Group messaging in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Identify the correct architecture for a scalable group messaging system

You want to design a group messaging system that supports millions of users sending messages in real-time. Which architecture best supports scalability and low latency?

A A microservices architecture with separate services for user management, message routing, and storage, using message queues for communication.

B A single database storing all messages with clients polling the database every second for new messages.

C A peer-to-peer network where each client directly sends messages to all group members without servers.

D A monolithic server handling all message routing and storage for all groups.

💡 Hint

Think about how to separate concerns and handle high load efficiently.

Microservices with message queues allow scaling individual components and decouple message routing from storage, supporting millions of users with low latency.

❓ scaling

intermediate

Estimate the number of servers needed for message delivery

Your group messaging system expects 10 million active users sending an average of 20 messages per day. Assume average group size of 130 members. Each server can handle 1000 messages per second. How many servers are needed to handle peak load assuming peak traffic is 10% of daily messages concentrated in 1 hour?

A 1200 servers

B 720 servers

C 600 servers

D 360 servers

💡 Hint

Calculate total messages per hour during peak, apply fanout for deliveries, then divide by server capacity.

Daily sends = 10M × 20 = 200M. Peak hour sends = 10% = 20M sends/hour ≈ 20M / 3600 ≈ 5556 sends/sec. With average group size 130, peak deliveries/sec ≈ 5556 × 130 ≈ 722K. Servers needed ≈ 722K / 1000 ≈ 722 (720 servers).

❓ tradeoff

advanced

Choose the best data storage approach for group messages

Which data storage approach balances fast message retrieval and efficient storage for a group messaging system with millions of groups?

A Keep all messages in memory on a single server for fastest access.

B Store all messages in a single relational database table indexed by group ID and timestamp.

C Store messages as files on a shared network file system organized by group folders.

D Use a distributed NoSQL database partitioned by group ID with message data stored in append-only logs.

💡 Hint

Consider scalability and write/read patterns for group messaging.

NoSQL databases partitioned by group ID allow horizontal scaling and fast writes with append-only logs, which suit high write throughput and efficient retrieval.

🧠 Conceptual

advanced

Understand message delivery guarantees in group messaging

Which message delivery guarantee ensures that every message sent to a group is received exactly once by all group members, even if some servers fail?

A Exactly once delivery

B Best effort delivery

C At least once delivery

D At most once delivery

💡 Hint

Think about avoiding duplicates and message loss.

Exactly once delivery guarantees each message is received once and only once, preventing duplicates and losses even during failures.

❓ component

expert

Trace the request flow for sending a message in a large group

In a group messaging system, what is the correct sequence of components involved when a user sends a message to a large group?

A 1,3,2,4

B 2,1,3,4

C 1,2,3,4

D 1,2,4,3

💡 Hint

Follow the logical flow from client to delivery.

The user sends the message to the API gateway, which forwards it to the routing service. The routing service stores and publishes the message, then the delivery service pushes it to members.