chat

Demo application for asyncio, for teaching purposes.

The goal is to show how to build a non-trivial application, since a common (and justified) criticism of many asyncio tutorials is that they're too simple to be useful for building real-world applications.

How To Run

Requires Python >= 3.7

1. Open a new terminal, run $ python srv.py
2. Open a new terminal, run $ python client.py
3. Open a second new terminal, run $ python client.py

There should be two GUI windows open, and if you type a message into the bottom edit box and hit ENTER, you should see the message appear in the other GUI.

Demonstrated Techniques

This is a chat client/server application, perhaps like IRC or Slack. These are the specific features that are demonstrated in the demo code:

• server application with many concurrent connections
  • using asyncio.start_server()
• separate client application that connects to the server. Multiple client instances can be started.
  • using asyncio.open_connection()
  • handle server outage with a reconnection loop
• new asyncio APIs like asyncio.run() and asyncio.create_task
• Using the asyncio Streams API
• decoupling tasks for sending and receiving
• SSL connections
• client application has a Tkinter GUI.
  • show how to run the asyncio loop in a separate thread
  • Tkinter UI loop runs in the main thread
  • This is a similar problem you will encounter with many game frameworks
• Signal handling to control shutdown
  • Linux: SIGINT and SIGTERM
  • Windows: SIGINT (actually CTRL-C) and SIGBREAK
• Keeping track of current connections/clients, using weakref containers to make it easier to stay up-to-date.
• Using queues in the client UI application to pass data between the main thread (UI) and the asyncio thread (IO)
• Creating a message protocol, by using a size prefix, and converting from the stream bytes to an integer using int.frombytes()
• Passing JSON between client and server
• A Tkinter GUI app that can be sized and maximised

ssl

See Doug Hellman's block for how to set up the SSL connection. In particular, for generating the cert and key:

$ openssl req -newkey rsa:2048 -nodes -keyout chat.key \
    -x509 -days 365 -out chat.crt

This creates chat.key and chat.crt in the current dir.