🚀 Deploying Applications on Docker Swarm #

Docker Swarm is a native container orchestration tool that allows you to deploy and manage containerized applications across a cluster of Docker nodes with ease. Understanding the core concepts helps you grasp how your application runs inside the Swarm.

Source: Docker Documentation

📦 Core Concepts #

Docker Swarm operates with four key hierarchical components:

1. Container #

• The actual running instance of your application image
• Similar to a pod container in Kubernetes
• Executes the application code and its dependencies
• Isolated from other containers but shares the host OS kernel

2. Task #

• A Swarm-managed unit of work that runs exactly one container
• The atomic scheduling unit in Swarm
• Has a lifecycle: assigned → prepared → running → complete (or failed)
• If a task fails, the scheduler creates a new task to maintain the desired state

3. Service #

• A declarative definition of an application running in the Swarm
• Specifies how many copies (tasks) of a container to run
• Defines the container image, ports, networks, volumes, and resource constraints
• Two service modes:
  • Replicated: Run a specified number of replicas across the cluster
  • Global: Run one task on every node in the cluster

4. Stack #

• A collection of related services deployed together
• Defined by a single Docker Compose file
• Simplifies managing multiple services as a single unit
• Includes networks and volumes required by the services

🔄 Service Deployment Workflow #

When you deploy a service to Docker Swarm, the following happens:

1. Define Service: You specify the container image, number of replicas, and other parameters
2. Service Creation: The Swarm manager accepts the service definition
3. Task Scheduling: The scheduler assigns tasks to available worker nodes
4. Container Creation: Each worker node creates containers for its assigned tasks
5. Service Discovery: The internal DNS registers the service for discovery by other services
6. Load Balancing: Incoming requests are distributed across all tasks of the service

📝 Example: Deploying a Web Application #

# docker-compose.yml for a web application stack
version: '3.8'

services:
  web:
    image: nginx:latest
    deploy:
      replicas: 3
      update_config:
        parallelism: 1
        delay: 10s
      restart_policy:
        condition: on-failure
    ports:
      - "80:80"
    networks:
      - webnet

  database:
    image: postgres:13
    environment:
      POSTGRES_PASSWORD: example
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - webnet
    deploy:
      placement:
        constraints:
          - "node.role==worker"

networks:
  webnet:

volumes:
  db-data:

🛠️ Common Commands #

# Deploy a stack
docker stack deploy -c docker-compose.yml myapp

# List all services in a stack
docker stack services myapp

# Scale a service
docker service scale myapp_web=5

# Update a service
docker service update --image nginx:1.21 myapp_web

# Remove a stack
docker stack rm myapp

🔍 Service Inspection and Troubleshooting #

To inspect and troubleshoot services:

# View service details
docker service inspect myapp_web

# View service logs
docker service logs myapp_web

# List tasks for a service
docker service ps myapp_web

# View task logs
docker logs <task-container-id>

Understanding these concepts and commands will help you effectively deploy, manage, and troubleshoot applications on Docker Swarm.