Docker Swarm is a native clustering and orchestration solution for Docker. It turns a pool of Docker hosts into a single, virtual Docker host, allowing you to deploy and manage applications across multiple machines with ease.
Key Concepts
Before diving into examples, let's familiarize ourselves with some key concepts:
Node: A machine in the Swarm cluster (can be a manager or worker)
Manager Node: Responsible for cluster management and orchestration
Worker Node: Executes containers
Service: The definition of tasks to execute on nodes
Task: A Docker container and the commands to run inside it
Setting Up a Docker Swarm
Let's start by creating a simple Swarm cluster:
# Initialize a swarm on the manager node
docker swarm init --advertise-addr <MANAGER-IP>
# This command will output a token to add worker nodes
# On each worker node, run:
docker swarm join --token <TOKEN> <MANAGER-IP>:2377
# To view nodes in the swarm (run on manager)
docker node ls
Deploying a Service
Now that we have our Swarm set up, let's deploy a simple web service:
# Create a service with 3 replicas
docker service create --name my-web-app --replicas 3 -p 80:80 nginx
# List services
docker service ls
# Inspect the service
docker service ps my-web-app
Scaling a Service
One of the key benefits of Docker Swarm is easy scaling:
# Scale the service to 5 replicas
docker service scale my-web-app=5
# Verify the scaling
docker service ps my-web-app
Rolling Updates
Docker Swarm makes it easy to perform rolling updates:
# Update the image of our service
docker service update --image nginx:alpine my-web-app
# Watch the rolling update in action
docker service ps my-web-app
Creating a Multi-Service Stack
For more complex applications, we can use Docker Compose with Swarm mode:
version: '3'
services:
web:
image: nginx
deploy:
replicas: 3
update_config:
parallelism: 1
delay: 10s
restart_policy:
condition: on-failure
ports:
- "80:80"
visualizer:
image: dockersamples/visualizer
ports:
- "8080:8080"
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
deploy:
placement:
constraints: [node.role == manager]
Save this as docker-compose.yml and deploy with:
docker stack deploy -c docker-compose.yml my-stack
Conclusion
Docker Swarm provides a robust and user-friendly solution for container orchestration. With its built-in load balancing, service discovery, and rolling updates, it's an excellent choice for teams looking to scale their containerized applications.