One-to-one messaging in HLD - Architecture Diagram

System Overview - One-to-one messaging

This system enables two users to send messages directly to each other in real time. It must ensure message delivery, ordering, and persistence while handling many users concurrently.

Architecture Diagram

User A  User B
  |       |
  v       v
+-----------------+
|   Load Balancer  |
+-----------------+
         |
         v
+-----------------+
|   API Gateway   |
+-----------------+
         |
         v
+-----------------+      +-------------+
| Messaging Service|<----| Message DB  |
+-----------------+      +-------------+
         |
         v
+-----------------+
|   Cache Layer   |
+-----------------+

Components

User A

user

Sends and receives messages

User B

user

Sends and receives messages

Load Balancer

load_balancer

Distributes incoming user connections evenly to API Gateway instances

API Gateway

api_gateway

Handles client requests, authentication, and routes to Messaging Service

Messaging Service

service

Processes messages, ensures delivery, ordering, and stores messages

Message DB

database

Stores message history persistently

Cache Layer

cache

Caches recent messages for fast retrieval

Request Flow - 10 Hops

User A → Load Balancer

Load Balancer → API Gateway

API Gateway → Messaging Service

Messaging Service → Cache Layer

Messaging Service → Message DB

Messaging Service → Cache Layer

Messaging Service → API Gateway

API Gateway → Load Balancer

Load Balancer → User A

Messaging Service → User B

Failure Scenario

Component Fails:Message DB

Impact:New messages cannot be stored persistently; message history retrieval fails. Recent messages may still be served from cache.

Mitigation:Use database replication and failover to a standby DB; cache serves recent messages temporarily; alert system triggers for manual intervention.

Architecture Quiz - 3 Questions

Which component is responsible for distributing incoming user connections?

A Load Balancer

B API Gateway

C Messaging Service

D Cache Layer

Design Principle

This design uses a layered approach with load balancing, API gateway for routing and authentication, a dedicated messaging service for business logic, caching for performance, and a persistent database for durability. It ensures scalability, fault tolerance, and low latency for one-to-one messaging.

Practice

1. What is the primary role of the server in a one-to-one messaging system?

easy

A. To allow users to send messages directly without any intermediary

B. To store all messages permanently without delivering them

C. To receive messages from the sender and deliver them to the receiver

D. To broadcast messages to all users in the system

Solution

1. Step 1: Understand message flow in one-to-one messaging

   Messages are sent from one user to another through a server acting as an intermediary.

2. Step 2: Identify server's role

   The server receives messages from the sender and forwards them to the intended receiver, ensuring delivery and possibly storing temporarily.

3. Final Answer:

   To receive messages from the sender and deliver them to the receiver -> Option C

4. Quick Check:

   Server acts as message relay = B [OK]

Hint: Server relays messages between users, not direct connection [OK]

Common Mistakes:

• Thinking users connect directly without server
• Assuming server only stores without forwarding
• Confusing one-to-one with broadcast messaging

2. Which component is essential to ensure message privacy in a one-to-one messaging system?

easy

A. End-to-end encryption

B. Message queue

C. Load balancer

D. Caching layer

Solution

1. Step 1: Identify privacy needs in messaging

   Privacy means only sender and receiver can read the message content.

2. Step 2: Match privacy solution

   End-to-end encryption encrypts messages so only sender and receiver can decrypt them, ensuring privacy.

3. Final Answer:

   End-to-end encryption -> Option A

4. Quick Check:

   Privacy needs encryption = C [OK]

Hint: Privacy means encrypt messages end-to-end [OK]

Common Mistakes:

• Confusing load balancer as privacy tool
• Thinking caching secures messages
• Assuming message queue provides privacy

3. Consider this simplified message flow code snippet for one-to-one messaging:

def send_message(sender, receiver, message):
    server.store_message(sender, receiver, message)
    server.deliver_message(receiver)

send_message('Alice', 'Bob', 'Hello')

What is the expected output or result of this code?

medium

A. Syntax error due to missing parameters

B. Message is delivered to Alice instead of Bob

C. Message is stored but never delivered

D. Message 'Hello' is stored and delivered to Bob

Solution

1. Step 1: Analyze function calls

   The function stores the message from Alice to Bob, then delivers it to Bob.

2. Step 2: Check message flow correctness

   Message is stored correctly and delivery is triggered for Bob, the intended receiver.

3. Final Answer:

   Message 'Hello' is stored and delivered to Bob -> Option D

4. Quick Check:

   Store then deliver to receiver = D [OK]

Hint: Store then deliver to receiver means message sent correctly [OK]

Common Mistakes:

• Confusing sender and receiver in delivery
• Assuming message is not delivered after storing
• Thinking code has syntax errors

4. In a one-to-one messaging system, this code snippet causes messages to never reach the receiver:

def deliver_message(receiver, message):
    if receiver.is_online == False:
        return
    send_to_client(receiver, message)

What is the main issue causing message delivery failure?

medium

A. The function sends messages to the wrong user

B. Messages are dropped if receiver is offline without queuing

C. There is a syntax error in the if condition

D. The message variable is undefined

Solution

1. Step 1: Understand delivery condition

   The function returns early if the receiver is offline, skipping delivery.

2. Step 2: Identify missing handling

   Messages are not queued or stored for later delivery, so offline users never get messages.

3. Final Answer:

   Messages are dropped if receiver is offline without queuing -> Option B

4. Quick Check:

   Offline receiver drops messages = A [OK]

Hint: Offline users need message queue to avoid drops [OK]

Common Mistakes:

• Thinking syntax error causes failure
• Assuming wrong user receives message
• Assuming offline messages are automatically queued

5. You are designing a scalable one-to-one messaging system for millions of users. Which approach best ensures message delivery even if the receiver is offline?

hard

A. Use a message queue to store undelivered messages and retry delivery when receiver is online

B. Drop messages if receiver is offline to save storage and bandwidth

C. Broadcast messages to all users and let receiver filter them

D. Send messages only when both users are online simultaneously

Solution

1. Step 1: Understand offline delivery challenge

   Receivers may be offline, so messages must be stored to avoid loss.

2. Step 2: Choose scalable solution

   Message queues store undelivered messages and retry delivery when receiver reconnects, ensuring reliability and scalability.

3. Final Answer:

   Use a message queue to store undelivered messages and retry delivery when receiver is online -> Option A

4. Quick Check:

   Queue undelivered messages for offline users = A [OK]

Hint: Queue messages for offline users to ensure delivery [OK]

Common Mistakes:

• Dropping messages loses data
• Broadcast wastes resources and breaks privacy
• Requiring both online limits usability