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Docker Swarm

What is Docker Swarm?

Docker Swarm is Docker’s native container orchestration tool.
It allows you to manage multiple Docker hosts (nodes) as a single virtual system.

With Swarm, you can deploy and scale applications across a cluster, ensuring high availability and load balancing.

Why Use Docker Swarm?

  • Native integration with Docker CLI and Docker Compose.
  • Easy setup compared to Kubernetes.
  • Built-in service discovery and load balancing.
  • Scaling and self-healing of services.
  • Secure cluster communication using TLS.

Swarm Architecture

  • Manager Nodes: Responsible for managing the cluster and scheduling tasks.
  • Worker Nodes: Run containers assigned by the managers.
  • Services: High-level definition of containers in Swarm.
  • Tasks: The actual running containers.

Initializing a Swarm

To create a new Swarm cluster:

docker swarm init

This command outputs a join token for adding worker or manager nodes:

docker swarm join --token <token> <manager-ip>:2377

Creating a Service

Example: Run an Nginx service with 3 replicas.

docker service create --name web --replicas 3 -p 8080:80 nginx

Check running services:

docker service ls

Inspect tasks of a service:

docker service ps web

Scale a service:

docker service scale web=5

Docker Stack

Stacks let you deploy a full application defined in a docker-compose.yml.

Deploy a stack:

docker stack deploy -c docker-compose.yml mystack

Check stacks:

docker stack ls

Remove a stack:

docker stack rm mystack

Networking in Swarm

Swarm supports an Overlay Network to enable communication between services across nodes.

Create an overlay network:

docker network create --driver overlay mynet

Use it in a stack or service:

docker service create --name api --network mynet myimage

Secrets Management

Swarm provides a way to securely store sensitive data.

Create a secret:

echo "mydbpassword" | docker secret create db_pass -

Use secret in a service:

docker service create --name app --secret db_pass myimage

List secrets:

docker secret ls

Swarm Node Management

List Nodes

List all nodes in the Swarm:

docker node ls

Example output:

ID                            HOSTNAME  STATUS  AVAILABILITY  MANAGER STATUS
abcd1234efgh *                manager1  Ready   Active        Leader
ijkl5678mnop                  worker1   Ready   Active        
qrst9012uvwx                  worker2   Ready   Active

Inspect Node

Inspect a node:

docker node inspect <node-id>

Promote a Node to Manager

Promote an existing worker to a manager:

docker node promote <node-id>

Example:

docker node promote ijkl5678mnop

Now worker1 becomes a manager.

Demote a Manager to Worker

Demote a manager back to a worker:

docker node demote <node-id>

Add a New Manager Node

From an existing manager, get the join command for a new manager:

docker swarm join-token manager

Output example:

docker swarm join --token SWMTKN-1-xyz <manager-ip>:2377

Run this on the new node to join the Swarm as a manager.

Node Availability

  • Active: Node can accept tasks.
  • Drain: Node won’t accept new tasks (used for maintenance). Draining a node prevents new tasks from running on it.
  • Pause: Node temporarily stops tasks.

Set node availability:

docker node update --availability drain <node-id>
docker node update --availability active <node-id>

Monitoring and Logging

Get real-time stats:

docker stats

Check service logs:

docker service logs web

Limitations of Docker Swarm

  • Not as feature-rich as Kubernetes.
  • Less adoption in large-scale production systems.
  • Limited ecosystem support.

References