Doubts about communication between containers

Question

I have a question about the communication between containers, in my Docker-Compose I am creating a link between the container web and mysql. This link would be an internal network?

If not, it would be more performative to create an internal network?

Docker-Compose.yml

version: "3.3"
services:
  mysql:
    container_name: mysql
    image: mysql:5.7
    environment:
      MYSQL_ROOT_PASSWORD: SENHA
      MYSQL_DATABASE: webApp
      MYSQL_USER: root
      MYSQL_PASSWORD: senha
    restart: always
    ports:
      - 3306:3306

  web:
    container_name: web
    image: web_dev
    build:
      context: .
      dockerfile: Dockerfile-web
    volumes:
      - ../Desenvolvimento Web/:/var/www
      - ./Apache/:/etc/apache2/sites-enabled/
    working_dir: /var/www
    depends_on:
      - mysql
    links:
      - mysql
    restart: always
    ports:
      - 80:80

Answer 1

The link simply represents an entry into the HOSTS of your container. As the link information goes to the container metadata, whenever you go up this container Docker discovers the correct IP of the "linked" container and hits the HOSTS file of the container that depends on the link. This process makes your container not need to resolve the name on an IP, it simply "knows".

A network on Docker is literally a virtual network. The bridge network for example, (except the default) does name resolution automatically, it means that instead of your container simply knowing which is the IP, it is the network drive that responds with the current information.

You can inspect your networks using the docker network, as docker network ls for example..