Deploy to Heroku

This guide describes how to deploy a websockets server to Heroku. The same principles should apply to other Platform as a Service providers.

We’re going to deploy a very simple app. The process would be identical for a more realistic app.

Create application

Deploying to Heroku requires a git repository. Let’s initialize one:

$ mkdir websockets-echo
$ cd websockets-echo
$ git init -b main
Initialized empty Git repository in websockets-echo/.git/
$ git commit --allow-empty -m "Initial commit."
[main (root-commit) 1e7947d] Initial commit.

Follow the set-up instructions to install the Heroku CLI and to log in, if you haven’t done that yet.

Then, create a Heroku app — if you follow these instructions step-by-step, you’ll have to pick a different name because I’m already using websockets-echo on Heroku:

$ heroku create websockets-echo
Creating ⬢ websockets-echo... done
https://websockets-echo.herokuapp.com/ | https://git.heroku.com/websockets-echo.git

Here’s the implementation of the app, an echo server. Save it in a file called app.py:

#!/usr/bin/env python

import asyncio
import signal
import os

import websockets


async def echo(websocket):
    async for message in websocket:
        await websocket.send(message)


async def main():
    # Set the stop condition when receiving SIGTERM.
    loop = asyncio.get_running_loop()
    stop = loop.create_future()
    loop.add_signal_handler(signal.SIGTERM, stop.set_result, None)

    async with websockets.serve(
        echo,
        host="",
        port=int(os.environ["PORT"]),
    ):
        await stop


if __name__ == "__main__":
    asyncio.run(main())

Heroku expects the server to listen on a specific port, which is provided in the $PORT environment variable. The app reads it and passes it to serve().

Heroku sends a SIGTERM signal to all processes when shutting down a dyno. When the app receives this signal, it closes connections and exits cleanly.

Deploy application

In order to build the app, Heroku needs to know that it depends on websockets. Create a requirements.txt file containing this line:

websockets

Heroku also needs to know how to run the app. Create a Procfile with this content:

web: python app.py

Confirm that you created the correct files and commit them to git:

$ ls
Procfile         app.py           requirements.txt
$ git add .
$ git commit -m "Deploy echo server to Heroku."
[main 8418c62] Deploy echo server to Heroku.
 3 files changed, 32 insertions(+)
 create mode 100644 Procfile
 create mode 100644 app.py
 create mode 100644 requirements.txt

The app is ready. Let’s deploy it!

$ git push heroku

... lots of output...

remote: -----> Launching...
remote:        Released v1
remote:        https://websockets-echo.herokuapp.com/ deployed to Heroku
remote:
remote: Verifying deploy... done.
To https://git.heroku.com/websockets-echo.git
 * [new branch]      main -> main

Validate deployment

Of course you’d like to confirm that your application is running as expected!

Since it’s a WebSocket server, you need a WebSocket client, such as the interactive client that comes with websockets.

If you’re currently building a websockets server, perhaps you’re already in a virtualenv where websockets is installed. If not, you can install it in a new virtualenv as follows:

$ python -m venv websockets-client
$ . websockets-client/bin/activate
$ pip install websockets

Connect the interactive client — using the name of your Heroku app instead of websockets-echo:

$ python -m websockets wss://websockets-echo.herokuapp.com/
Connected to wss://websockets-echo.herokuapp.com/.
>

Great! Your app is running!

In this example, I used a secure connection (wss://). It worked because Heroku served a valid TLS certificate for websockets-echo.herokuapp.com. An insecure connection (ws://) would also work.

Once you’re connected, you can send any message and the server will echo it, then press Ctrl-D to terminate the connection:

> Hello!
< Hello!
Connection closed: 1000 (OK).

You can also confirm that your application shuts down gracefully. Connect an interactive client again — remember to replace websockets-echo with your app:

$ python -m websockets wss://websockets-echo.herokuapp.com/
Connected to wss://websockets-echo.herokuapp.com/.
>

In another shell, restart the dyno — again, replace websockets-echo with your app:

$ heroku dyno:restart -a websockets-echo
Restarting dynos on ⬢ websockets-echo... done

Go back to the first shell. The connection is closed with code 1001 (going away).

$ python -m websockets wss://websockets-echo.herokuapp.com/
Connected to wss://websockets-echo.herokuapp.com/.
Connection closed: 1001 (going away).

If graceful shutdown wasn’t working, the server wouldn’t perform a closing handshake and the connection would be closed with code 1006 (connection closed abnormally).