As it was pretty obvious from just a few paragraphs before, a manual setup of these services seems somewhat of a pain, so here we introduce a new tool that can help us do this much easier: Docker Stack. This tool uses a YAML file to get things to deploy all the services easily and repeatedly.

First we will clean up our old exercise before trying to use Docker stack configuration:

$ docker service ls -q | xargs docker service rm
pilssv8du68r
pue2ant1lg2u
swi95q7z38i2

$ docker network rm service_network
service_network

Now we can write our YAML configuration file--you can easily notice the parallels that the CLI has to this configuration file:

version: "3"
services:
  local-database:
    image: local_database
    networks:
      - service_network
    deploy:
      replicas: 1
      restart_policy:
        condition: on-failure
    volumes:
      - database_volume:/data/db

  application-server:
    image: application_server
    networks:
      - service_network
    depends_on:
      - local-database
    environment:
      - DB_HOST=local-database
    deploy:
      replicas: 3
      restart_policy:
        condition: on-failure

  web-server:
    image: web_server
    networks:
      - service_network
    ports:
      - 8080:8080
    depends_on:
      - application-server
    environment:
      - APP_NAME=application-server
    deploy:
      replicas: 3
      restart_policy:
        condition: on-failure

networks:
  service_network:

volumes:
  database_volume:

What about starting our stack? That's easy too! Stack has almost the same commands as docker services:

$ docker stack deploy --compose-file swarm_application.yml swarm_test
Creating network swarm_test_service_network
Creating service swarm_test_local-database
Creating service swarm_test_application-server
Creating service swarm_test_web-server

$ # Sanity checks
$ docker stack ls
NAME        SERVICES
swarm_test  3

$ docker stack services swarm_test
ID           NAME                          MODE       REPLICAS            IMAGE                PORTS
n5qnthc6031k swarm_test_application-server replicated 3/3                 application_server   
v9ho17uniwc4 swarm_test_web-server         replicated 3/3                 web_server           *:8080->8080/tcp
vu06jxakqn6o swarm_test_local-database     replicated 1/1                 local_database

$ docker ps --format 'table {{.ID}}  {{.Image}}	  {{.Ports}}'
CONTAINER ID  IMAGE                         PORTS
afb936897b0d  application_server:latest     8000/tcp
d9c6bab2453a  web_server:latest             80/tcp, 8080/tcp
5e6591ee608b  web_server:latest             80/tcp, 8080/tcp
c8a8dc620023  web_server:latest             80/tcp, 8080/tcp
5db03c196fda  application_server:latest     8000/tcp
d2bf613ecae0  application_server:latest     8000/tcp
369c86b73ae1  local_database:latest         27017/tcp

If you go to http://127.0.0.1:8080 in your browser again, you will see that our app works just like before! We have managed to deploy our whole cluster worth of images with a single file on a Docker Swarm cluster!