Mounting volumes with docker-compose - docker-compose

Question

I'm looking into how to mount volumes with docker-compose for data persistence but I'm having trouble understanding all the examples I read.
https://www.linux.com/learn/docker-volumes-and-networks-compose
version: '2'
services:
mysql:
image: mysql
container_name: mysql
volumes:
- mysql:/var/lib/mysql
...
volumes:
mysql:
Ok so this defines a volume named mysql at the bottom and it references this volume in
- mysql:/var/lib/mysql
How will the mysql image know to look in this volume named mysql? Is it just designed to look in all the volumes it has to store data or something?
Then in other examples I see the following:
services:
nginx:
image: nginx
depends_on:
- ghost
volumes:
- ./default.conf:/etc/nginx/conf.d/default.conf
ports:
- "80:80"
networks:
- proxy
This example doesn't need to define a volume, why is that?

Answer 1

your MySQL data will be stored in the named volume mysql which is created by:
volumes:
mysql:
You can list the docker volumes using docker volume ls and the 'path' will be something like: /var/lib/docker/volumes/mysql/date. When you cd in this folder you will see the same data as the data which is in your mysql container on path: /var/lib/mysql. If you exec inside your container you will see the same data.
How does it know how to use this path?
Well check the Dockerfile of mysql. Here is:
VOLUME /var/lib/mysql
In short: all the data of your mysql is stored in /var/lib/mysql inside your container and mounted to your named docker volume mysql on your host, which path is something like /var/lib/docker/volumes/mysql/data/.
The next part is mounting ./default.conf (on your host, relative path) on the path /etc/nginx/conf.d/default.conf inside your nginx container.
Nginx and ghost don't need a named volume in this case because they don't need to keep specific data. When you create your environment you will add data using Ghost (write blogs), but the data itself will be stored in the mysql database. Not in the Ghost container.
Remark (if your second example has nothing to do with the mysql example): the default image of ghost is working with the sqlite3 db which is inside the same container (=! microservice for each container so this is fine to develop, not in production). But if you would use this setup you need to create a named volume for your sqlite which is in the same container as ghost. Take a look to the dockerfile of ghost.
If you want to use mysql you probably need to mount a config file to your ghost container to tell the container: use mysql, you will not need a named docker volume for ghost then, because data won't be stored in the ghost container but in the mysql container.
To keep your last example persistent without using mysql with a named volume you have to add a volume for the sqlite db which is inside the ghost container for this path: /var/lib/ghost/content. Check the Dockerfile again to see this path.
This blog post explains how to setup ghost with mysql in docker-compose