What is the difference between volumes-from and volumes? - docker

Question

I saw the docker-compose patterns but I'm confused. What is the best way to make composed containers.
When should I use link, or volumes_from.
When should I use volumes_from, volumes
#1 app-db-data
app:
image: someimage
link:
- db // data volume container name
db:
image: mysql
volumes_from:
- data // data volume name
data:
image: someimage
volumes:
- {host data}:{guest data}
#2 app-db+data
app:
image: someimage
link:
- db // data volume container name
db:
image: mysql
volumes:
- data // data file name
app
#1 app-service-data
app:
image: someimage
volumes_from:
- service // service container name
service:
image: mysql
volumes_from:
- data // image container name
data:
image: someimage
volumes:
- {host data}:{guest data}
#2 app-service+data
app:
image: someimage
volumes_from:
- service // service container name
service:
image: mysql
volumes:
- data // mounted file
Thanks

Answer 1

In short:
volumes_from mounts from other containers.
volumes mounts defined inline.
links connects containers.
A little bit more explained:
volumes_from mounts volumes from other containers. For example if you have data only containers and you want to mount these data only containers in the container that has your application code.
volumes is a the inline way to define and mount volumes. If you read #17798 you can see that named volumes can replace data only containers in most cases.
The simplest is then to use volumes. Since you can reuse them by naming them.
links is different. Because it does not mount. Instead it connects containers. So if you do:
app:
container_name: app_container
links:
- db
That means that if you connect to app_container with docker exec -it app_container bash and try ping db you will see that container is able to resolve ip for db.
This is because docker creates a network between containers.

Answer 2

Link and volumes_from are different concepts. Links are used when you need to connect (by network) two containers. In this case if you want to connect an App to the Database, the way to do this is by using a link, since applications use a port and host to connect to a database (not a directory on the filesystem).
Volumes and volumes_from differ in that the first one only declares volumes that docker will make persistent or host:guest mounts, but volumes_from tells docker to use a volumes that is already declared on another host (making it available to this host).
Of those 4 cases that you present, I think that the first and second are good choices. In the first you are creating a data only container, and make the mysql container use it. In the second case the data and the mysql container are the same.
Links and volumes are perfectly explained in the docker documentation.
Hope it helps.

Answer 3

Addition: Volumes_from is used when you want to mount all anon-volumes of a container - named volumes could have been mounted directly since the early days.
AFAICs https://docs.docker.com/compose/compose-file/#volumes . docker-compose has removed this functionality entirely, not sure how and why and if there is an alternative. But assume, you have an app container and you have a httpd container. Usually you would define the codebase folder, /var/www, as an anon volume and then mount it in httpd to be to serve static files using the httpd service, while passing all dynamic files like ruby/php/java to an upstream backend on app.
The point in using a anon volume and not a named volume is, that actually you want to be able to redeploy app and change the codebase ( app update ) which would not work, if app would have a named volume. That said, anon volumes are doing exactly that and thats why volumes_from is used here - using named volumes is no option is this case ( as it is very practical in a a lot of other cases ).
For the reference the upgrade guides for volumes_from:
https://docs.docker.com/compose/compose-file/compose-versioning/#upgrading
So volumes_from usually is used in a different context / scenario and named-volumes are the standard in a ll other cases as explained above. A brief post about that is https://stackoverflow.com/a/44744861/3625317