Docker Swarm

Shardul | Apr 4, 2025 min read
Docker Swarm

Docker Swarm is Docker’s native container orchestration tool. It simplifies deployment and management of containers by allowing users to configure and manage clusters of Docker nodes as a single virtual system. With Docker Swarm, you can scale containers and ensure high availability.

What is Docker Swarm?

  • A Swarm is a cluster of Docker nodes (machines).
  • There are two types of nodes:
    • Manager nodes: Orchestrate and manage the Swarm.
    • Worker nodes: Execute tasks assigned by the manager.
  • Swarm mode supports load balancing, service discovery, rolling updates, and high availability out of the box.

Install Docker on All Nodes

Install Docker on each machine (Ubuntu/CentOS/etc.), using:

Docker installation for Linux

Create Swarm on Master

On the manager node, run:

docker swarm init

or

docker swarm init --advertise-addr <MANAGER-IP>
#example:
docker swarm init --advertise-addr 192.168.122.19

You’ll get output like this:

Swarm initialized: current node (abc123...) is now a manager.

To add a worker to this swarm, run the following command:
  "docker swarm join --token SWMTKN-1-4utn2rljj43dnixqbgpnwv8peuid25m5j0od2oeahjcdw123i9-72pv0zjr14a3qt6karbfmet4d 192.168.122.19:2377"

To add a manager to this swarm, run 'docker swarm join-token manager' and follow the instructions.

Join Worker Nodes

On each worker node, run the docker swarm join command you got above:

docker swarm join --token SWMTKN-1-4utn2rljj43dnixqbgpnwv8peuid25m5j0od2oeahjcdw123i9-72pv0zjr14a3qt6karbfmet4d 192.168.122.19:2377

To retrieve the join command including the join token for worker nodes, run command on a manager node:

docker swarm join-token worker

You’ll get output like this:

To add a worker to this swarm, run the following command:

    docker swarm join \
    --token SWMTKN-1-49nj1cmql0jkz5s954yi3oex3nedyz0fb0xx14ie39trti4wxv-8vxv8rssmk743ojnwacrr2e7c \
    192.168.122.19:2377

This node joined a swarm as a worker.

Join Manager nodes

To view the join command and token for manager nodes, run:

docker swarm join-token manager

You’ll get output like this:

To add a manager to this swarm, run the following command:

    docker swarm join \
    --token SWMTKN-1-59egwe8qangbzbqb3ryawxzk3jn97ifahlsrw01yar60pmkr90-bdjfnkcflhooyafetgjod97sz \
    192.168.99.100:2377

Run the output command on the new node that needs to be joined in the swarm as manager node.

node on the swarm

Now the nodes are the part of the Swarm. Verify that all your nodes are actively available.

docker node ls

You’ll get output like this:

ID                           HOSTNAME  STATUS  AVAILABILITY  MANAGER STATUS
1bcef6utixb0l0ca7gxuivsj0    worker2   Ready   Active
38ciaotwjuritcdtn9npbnkuz    worker1   Ready   Active
e216jshn25ckzbvmwlnh5jr3g *  manager1  Ready   Active        Leader

Make node Manager

Promotes a node to manager. This command can only be executed on a manager node.

docker node promote `<worker node>`

or

docker node update --role manager

Remove from manager

Demote a node from manager to worker. This command can only be executed on a manager node.

docker node demote `<worker node>`

or

docker node update --role worker

Remove node from cluster

Removes the specified nodes from a swarm. A manager node must be demoted to a worker node (using docker node demote) before you can remove it from the swarm.

 $ docker node rm `<node>`

When you run this command on a worker, that worker leaves the swarm.

docker swarm leave

Drain a node

Sometimes, such as planned maintenance times, you need to set a node to Drain availability. Drain availability prevents a node from receiving new tasks from the swarm manager. It also means the manager stops tasks running on the node and launches replica tasks on a node with Active availability.

docker node update --availability drain worker1

Inspect the node to check its availability:

docker node inspect --pretty worker1

ID:			38ciaotwjuritcdtn9npbnkuz
Hostname:		worker1
Status:
 State:			Ready
 Availability:		Drain
...snip...

Monitor swarm Health

From the command line, run docker node inspect to query the nodes. For instance, to query the reachability of the node as a manager:

docker node inspect manager1 --format "{{ .ManagerStatus.Reachability }}"
reachable

To query the status of the node as a worker that accept tasks:

docker node inspect manager1 --format "{{ .Status.State }}"
 ready




Let me know if you need help with anything else related to Docker!