If you have a Ruby environment available, you can install Kamal globally with:

gem install kamal

Otherwise, you can run a dockerized version via an alias (add this to your ~/.bashrc or similar to simplify re-use).

On macOS, use:

alias kamal='docker run -it --rm -v "${PWD}:/workdir" -v "/run/host-services/ssh-auth.sock:/run/host-services/ssh-auth.sock" -e SSH_AUTH_SOCK="/run/host-services/ssh-auth.sock" -v /var/run/docker.sock:/var/run/docker.sock'

On Linux, use:

alias kamal='docker run -it --rm -v "${PWD}:/workdir" -v "${SSH_AUTH_SOCK}:/ssh-agent" -v /var/run/docker.sock:/var/run/docker.sock -e "SSH_AUTH_SOCK=/ssh-agent"'

Then, inside your app directory, run kamal init. Now edit the new file config/deploy.yml. It could look as simple as this:

service: hey
image: 37s/hey
  username: registry-user-name

Then edit your .env file to add your registry password as KAMAL_REGISTRY_PASSWORD (and your RAILS_MASTER_KEY for production with a Rails app).

Now you’re ready to deploy to the servers:

kamal setup

This will:

  1. Connect to the servers over SSH (using root by default, authenticated by your ssh key).
  2. Install Docker and curl on any server that might be missing it (using apt-get): root access is needed via ssh for this.
  3. Log into the registry both locally and remotely.
  4. Build the image using the standard Dockerfile in the root of the application.
  5. Push the image to the registry.
  6. Pull the image from the registry onto the servers.
  7. Push the ENV variables from .env onto the servers.
  8. Ensure Traefik is running and accepting traffic on port 80.
  9. Ensure your app responds with 200 OK to GET /up (you must have curl installed inside your app image!).
  10. Start a new container with the version of the app that matches the current git version hash.
  11. Stop the old container running the previous version of the app.
  12. Prune unused images and stopped containers to ensure servers don’t fill up.

Voila! All the servers are now serving the app on port 80. If you’re just running a single server, you’re ready to go. If you’re running multiple servers, you need to put a load balancer in front of them. For subsequent deploys, or if your servers already have Docker and curl installed, you can just run kamal deploy.