Moving to Docker Swarm

Now that we’re aware of the advantages of Swarm, let’s migrate our Compose setup to Swarm step by step. In each step, we’ll take advantage of Swarm’s features while adapting our existing configuration.

Making the Compose File Swarm-Ready

Before we can deploy our stack, we need to make our current Compose file Swarm-compliant. Start by renaming your existing Compose file to something more appropriate for stacks:

mv docker-compose.yml docker-stack.yml

Next, we’ll need to recreate the home network that we originally set up in Connecting Services with Docker Networking. This is because it used the default bridge driver. However, Swarm requires an overlay driver to enable multi-host networking across nodes.

Run the following commands to remove the old network and create a new one for Swarm:

docker network rm home
docker network create --driver overlay home

After that, we can make our Stack file compliant with the standard format. The only section we need to update is ports, since Stack files use a different syntax.

So instead of:

ports:
  - "5432:5432"

We write:

ports:
- target: 5432
    published: 5432
    protocol: tcp
    mode: host

This is because Stack deployments require explicit definitions for each port mapping, including the target, published port, protocol, and mode.

To test it out, we can now deploy the stack using the following command. In this case, home is the name of the stack:

docker stack deploy -c docker-stack.yml home

And view the status of the services with:

docker service ls