored.

D. Clients will automatically queue messages on their side until server is free.

Solution

1. Step 1: Analyze server load with simultaneous messages

   When many clients send messages at once, the server must process and broadcast each, increasing CPU and network load.

2. Step 2: Understand server resource limits

   Without scaling or message queuing, the server can get overwhelmed, causing delays or dropped connections.

3. Final Answer:

   Server may become overwhelmed, causing message delays or dropped connections. -> Option A

4. Quick Check:

   High load without scaling = delays/drops [OK]

Hint: High simultaneous messages can overload server without scaling [OK]

Common Mistakes:

• Assuming instant broadcast without delay
• Thinking clients queue messages automatically
• Believing only first message is processed

4. A developer wrote this WebSocket server snippet:

ws.on('message', (msg) => {
  ws.send(msg);
});

Why might this cause problems in a multi-client chat application?

medium

A. It closes the connection after sending the message.

B. It broadcasts the message to all clients, causing duplicates.

C. It causes syntax errors due to incorrect event name.

D. It only sends the message back to the sender, not to other clients.

Solution

1. Step 1: Understand the code behavior

   The code listens for a message from a client and sends the same message back only to that client (ws.send(msg)).

2. Step 2: Identify issue in multi-client context

   Other connected clients do not receive the message, so chat messages are not broadcasted properly.

3. Final Answer:

   It only sends the message back to the sender, not to other clients. -> Option D

4. Quick Check:

   ws.send(msg) sends only to sender [OK]

Hint: ws.send(msg) sends only to sender, not all clients [OK]

Common Mistakes:

• Assuming ws.send broadcasts to all clients
• Thinking event name 'message' is incorrect
• Believing connection closes after send

5. You are designing a scalable chat app using WebSocket. Which architectural component helps efficiently distribute messages to thousands of clients without overloading the main server?

hard

A. A message broker (e.g., Redis Pub/Sub) between WebSocket servers and clients.

B. A single powerful WebSocket server handling all connections directly.

C. Using HTTP long polling instead of WebSocket for all clients.

D. Storing all messages in a database before sending to clients.

Solution

1. Step 1: Understand scaling challenges with many clients

   A single server can become a bottleneck when handling thousands of WebSocket connections and broadcasting messages.

2. Step 2: Identify role of message broker

   A message broker like Redis Pub/Sub can distribute messages efficiently across multiple WebSocket servers, balancing load and improving scalability.

3. Final Answer:

   A message broker (e.g., Redis Pub/Sub) between WebSocket servers and clients. -> Option A

4. Quick Check:

   Message broker enables scalable message distribution [OK]

Hint: Use message broker to scale WebSocket message distribution [OK]

Common Mistakes:

• Relying on a single server for all connections
• Switching to HTTP long polling loses real-time benefits
• Storing messages in DB adds latency, not scaling