Group messaging in HLD - Architecture Diagram

System Overview - Group messaging

This system allows multiple users to send and receive messages within groups. It supports real-time message delivery, message storage, and offline message retrieval. The system must be scalable to handle many users and groups simultaneously.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Message Service <--> Message Queue
  |                 |
  v                 v
Cache             Database
  |
  v
Push Notification Service

Components

User

client

End user sending and receiving group messages

Load Balancer

load_balancer

Distributes incoming user requests evenly to API Gateway instances

API Gateway

api_gateway

Handles client requests, authentication, and routes to backend services

Message Service

service

Processes group messages, manages group membership, and interacts with storage

Message Queue

message_queue

Buffers messages for asynchronous processing and delivery

Cache

cache

Stores recent messages for fast retrieval and reduces database load

Database

database

Persists all group messages and user/group metadata reliably

Push Notification Service

service

Sends real-time notifications to users about new messages

Request Flow - 11 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Message Service

Message Service → Cache

Cache → Message Service

Message Service → Database

Database → Message Service

Message Service → Cache

Message Service → Message Queue

Message Queue → Push Notification Service

Push Notification Service → User

Failure Scenario

Component Fails:Database

Impact:New messages cannot be stored; message fetches may fail if cache misses occur

Mitigation:Reads served from cache if available; writes queued and retried later; database replication and failover configured

Architecture Quiz - 3 Questions

Which component handles user authentication and routes requests to backend services?

A API Gateway

B Load Balancer

C Message Service

D Cache

Design Principle

This architecture uses caching to speed up message retrieval and a message queue to decouple message processing from delivery, improving scalability and responsiveness. The API Gateway centralizes authentication and routing, while the load balancer ensures even distribution of user requests.

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