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Using Docker Swarm

Updated Jul 07, 2022 ·
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These are optional, and you can setup a basic Docker Swarm after installing the necessary tools. For a quick setup, please see:

Below are some recommended settings when setting up a Docker Swarm.

  • Number of nodes

    • At least 3 Linux nodes (VMs or physical machines)
    • 1 manager node
    • 2+ worker nodes

  • Recommended OS

    • Ubuntu 20.04+
    • CentOS 7+
    • Rocky Linux
    • AlmaLinux

  • CPU and RAM

    • Each node should have minimum 1 CPU and 1GB RAM
    • Docker Engine installed (version 20.10+ recommended)
    • Static IP or properly configured networking (especially if across machines)

  • Network Requirements

    • All nodes must be reachable over the network

    • Especially on these ports:

      • 2377/tcp – for cluster management
      • 7946/tcp and 7946/udp – for node communication
      • 4789/udp – for overlay network (VXLAN)

    • No firewalls blocking the required ports between nodes

    • Consistent DNS or /etc/hosts entries for node name resolution (if no DNS)

  • User & Access

    • SSH access between nodes (especially for setup convenience)
    • User with sudo privileges on all nodes

Tools​

  • Install Docker:

    curl -fsSL https://get.docker.com | bash
    sudo usermod -aG docker $USER

  • Enable and start Docker:

    sudo systemctl enable docker
    sudo systemctl start docker

  • Install Docker Compose plugin:

    For Ubuntu/Debian:

    sudo apt-get update
    sudo apt-get install docker-compose-plugin

    For CentOS/RHEL:

    sudo yum install docker-compose-plugin

  • Verify installation

    docker compose version

Docker Swarm with Multiple Nodes​

  1. On the intended manager node, initialize the swarm:

    docker swarm init

  2. Optional: Bind it to a specific IP (recommended in multi-node labs):

    docker swarm init --advertise-addr <MANAGER-IP>

  3. Get the join command:

    docker swarm join-token worker

  4. On the worker node, run the output command (something like):

    docker swarm join --token SWMTKN-xxxx <MANAGER-IP>:2377

  5. To verify, run docker stack deploy on a manager node.

    docker info | grep 'Swarm'

    Expected output if it's a manager:

    Swarm: active
    Is Manager: true

    If it shows inactive, or Is Manager: false, you're not on the right node.

Docker Swarm with One Node​

A few notes:

  • A single-node swarm works just like a multi-node one (for testing).
  • You can run both manager and worker roles on the same machine.

Ideal for testing features like:

  • docker stack deploy
  • docker service ls
  • Rolling updates, constraints, placement preferences (in limited form)

Steps:

  1. Initialize the swarm:

    docker swarm init

    This makes your local machine the manager node, and it's automatically part of the swarm.

    Swarm initialized: current node (abcdefgh12345) is now a manager.

  2. Verify it’s active:

    docker info | grep Swarm

    You should see:

    Swarm: active
    Is Manager: true

Optional but Useful Tools​