How containers are linked to each other in docker-compose.yml - docker

Question

To build a web-server, i'm trying to understand how containers are attached to each other, and i really need some quick answers.
So, if we take this docker-compose.yml file as an exemple:
version: '2'
services:
# APP
nginx:
build: docker/nginx
volumes_from:
- php
links:
- php
depends_on:
- php
php:
build: docker/php
volumes:
- ${SYMFONY_APP_PATH}:/symfony
links:
- mysql
- faye
- rabbitmq
- elasticsearch
client:
image: node:8.9.4
volumes_from:
- php
working_dir: /symfony
user: 1000:1000
command: "npm run dev"
ports:
- "${LIVERELOAD_PORT}:35729"
environment:
LIVERELOAD_PORT: ${LIVERELOAD_PORT}
mysql:
build: docker/mysql
environment:
MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
MYSQL_DATABASE: ${MYSQL_DATABASE}
MYSQL_USER: ${MYSQL_USER}
MYSQL_PASSWORD: ${MYSQL_PASSWORD}
volumes:
- ${SYMFONY_APP_PATH}:/symfony
- "mysql:/var/lib/mysql"
rabbitmq:
image: rabbitmq:3.4-management
volumes:
- "rabbitmq:/var/lib/rabbitmq"
volumes:
- "elasticsearch5:/usr/share/elasticsearch/data"
- ${SYMFONY_APP_PATH}:/symfony
volumes:
mysql: ~
elasticsearch5: ~
rabbitmq: ~
What is the difference between volumes_from, links, and depends_on ?
If the idea is to attach each container with the other why we don't use only links. what is the difference between volumes_from, links, and depends_on.
Why in my example ngnix depend/linked to php container? why not the opposite? At the file footer, there's a volume configuration volumes:
mysql: ~ elasticsearch5: ~ rabbitmq: ~ but I think we already defined as the volume of each container, so what's is the main reason of that config?
And why not we dont use only one web container that use php, ngnix, and mysqld why we seperate them?

Answer 1

What is the difference between volumes_from, links, and depends_on?
Both links and depends_on provide a way for a container to communicate with each other.
links is a legacy feature and will be deprecated in the future, so avoid using links whenever possible.
volumes_from is used for other purpose, and it has nothing to do with links and depends_on.
Why in my example ngnix depend/linked to php container? why not the opposite?
depends_on defines order of services starting. In your example, you're using Nginx as a proxy server to the PHP service. So you might want the PHP service to start before Nginx.
And why not we dont use only one web container that use php, ngnix, and mysqld why we seperate them?
One of Docker's best practices is to keep each container simple enough to do only one job. Much like Unix's "do one thing and do it well" philosophy.
Single responsibility principle is a good thing, embrace it.