Docker Cookbook

See Also
•
Managing data in containers
32 | Chapter 1: Getting Started with Docker

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•
Understanding volumes
• Data 
container
• Docker 
volumes
1.19 Sharing Data Between Containers
Problem
You know how to mount a host volume into a running container, but you would like
to share a volume defined in a container with other containers. This would have the
benefit of letting Docker manage the volumes and support the principle of single
responsibility.
Solution
Use data containers. In 
Recipe 1.18
, you saw how to mount a host volume into a con‐
tainer. You used the 
-v
option of 
docker run
, specifying a host volume and a path
within a container to mount that volume to. If the host path is omitted, you create a
data container
. The volume specified is created inside the container as a read-write
filesystem not layered on top of the read-only layers used to create the container
image. Docker manages that filesystem, but you can read and write to it from the
host. Let’s illustrate this (with a truncated ID of the volumes for brevity):
$ docker run -ti -v /cookbook ubuntu:14.04 /bin/bash
root@b5835d2b951e:/# touch /cookbook/foobar
root@b5835d2b951e:/# ls cookbook/
foobar
root@b5835d2b951e:/# exit
exit
bash-4.3$ docker inspect -f {{.Mounts}} b5835d2b951e
[{dbba7caf8d07b862b61b39... /var/lib/docker/volumes/dbba7caf8d07b862b61b39... \
/_data /cookbook local true}]
$ sudo ls /var/lib/docker/volumes/dbba7caf8d07b862b61b39...
foobar
The directory created by the Docker engine to store the data in the
volume defined resides on the Docker host. If you are using
Docker Machine to set up a remote Docker host, you will need to
connect to this remote Docker host to see the data path of the
Docker volumes.
When the container is started, Docker creates the 
/cookbook
directory. From within
the container, you can read and write to this directory. Once you exit the container,
you can use 
inspect
(see 
Recipe 9.1
) to know where the volume has been created on