Sample app overview

This example is built around a web-based voting application that collects, tallies, and returns the results of votes (for cats and dogs, or other choices you specify). The voting app includes several services, each one running in its own container. We’ll deploy the app as a stack to introduce some new concepts surfaced in Compose Version 3, and also use swarm mode, which is cluster management and orchestration capability built into Docker Engine.

Got Docker?

If you haven’t yet downloaded Docker or installed it, go to Get Docker and grab Docker for your platform. You can follow along and run this example using Docker for Mac, Docker for Windows or Docker for Linux.

Once you have Docker installed, you can run docker run hello-world or other commands described in the Get Started with Docker tutorial to verify your installation. If you are totally new to Docker, you might continue through the full Get Started with Docker tutorial first, then come back.

What you’ll learn and do

In this tutorial, you’ll learn how to:

• Use docker machine to create multiple virtual local hosts or dockerized cloud servers
• Use docker commands to set up and run a swarm with manager and worker nodes
• Deploy the vote app services across the two nodes by feeding our example docker-stack.yml file to the docker stack deploy command
• Test the app by voting for cats and dogs, and view the results
• Use the visualizer to explore and understand the runtime app and services
• Update the docker-stack.yml and redeploy the app using a different vote image to implement a poll on different choices
• Use features new in Compose Version 3, highlighted in the sample app

Services and images overview

A service is a bit of executable code designed to accomplish a specific task. A service can run in one or more containers. Defining a service configuration for your app (above and beyond docker run commands in a Dockerfile) enables you to deploy the app to a swarm and manage it as a distributed, multi-container application.

The voting app you are about to deploy is composed of several services, each based on an image:

Service	Description	Base Image
vote	Presents the voting interface via port 5000. Viewable at <manager-IP>:5000	Based on a Python image, dockersamples/examplevotingapp_vote
result	Displays the voting results via port 5001. Viewable at <manager-IP>:5001	Based on a Node.js image, dockersamples/examplevotingapp_result
visualizer	A web app that shows a map of the deployment of the various services across the available nodes via port 8080. Viewable at <manager-IP>:8080	Based on a Node.js image, dockersamples/visualizer
redis	Collects raw voting data and stores it in a key/value queue	Based on a redis image, redis:alpine
db	A PostgreSQL service which provides permanent storage on a host volume	Based on a postgres image, postgres:9.4
worker	A background service that transfers votes from the queue to permanent storage	Based on a .NET image, dockersamples/examplevotingapp_worker

Each service will run in its own container. Using swarm mode, we can also scale the application to deploy replicas of containerized services distributed across multiple nodes.

Here is an example of one of the services fully defined:

vote:
  image: dockersamples/examplevotingapp_vote:before
  ports:
    - 5000:80
  networks:
    - frontend
  depends_on:
    - redis
  deploy:
    replicas: 2
    update_config:
      parallelism: 2
    restart_policy:
      condition: on-failure

The image key defines which image the service will use. The vote service uses dockersamples/examplevotingapp_vote:before.

The depends_on key allows you to specify that a service is only deployed after another service. In our example, vote only deploys after redis.

The deploy key specifies aspects of a swarm deployment. For example, in this configuration we create replicas of the vote and result services (2 containers of each will be deployed to the swarm), and we constrain some services (db and visualizer) to run only on a manager node.

docker-stack.yml deployment configuration file

In addition to defining a set of build and run commands in a Dockerfile, you can define services in a Compose file, along with details about how and where those services will run. You can use Compose files to kick off multiple Dockerfiles, or use Compose files independently of Dockerfiles.

In the Getting Started with Docker tutorial, you wrote a Dockerfile for the whalesay app then used it to build a single image and run it as a single container.

For this tutorial, the images are pre-built, and we will use docker-stack.yml (a Version 3 Compose file) instead of a Dockerfile to run the images. When we deploy, each image will run as a service in a container (or in multiple containers, for those that have replicas defined to scale the app). This example relies on Compose version 3, which is designed to be compatible with Docker Engine swarm mode.

To follow along with the example, you need only have Docker running and the copy of docker-stack.yml we provide here. This file defines all the services shown in the table above, their base images, configuration details such as ports, networks, volumes, application dependencies, and the swarm configuration.

version: "3"
services:

  redis:
    image: redis:alpine
    ports:
      - "6379"
    networks:
      - frontend
    deploy:
      replicas: 2
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure
  db:
    image: postgres:9.4
    volumes:
      - db-data:/var/lib/postgresql/data
    networks:
      - backend
    deploy:
      placement:
        constraints: [node.role == manager]
  vote:
    image: dockersamples/examplevotingapp_vote:before
    ports:
      - 5000:80
    networks:
      - frontend
    depends_on:
      - redis
    deploy:
      replicas: 2
      update_config:
        parallelism: 2
      restart_policy:
        condition: on-failure
  result:
    image: dockersamples/examplevotingapp_result:before
    ports:
      - 5001:80
    networks:
      - backend
    depends_on:
      - db
    deploy:
      replicas: 1
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure

  worker:
    image: dockersamples/examplevotingapp_worker
    networks:
      - frontend
      - backend
    deploy:
      mode: replicated
      replicas: 1
      labels: [APP=VOTING]
      restart_policy:
        condition: on-failure
        delay: 10s
        max_attempts: 3
        window: 120s
      placement:
        constraints: [node.role == manager]

  visualizer:
    image: dockersamples/visualizer:stable
    ports:
      - "8080:8080"
    stop_grace_period: 1m30s
    volumes:
      - "/var/run/docker.sock:/var/run/docker.sock"
    deploy:
      placement:
        constraints: [node.role == manager]

networks:
  frontend:
  backend:

volumes:
  db-data:

Docker stacks and services

To deploy the voting app, we will run the docker stack deploy command with appropriate options using this docker-stack.yml file to pull the referenced images and launch the services in a swarm. This allows us to run the application across multiple servers, and use swarm mode for load balancing and performance. Rather than thinking about running individual containers, we can start to model deployments as application stacks and services.

If you are interested in learning more about new Compose version 3.x features, Docker Engine 1.13.x, and swarm mode integration, check out the list of resources at the end of this tutorial.

What’s next?

In the next step, we’ll set up two Dockerized hosts.