As we did for our simple web server, we will begin by creating another Swarm cluster:

$ docker swarm init
Swarm initialized: current node (1y1h7rgpxbsfqryvrxa04rvcp) is now a manager.

To add a worker to this swarm, run the following command:

    docker swarm join 
    --token SWMTKN-1-36flmf9vnika6x5mbxx7vf9kldqaw6bq8lxtkeyaj4r5s461ln-aiqlw49iufv3s6po4z2fytos1 
    192.168.4.128:2377

Then, we need to create our overlay network for the service-discovery hostname resolution to work. You don't need to know much about this other than it creates an isolated network that we will add all the services to:

$ docker network create --driver overlay service_network
44cyg4vsitbx81p208vslp0rx

Finally, we will build and launch our containers:

$ cd ../database
$ docker build . -t local_database
$ docker service create -d --replicas 1 
                           --name local-database 
                           --network service_network 
                           --mount type=volume,source=database_volume,destination=/data/db 
                           local_database
<snip>
pilssv8du68rg0oztm6gdsqse

$ cd ../application_server
$ docker build -t application_server .
$ docker service create -d -e DB_HOST=local-database 
                           --replicas 3 
                           --network service_network 
                           --name application-server 
                           application_server
<snip>
pue2ant1lg2u8ejocbsovsxy3

$ cd ../web_server
$ docker build -t web_server .
$ docker service create -d --name web-server 
                           --network service_network 
                           --replicas 3 
                           -e APP_NAME=application-server 
                           -p 8080:8080 
                           web_server
<snip>
swi95q7z38i2wepmdzoiuudv7

$ # Sanity checks
$ docker service ls
ID           NAME               MODE       REPLICAS IMAGE                PORTS
pilssv8du68r local-database     replicated 1/1      local_database       
pue2ant1lg2u application-server replicated 3/3      application_server
swi95q7z38i2 web-server         replicated 3/3      web_server            *:8080->8080/tcp

$ docker ps --format 'table {{.ID}}  {{.Image}}	  {{.Ports}}'
CONTAINER ID  IMAGE                         PORTS
8cdbec233de7  application_server:latest     8000/tcp
372c0b3195cd  application_server:latest     8000/tcp
6be2d6e9ce77  web_server:latest             80/tcp, 8080/tcp
7aca0c1564f0  web_server:latest             80/tcp, 8080/tcp
3d621c697ed0  web_server:latest             80/tcp, 8080/tcp
d3dad64c4837  application_server:latest     8000/tcp
aab4b2e62952  local_database:latest         27017/tcp

With these in place, we can now check whether our code works at http://127.0.0.1:8080 (username: user, password: test):

Looks like it is working! Once we put in our credentials, we should be redirected to the main application page:

Does the database work if we put in some words?

Indeed! We have really created a 1-node swarm-backed service, and it is scalable plus load balanced!