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In this lesson, we’ll deploy the Example Voting App using Docker Stacks on a Swarm cluster. We start by upgrading our Compose file to version 3, deploy it as a stack, verify running services, and then scale the voting service.

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1. Prepare the Docker Compose File (Version 3)

Below is the upgraded Compose file in version 3 format. Save this as docker-stack.yml on your Swarm manager: 
version: '3'
services:
  redis:
    image: redis
  db:
    image: postgres:9.4
  vote:
    image: dockersamples/examplevotingapp_vote
    ports:
      - "5000:80"
  worker:
    image: dockersamples/examplevotingapp_worker
  result:
    image: dockersamples/examplevotingapp_result
    ports:
      - "5001:80"
The dockersamples images are available on Docker Hub.
The image shows a Docker Hub profile page for "dockersamples," displaying a list of public repositories with details such as stars and pull counts.
Make sure Docker Swarm is initialized on your manager node:
docker swarm init

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2. Deploy the Stack

  1. SSH into your Swarm manager and create a project directory: 
    mkdir ~/voting-app-stack && cd ~/voting-app-stack
  2. Place the docker-stack.yml file in this directory.
  3. Deploy the stack: 
    docker stack deploy voting-app-stack --compose-file docker-stack.yml
    You should see output similar to:
    Compose
    Creating network voting-app-stack_default
Creating service voting-app-stack_redis
Creating service voting-app-stack_db
Creating service voting-app-stack_vote
Creating service voting-app-stack_worker
Creating service voting-app-stack_result
Verify all services are up: 
docker service ls
Service NameReplicasImage
voting-app-stack_redis1/1redis:latest
voting-app-stack_db1/1postgres:9.4
voting-app-stack_vote1/1dockersamples/examplevotingapp_vote:latest
voting-app-stack_worker0/1dockersamples/examplevotingapp_worker:latest
voting-app-stack_result1/1dockersamples/examplevotingapp_result:latest

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3. Inspect Running Tasks

On any manager or worker node, list the active containers to ensure tasks are distributed: 
docker ps
Example output: 
CONTAINER ID  IMAGE                                           NAMES
3940e228ce02  dockersamples/examplevotingapp_result:latest    voting-app-stack_result.1.abcde
9348d1d3d7f5  postgres:9.4                                    voting-app-stack_db.1.fghij

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4. Access the Voting Application

Open a browser and navigate to:
  • Voting interface: http://<manager-ip>:5000
  • Results dashboard: http://<manager-ip>:5001
The image shows a web page titled "Cats vs Dogs!" with options to vote for either "CATS" or "DOGS," and the "CATS" option is selected.

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5. Scale the Voting Service

To handle higher traffic, update docker-stack.yml by adding the deploy.replicas setting under the vote service: 
services:
  vote:
    image: dockersamples/examplevotingapp_vote
    ports:
      - "5000:80"
    deploy:
      replicas: 2
Redeploy the updated stack: 
docker stack deploy voting-app-stack --compose-file docker-stack.yml
Confirm that the voting service now has two replicas: 
docker service ls
Service NameReplicasImage
voting-app-stack_vote2/2dockersamples/examplevotingapp_vote:latest
voting-app-stack_db1/1postgres:9.4
voting-app-stack_redis1/1redis:latest
voting-app-stack_result1/1dockersamples/examplevotingapp_result:latest
voting-app-stack_worker1/1dockersamples/examplevotingapp_worker:latest
Inspect each task: 
docker service ps voting-app-stack_vote
This demonstrates how to deploy, verify, and scale services using Docker Stacks in a Swarm cluster.

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