Docker Swarm (Orchestration)

Introduction to Docker Swarm

A Swarm is a group of Docker hosts (nodes) running in cluster mode. One or more manager nodes handle orchestration, while worker nodes execute containers.

• Built directly into Docker Engine
• Supports service discovery and load balancing
• Enables high availability and failover

Setting Up a Swarm Cluster

Initialize the Swarm on the manager node:

docker swarm init

Add worker nodes using the generated token:

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

List all nodes in the Swarm:

docker node ls

Deploying Services in Swarm

Use the docker service command to deploy containers as Swarm services:

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

This runs 3 replicas of an NGINX container load-balanced across the cluster.

Check running services:

docker service ls

Inspect a specific service:

docker service ps web

Scaling Containers

Change the number of replicas dynamically:

docker service scale web=5

This command increases the number of running containers from 3 to 5.

Rolling Updates & Rollbacks

Update the image version with zero downtime:

docker service update --image nginx:1.25 web

To rollback in case of a failure:

docker service rollback web

Docker Swarm makes it easy to orchestrate container workloads with built-in clustering, load balancing, scaling, and self-healing. It’s a great choice for simpler, lightweight orchestration needs before moving to Kubernetes.