Connect two docker deployments

Sometimes you have to connect two docker deployments together, so today we will look at how to achieve that connection using docker networks.

The problem

Sometimes you have to connect two docker deployments together, so today we will look at how to achieve that connection using docker networks.

In this example we will have two deployments, backend.yml and frontend.yml

The backend.yml looks like this:

# backend.yml
version: '3'

services:
   application:
      image: "application"
      container_name: "application"
      restart: unless-stopped

And the frontend.yml looks like this:

# frontend.yml
version: '3'

services:
   webserver:
      image: nginx:1.23.3-alpine
      container_name: webserver
      restart: unless-stopped
      ports:
       - "80:80"
       - "443:443"

Now, the problem is that if we try to route traffic from the nginx on the frontend.yml deployment to let's say the "application" container from the backend.yml all those packages will fail.

The solution

To solve this, we will need to create a docker network. We do this by modifying backend.yml to look like this

# backend.yml
version: '3'

services:
   application:
      image: "application"
      container_name: "application"
      restart: unless-stopped
      networks:
       - app-network

networks:
   app-network:
      external: false

As you can see we are declaring an explicit network that will be created by docker for us when we do docker-compose up

Now we need to tell the frontend.yml that such network exists and that the nginx container can have access to it. By doing this:

# frontend.yml
version: '3'
webserver:
   image: nginx:1.23.3-alpine
   container_name: webserver
   restart: unless-stopped
   ports:
     - "80:80"
     - "443:443"
   networks:
     - app-network_backend

networks:
  app-network_backend:
     external: true

See how the name of the network is not the same as we defined on the backend.yml file, that is because docker creates a unique name for every network (unless you explicitly define one), so please make sure to use the following command to check your networks

docker networks ls

Which gives the following output:

NETWORK ID     NAME                        DRIVER    SCOPE
f3b539b9ccd2   bridge                      bridge    local
e82e0b83d27f   host                        host      local
0af007240301   none                        null      local
93e728e8ec71   app-network_backend         bridge    local

And after restarting both deployments, ngnix will be able to route traffic to the containers on the backend.yml