Real-time features in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Designing a scalable chat system with real-time messaging

You are tasked with designing a chat system that supports real-time messaging between millions of users. Which architectural component is essential to ensure messages are delivered instantly and reliably?

A A batch processing system to send messages every few minutes

B A relational database to store all messages synchronously

C A message broker or pub/sub system to handle message distribution

D A static file server to serve chat logs

💡 Hint

Think about how messages can be pushed to many users instantly without delay.

A message broker or pub/sub system allows real-time distribution of messages to connected clients, ensuring low latency and scalability. Databases and batch processing are too slow for instant delivery, and static file servers cannot push updates.

❓ scaling

intermediate

Scaling WebSocket connections for real-time updates

Your application uses WebSocket connections to push real-time updates to users. As user count grows to millions, what is the best approach to scale WebSocket servers?

A Distribute connections across multiple servers with a load balancer and share state via a centralized message broker

B Use a single powerful server to handle all WebSocket connections

C Switch to HTTP polling to reduce server load

D Store all WebSocket messages in a database before sending

💡 Hint

Consider how to handle many simultaneous connections and keep clients updated consistently.

Scaling WebSocket connections requires multiple servers behind a load balancer. A centralized message broker helps synchronize messages across servers so clients receive consistent updates. A single server cannot handle millions of connections, polling is inefficient, and databases add latency.

❓ tradeoff

advanced

Choosing between WebSocket and Server-Sent Events (SSE)

You need to implement a real-time notification system for a web app. Which tradeoff is true when choosing WebSocket over SSE?

A SSE supports binary data natively, WebSocket only supports text

B WebSocket supports full-duplex communication, allowing client and server to send messages independently, while SSE is unidirectional from server to client

C WebSocket connections are stateless, SSE connections maintain state

D SSE requires a separate protocol, WebSocket uses standard HTTP

💡 Hint

Think about the direction and type of communication each protocol supports.

WebSocket allows two-way communication (full-duplex), so both client and server can send messages anytime. SSE only allows server-to-client messages (unidirectional). WebSocket supports binary and text, SSE supports only text. Both use HTTP for connection setup.

🧠 Conceptual

advanced

Ensuring message ordering in a distributed real-time system

In a distributed real-time messaging system, what is the best approach to guarantee that messages are delivered to clients in the exact order they were sent?

A Use multiple message brokers without synchronization

B Send messages in parallel from different servers to reduce latency

C Allow each server to send messages independently without coordination

D Use a centralized message queue that preserves message order and assign sequence numbers to messages

💡 Hint

Think about how to keep track of message sequence across distributed components.

A centralized message queue that preserves order and sequence numbers ensures clients receive messages in the correct order. Independent servers or unsynchronized brokers can cause out-of-order delivery. Parallel sending without coordination risks message reordering.

❓ estimation

expert

Estimating server capacity for a real-time stock ticker system

You are designing a real-time stock ticker system that pushes price updates to 5 million users. Each user receives 10 updates per second. Each update message is approximately 500 bytes. Estimate the total outgoing bandwidth required per second.

A Approximately 25 gigabytes per second

B Approximately 2.5 gigabytes per second

C Approximately 250 megabytes per second

D Approximately 50 gigabytes per second

💡 Hint

Calculate total messages per second and multiply by message size, then convert bytes to gigabytes.

Total messages per second = 5,000,000 users * 10 updates = 50,000,000 messages/sec. Total bytes per second = 50,000,000 * 500 = 25,000,000,000 bytes/sec = 25 GB/sec.