More Docker networking

docker networking

Connecting two services such as a server and its database in docker can be achieved with docker-compose networks. In addition to starting services listed in docker-compose.yml the tool automatically creates and joins both containers into a network with a DNS. Each container service is named after their container name and as such containers can reference each other simply with their names.

Here are two services in a single network: webapp and webapp-helper. The webapp-helper has a server, listening for requests in port 3000, that webapp wants to access. Because they were defined in the same docker-compose.yml file the access is trivial. Docker-compose has already taken care of creating a network and webapp can simply send a request to webapp-helper:3000, the internal DNS will translate that to the correct access and ports do not have to be published outside of the network.

You can also manually define the network and its name. A major benefit of defining network is that it makes it easy to setup a configuration where other containers connect to an existing network as an external network. This is used when a container wishes to interact with a container defined in another docker-compose file.

Defining network in docker-compose.yml. Services can be added to networks by adding networks into the definition of the service:

version: "3.8"

services:
  db:
    image: postgres:13.2-alpine
    networks:
      - database-network # Name in this docker-compose file

networks:
  database-network: # Name in this docker-compose file
    name: database-network # Name that will be the actual name of the network

This defines a network called database-network which is created with docker-compose up and removed with docker-compose down.

To connect to an external network (possibly defined another docker-compose.yml):

version: "3.8"

services:
  db:
    image: backend-image
    networks:
      - database-network

networks:
  database-network:
    external:
      name: database-network # Must match the actual name of the network

By default all services are added to a network called default. The default network can be configured and this makes it possible to connect to an external network by default as well:

version: "3.8"

services:
  db:
    image: backend-image

networks:
  default:
    external:
      name: database-network # Must match the actual name of the network

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