Publish–Subscribe Notification System (Python Sockets)

A simple TCP socket-based publish/subscribe system where each client can act as:

• Subscriber (subscribe to topics and receive updates)
• Publisher (post messages to topics)

The server routes published messages to all subscribers of the selected topic.

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Features

• Multi-client support using threads
• Topic-based message distribution
• Shared topic definitions via topics.py
• Real-time notifications to subscribed clients
• Single client app that can both publish and subscribe

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Project Structure

publish_subscribe_notification/
├── server.py      # TCP server, subscription store, broadcast logic
├── client.py      # Interactive publisher/subscriber client
├── topics.py      # Allowed topics shared by server and client
└── README.md

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Requirements

• Python 3.8+
• No external dependencies (uses standard library only)

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You can access the website at : https://netthreads.up.railway.app/

How to Run

1) Start the server

python server.py

The server listens on:

• Host: 0.0.0.0
• Port: 5000

2) Start one or more clients

Open a new terminal for each client:

python client.py

By default, the client connects to 127.0.0.1:5000.

If your server is on another machine, update SERVER_HOST in client.py.

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Client Commands

Inside the client prompt:

• topics → list all available topics
• subscribe <topic> → subscribe to a topic
• unsubscribe <topic> → request unsubscribe from a topic
• post <topic> <message> → publish a message to a topic
• help → show command help
• quit → close client

Example:

> subscribe sports
> post sports Messi scores in stoppage time!

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Message Protocol (Client ↔ Server)

The system uses a simple |-separated command protocol:

• Subscribe: SUBSCRIBE|<topic>
• Unsubscribe: UNSUBSCRIBE|<topic>
• Post: POST|<topic>|<message>

Broadcast format from server to subscribers:

[TOPIC] message text

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Topics

Defined in topics.py:

• sports
• world_news
• national_news
• pesu_live
• music
• gaming
• hollywood
• bollybuzz
• stocks

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Demo Walkthrough (2 Subscribers + 1 Publisher)

Open 4 terminals total:

• Terminal 1: server
• Terminal 2: Client A (subscriber)
• Terminal 3: Client B (subscriber)
• Terminal 4: Client C (publisher)

Terminal 1 (Server)

python server.py
Connected: ('127.0.0.1', 54321)
Connected: ('127.0.0.1', 54322)
Connected: ('127.0.0.1', 54323)

Terminal 2 (Client A)

python client.py
> subscribe sports
[NOTIFICATION] Subscribed to sports

Terminal 3 (Client B)

python client.py
> subscribe sports
[NOTIFICATION] Subscribed to sports

Terminal 4 (Client C)

python client.py
> post sports Match starts at 7 PM today

What subscribers see

Both Client A and Client B receive:

[NOTIFICATION] [SPORTS] Match starts at 7 PM today

This confirms the publish/subscribe flow is working.

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Current Notes / Limitations

• The client sends UNSUBSCRIBE, but the server must implement handling for this command to fully support unsubscribe behavior.
• Current server message parsing is minimal and assumes valid command format.
• No authentication or encryption (for learning/demo use).

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Suggested Improvements

• Add UNSUBSCRIBE handling in server logic
• Prevent duplicate subscriptions per client/topic
• Improve input validation and error responses
• Add graceful shutdown and cleanup of disconnected sockets
• Add logging and unit tests

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License

This project is for educational use.