eugenhotaj / talko

A client-server chat app in Python.

🌮 Talko

A fully functioning chat application built entierly from scratch -- no HTTP, no requests, no frameworks.

Talko in action. (Left) The web application UI. (Right) The ncurses UI.

Talko began as an exercise to understand how the internet works by reinventing parts of it from the ground up. We use TCP sockets as the foundation and build all other components from first principles, including a simple transfer protocol, a request/response RPC protocol, and a message streaming protocol. We use these protocols to build a fully functioning chat application and hook up two front ends to it, an ncurses terminal UI, and a web application hosted here.

Installation

To try out Talko, clone the repository, run the commands below, then go to http://localhost:5000.

git clone http://www.github.com/eugenhotaj/talko
cd talko
pip install -r requirements.txt
python3 main.py \
    --ui_client=webapp \ 
    --db_path=database.db \
    --recreate_db=True \
    --insert_fake_chat=True

Design

High level architecture of the Talko application.

At a high level, Talko is designed as a client-server microservice architecture. The client communicates primarily with the DataServer to read and write chat conversation data. To handle real-time messaging, the client opens a TCP socket connection with the BroadcastServer. Although the socket stream is bi-directional, information only flows from the BroadcastServer to the client.

To send a chat message, the client issues an InsertMessageRequest to the DataServer. The DataServer first stores the message in the database then sends a BroadcastMessageRequest to the BroadcastServer with the user_ids of the message recipients. Finally, the BroadcastServer looks up the TCP socket of each connected recipient and broadcasts the message to the recipient.

In a previous design, the client would issues InsertMessageRequests to the BroadcastServer, which would then both store the new message (by forwarding the InsertMessageRequest to DataServer) as well as broadcast it to online recipients. This greatly confused the interactions between the clients and servers and lead to a huge bottleneck in the BroadcastServer as the server had to dedicate separate threads to each connected client, even if the client was idle.

We'll now go into more detail on each of the application components.

Storage

Talko uses a SQLite database for ease of experimentation. The full database schema is available in schema.sql. We store chat conversations across 3 tables, Chats, Messages, and Participants. The Participants table is a key-value map from chat_id -> user_id. Splitting out the participants from the chats allows us to easily handle both private (i.e one-on-one) chats and group chats.

Protocols

The client and servers communicate with each other by sending and receiving JSON-RCP messages. The communication protocol is built entirely on top of TCP sockets and is implemented in socket_lib.py. Each message consists of a 10 byte header followed by a utf-8 encoded binary payload. The header simply encodes the size of the payload in bytes. To receive a message, we first read 10 bytes from the socket stream then read the rest of the n byte payload (where n is the value encoded in the first 10 bytes).

TCP sockets are primarily designed to stream data bi-directionally and do not inherently have a concept of requests/responses. We enforce this aspect by ensuring that each client request receives exactly one server response, after which the server closes the connection with the client. To send further requests, the client must reconnect to the server using a new socket. However, the BroadcastServer keeps some connections open in order to broadcast chat messages in real-time.

The above more or less covers how the clients and servers communicate. What they communicate is defined by a custom RPC protocol, implemented in protocol.py.

Front Ends

A client library, implemented in client.py abstracts away communicating with both servers behind a single API. This API can then be used to build a variety of front end, such as an ncurses terminal front end (implemented in ui/curses.py), a web application frontend (implemented in ui/webapp), etc.

The client library supports both message streaming via (async) generators as well as long polling.

(Potential) Future Work

• Login screen with the ability to create new user accounts
• TLS and end-to-end encryption
• Robustification -- e.g. handling bad input, dropped requests, etc
• Production database and SQL tuning