A Simple Analogy

Docker Swarm is like a team of builders. One is the foreman (manager) coordinating work, and others are workers executing tasks.

---

Why Swarm?

• Orchestration: Manage multiple containers
• Load balancing: Distribute requests
• Fault tolerance: Restart failed containers
• Scaling: Add replicas easily
• Built-in: No external dependencies

---

Initialization

# Initialize swarm (first manager)
docker swarm init

# Get join token for workers
docker swarm join-token worker

# Add worker node (run on worker machine)
docker swarm join --token SWMTKN-xxx IP:2377

# List nodes
docker node ls

---

Deploying Services

# Create service
docker service create \
  --name web \
  --replicas 3 \
  -p 80:8080 \
  myapp:latest

# List services
docker service ls

# Scale service
docker service scale web=5

# Check service status
docker service ps web

# Update image
docker service update \
  --image myapp:v2 \
  web

---

Docker Compose Stack

version: '3.9'

services:
  web:
    image: myapp:latest
    ports:
      - "80:8080"
    deploy:
      replicas: 3
      resources:
        limits:
          cpus: '0.5'
          memory: 512M
        reservations:
          cpus: '0.25'
          memory: 256M
      restart_policy:
        condition: on-failure
        delay: 5s
        max_attempts: 3
    environment:
      - DB_HOST=db
  
  db:
    image: postgres:15
    environment:
      POSTGRES_PASSWORD: secure
    volumes:
      - db_data:/var/lib/postgresql/data
    deploy:
      replicas: 1
      placement:
        constraints: [node.role == manager]

volumes:
  db_data:

Deploy: docker stack deploy -c docker-compose.yml myapp

---

Networking

# Create overlay network (cross-node)
docker network create --driver overlay mynet

# Connect service to network
docker service create \
  --name web \
  --network mynet \
  myapp:latest

# Services can communicate by name
# Inside container: curl http://web:8080

---

Rolling Updates

deploy:
  update_config:
    parallelism: 1
    delay: 10s
    failure_action: rollback
  
  rollback_config:
    parallelism: 1
    delay: 10s

---

Best Practices

1. Pin image versions: Use tags, not latest
2. Health checks: Define startup/liveness
3. Resource limits: Prevent container sprawl
4. Backup: Persist important data
5. Monitor: Track service health

---

Related Concepts

• Kubernetes (more complex)
• Docker Compose (single host)
• Service discovery
• Load balancing

---

Summary

Docker Swarm simplifies multi-container orchestration. Use services, stacks, and overlay networks for scalable, fault-tolerant deployments.