Python Chat Room

Introduction

• Socket-based, multi-threaded chat system built with Python
• Server: uses socket and threading for concurrent client handling
• Client: built with Tkinter, featuring event-driven UI and background message receiver thread
• Data & Security: user authentication via SQLite + SQLAlchemy, secured with SSL/TLS
• Protocol: custom JSON messaging validated by Pydantic

Updates

• Lobby and chat presence now refresh in real time; no manual refresh required to see online status changes.
• Password storage now includes a per-user salt, hardening credential security.

Design Overview

Server

1. The server is implemented using a multi-threaded architecture. Each incoming connection spawns a new thread.

2. Each thread is managed by a ClientHandler, which operates as a state machine to handle the full client interaction lifecycle.

3. Concurrency Control

   To ensure data consistency in a multi-threaded environment, shared in-memory session data (e.g., online_users, chat_rooms) is managed through a ChatData structure protected by a reentrant mutex lock.

4. Database

   User credentials (username and password) are stored in a SQLite database, accessed via SQLAlchemy Core.

5. Security

   1. Passwords are hashed before being stored.
   2. All client-server communication is secured using SSL/TLS encryption.

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Client

In the diagram, blue arrows represent server responses/pushes, and red arrows represent user-triggered UI requests.

1. The client’s main thread runs a Tkinter event loop, which handles both UI events (e.g., button clicks, message sending) and server responses or push events.

2. Decoupled Class Design

   1. ServerHandler manages all server communication. To prevent blocking the Tkinter event loop, it runs a dedicated receiver thread that continuously listens for incoming messages from the server and places them into a queue, acting as the producer.
   2. App runs on the main thread, handling page transitions and serving as the consumer, retrieving messages from the queue and dispatching them to the Dispatcher.
   3. Dispatcher is responsible for event routing—it establishes relationships between events and callbacks without requiring awareness of individual UI pages. Each page (LoginPage, LobbyPage, ChatRoomPage) registers its callbacks with the dispatcher.

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Communication Protocol

1. Message Framing

   Because TCP may cause packet concatenation (the “sticky packet” issue), the protocol adopts a length-prefixed design. Each message begins with a 4-byte header indicating message length. The application maintains an internal buffer to reconstruct complete messages.

2. Message Format

   The client and server communicate using a custom JSON-based protocol, validated using Pydantic to ensure schema correctness.

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How to run it?

1. Generating a Self-Signed SSL/TLS Certificate

   This project uses a self-signed certificate for demonstration purposes. To generate server.key, server.csr, and server.crt, use the following commands:

   # 1. Generate a private key (server.key)
   openssl genrsa -out server.key 2048
   
   # 2. Create a certificate signing request (server.csr)
   openssl req -new -key server.key -out server.csr
   
   # 3. Generate a self-signed certificate valid for 365 days (server.crt)
   openssl x509 -req -days 365 -in server.csr -signkey server.key -out server.crt

2. Virtual environment

   pip install -r requirements.txt

3. Run the server and client

   python server.py
   python client.py

How to Use

1. Register an account and log in.

   

2. On the Lobby page, you can:

   

   • Create a new room
   • Enter an existing room
   • Log out
   • Click the Refresh button to update the list of online users and rooms.

3. On the Chatroom page, you can:

   

   • Send messages
   • Refresh the online user list
   • Return to the lobby
   • To send a private message to a specific user, use the format:

     \private <receiver> <your message>
     

Message Format References

Client → Server

1. Authentication

Used for user registration and login.

{ "type": "register", "data": { "username": "harper", "password": "1234" } }
{ "type": "login",    "data": { "username": "harper", "password": "1234" } }

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2. Lobby Operations

Commands available in the lobby state.

{ "type": "list" }                                       // List all online users
{ "type": "logout" }                                     // Log out from the server
{ "type": "enter",  "data": { "room": "tech_talk" } }    // Enter an existing chatroom
{ "type": "create", "data": { "room": "tech_talk" } }    // Create a new chatroom

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3. Chatroom Operations

Commands available after entering a chatroom.

{ "type": "exit" }                                       // Leave current chatroom
{ "type": "list" }                                       // List users in current room
{ "type": "msg", "data": { "from": "sender", "to": "receiver", "text": "Hello everyone!" } } // Send message

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Server → Client

1. Authentication Responses

1. Success

   {
     "type": "login|register",
     "status": "ok",
     "data": { "username": "harper", "chatroom": "lobby" },
     "message": "Login successful"
   }
   

2. Failure

   {
     "type": "login|register",
     "status": "error",
     "message": "Password incorrect"
   }
   

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2. Lobby Responses

1. User/Room List

   {
     "type": "list",
     "status": "ok",
     "data": {
       "room1": ["user1", "user2", "user3"],
       "room2": ["user4"],
       "room3": []
     }
   }
   

2. Enter Room – Success

   {
     "type": "enter",
     "status": "ok",
     "data": { "username": "username", "room": "chatroom" },
     "message": "Welcome to the chatroom"
   }
   

3. Enter Room – Failure

   {
     "type": "enter",
     "status": "error",
     "message": "Error message"
   }
   

4. Create Room – Success

   {
     "type": "create",
     "status": "ok",
     "message": "room_name created successfully"
   }
   

5. Create Room – Failure

   {
     "type": "create",
     "status": "error",
     "message": "Error message"
   }
   

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3. Chatroom Responses

1. Exit – Success

   {
     "type": "exit",
     "status": "ok",
     "message": "Exited chatroom, returned to lobby"
   }
   

2. Exit – Failure

   {
     "type": "exit",
     "status": "error",
     "message": "Error message"
   }
   

3. New Message (Broadcast or Private)

   Success:

   {
     "type": "msg",
     "status": "ok",
     "data": { "to": "public", "from": "sender", "text": "Hello everyone!" }
   }
   

   Error:

   {
     "type": "msg",
     "status": "error",
     "message": "Error message"
   }
   

TODO

1. Add friends