Auto-reloading of code changes with Django development in Docker with Gunicorn

Thanks to kikicarbonell, I looked into having a volume for my code, and after looking at the Docker Compose recommended Django setup, I added volumes: - .:/code to my web container in docker-compose.yml, and now any code changes I make automatically apply.

## docker-compose.yml:web:  restart: always  build: .  expose:    - "8000"  links:    - postgres:postgres  volumes:    - /usr/src/app/static    - .:/code  env_file: .env  command: /usr/local/bin/gunicorn myapp.wsgi:application -w 2 -b :8000 --reload

Update: for a thorough example of using Gunicorn and Django with Docker, checkout this example project from Rackspace, which also shows how to use docker-machine to launch the setup on remote servers like Rackspace Cloud.

Caveat: currently, this method does not work when your code is locally and the docker host is remote (e.g., on a cloud provider like Digital Ocean or Rackspace). This also applies to virtual machines if your local file system is not mounted on the VM. Note that there are separate volume drivers (e.g., flocker), and there might be something out there to address this need. For now, the "fix" is to rsync/scp your files up to a directory on the remote docker host. Then, the --reload flag will auto-reload gunicorn after any scp/rsync. Update: If pushing code to remove docker host, I find it far easier to just rebuild the docker container (e.g., docker-compose build web && docker-compose up -d). This can be slower though than the rsync approach if your src folder is large.

You have another problem- Docker caches each layer that it builds. You shouldn't have to re-run pip install every time!

ADD . /code/RUN pip install -r /code/requirements/docker.txt

This is your problem- Docker checks every ADD statement to see if any files have changed and invalidates the cache for it and every later step if it has. The correct way to do this is...

ADD ./requirements/docker.txt /code/requirements/RUN pip install -r /code/requirements/docker.txtADD ./code/

Which will only invalidate your pip install line if your requirements file changes!

Since I never found a desirable solution consider this interesting hack. Posting here I wanted to see if anyone has similar/good/bad experiences with this "work around".

To make code reload locally for development I simply created a View that immediately calls exit(). The exit will crash Django and a reload will occur where code changes are available. The reboot takes a fraction of a second and can be done via a tab in the browser, a requests.get call, or any other similar call. The reload is not automatic but it does skip any Docker lag such as a restart.

When the exit is called you will see the PID increment (if tailing logs):

web    | [2019-07-15 18:29:52 +0000] [22] [INFO] Worker exiting (pid: 22)web    | [2019-07-15 18:29:52 +0000] [24] [INFO] Booting worker with pid: 24

I hope this helps others and/or gets feed back on this approach.