WebSocket for real-time communication in HLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to specify the protocol used for real-time communication.

HLD

The client initiates a connection using the [1] protocol.

Drag options to blanks, or click blank then click option'

AWebSocket

BHTTP

CFTP

DSMTP

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing HTTP which is request-response only.

Selecting FTP or SMTP which are unrelated to real-time web communication.

The WebSocket protocol enables full-duplex communication channels over a single TCP connection, ideal for real-time data exchange.

2fill in blank

medium

Complete the code to identify the server component that handles WebSocket connections.

HLD

The [1] manages incoming WebSocket connections and message routing.

Drag options to blanks, or click blank then click option'

AWebSocket server

BLoad balancer

CDatabase server

DCache server

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing load balancer with connection handler.

Selecting database or cache servers which do not manage WebSocket connections.

The WebSocket server is responsible for accepting connections and managing message flow between clients.

3fill in blank

hard

Fix the error in the connection upgrade process code snippet.

HLD

HTTP request header must include 'Upgrade: [1]' to switch protocols.

Drag options to blanks, or click blank then click option'

AHTTP/2

BTLS

CWebSocket

DFTP

Attempts:

3 left

💡 Hint

Common Mistakes

Using HTTP/2 which is a different protocol.

Using TLS or FTP which are unrelated to WebSocket upgrade.

The 'Upgrade' header must specify 'websocket' to initiate the protocol switch from HTTP to WebSocket.

4fill in blank

hard

Fill both blanks to complete the message flow in WebSocket communication.

HLD

Client sends a [1] frame, and server responds with a [2] frame.

Drag options to blanks, or click blank then click option'

Atext

Bbinary

Cpong

Dping

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'ping' and 'pong' frames.

Using 'text' or 'binary' frames for heartbeat messages.

In WebSocket, a client sends a 'ping' frame to check connection health, and the server replies with a 'pong' frame.

5fill in blank

hard

Fill all three blanks to complete the architecture components for scalable WebSocket service.

HLD

Use a [1] to distribute connections, a [2] to maintain session state, and a [3] to handle message broadcasting.

Drag options to blanks, or click blank then click option'

Aload balancer

Bsession store

Cmessage broker

Ddatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Using database instead of message broker for broadcasting.

Confusing session store with load balancer.

A load balancer distributes client connections across servers, a session store keeps track of user sessions, and a message broker efficiently broadcasts messages to clients.

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