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.

Practice

1. What is the primary purpose of a group messaging system?

easy

A. To allow multiple users to send and receive messages in a shared conversation

B. To store user passwords securely

C. To manage user profile pictures

D. To provide video streaming services

Solution

1. Step 1: Understand group messaging basics

   Group messaging connects multiple users so they can communicate together in one conversation.

2. Step 2: Identify the main function

   The main function is sending and receiving messages among group members, not unrelated features like password storage or video streaming.

3. Final Answer:

   To allow multiple users to send and receive messages in a shared conversation -> Option A

4. Quick Check:

   Group messaging = shared conversation [OK]

Hint: Focus on the core feature: multi-user message exchange [OK]

Common Mistakes:

• Confusing group messaging with unrelated features
• Thinking it only supports one-to-one chat
• Ignoring the shared conversation aspect

2. Which component is essential for managing who belongs to a group in a group messaging system?

easy

A. Group management

B. Notification service

C. Message storage

D. Media transcoding

Solution

1. Step 1: Identify components related to user membership

   Group management handles adding, removing, and listing members in a group.

2. Step 2: Exclude unrelated components

   Message storage saves messages, notification service alerts users, and media transcoding processes media, none manage group membership.

3. Final Answer:

   Group management -> Option A

4. Quick Check:

   Group membership = Group management [OK]

Hint: Group membership is handled by group management component [OK]

Common Mistakes:

• Confusing message storage with membership control
• Assuming notification service manages members
• Mixing media processing with group functions

3. Consider a group messaging system where each message is sent to all group members. If a group has 100 members and one message is sent, how many message deliveries occur?

medium

A. 1

B. 50

C. 101

D. 100

Solution

1. Step 1: Understand message delivery in group messaging

   Each message is delivered to every member of the group.

2. Step 2: Calculate total deliveries

   With 100 members, one message results in 100 deliveries (one per member).

3. Final Answer:

   100 -> Option D

4. Quick Check:

   Deliveries = group size = 100 [OK]

Hint: One message reaches all members, so deliveries = group size [OK]

Common Mistakes:

• Counting the sender as extra delivery
• Assuming half the group receives the message
• Confusing message count with delivery count

4. A group messaging system stores messages but users report delays in receiving messages. Which issue is most likely causing this delay?

medium

A. Message storage is too fast

B. Notification service is slow or failing

C. Group management is adding too many users

D. User profile pictures are too large

Solution

1. Step 1: Identify components involved in message delivery

   Notification service alerts users about new messages; if slow, users get delayed messages.

2. Step 2: Exclude unrelated causes

   Message storage speed does not cause delay in delivery; group management and profile pictures do not affect message delivery timing.

3. Final Answer:

   Notification service is slow or failing -> Option B

4. Quick Check:

   Delivery delay = notification issue [OK]

Hint: Delivery delays usually come from notification failures [OK]

Common Mistakes:

• Blaming message storage speed
• Thinking group size causes delay directly
• Ignoring notification service role

5. You are designing a scalable group messaging system for millions of users. Which approach best ensures message delivery without overloading servers?

hard

A. Send each message directly from sender to every recipient synchronously

B. Store messages only on sender's device and rely on manual forwarding

C. Use a message queue to asynchronously distribute messages to group members

D. Limit group size to 10 users to reduce load

Solution

1. Step 1: Understand scalability challenges

   Direct synchronous sending to many users overloads servers and causes delays.

2. Step 2: Identify scalable solution

   Using message queues allows asynchronous, reliable, and scalable message distribution without blocking sender or servers.

3. Step 3: Exclude impractical options

   Storing messages only on sender device or limiting group size reduces usability and scalability.

4. Final Answer:

   Use a message queue to asynchronously distribute messages to group members -> Option C

5. Quick Check:

   Scalable delivery = asynchronous queue [OK]

Hint: Use asynchronous queues for scalable message delivery [OK]

Common Mistakes:

• Trying synchronous delivery to all users
• Ignoring asynchronous processing benefits
• Reducing group size instead of scaling design