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How To Share Data between Docker Containers

PostedNovember 11, 2016 5.9k views Docker Ubuntu Ubuntu 16.04

Introduction

Docker is a popular containerization tool used to provide software applications with a filesystem that contains everything they need to run. Using Docker containers ensures that the software will behave the same way, regardless of where it is deployed, because its run-time environment is ruthlessly consistent.

In general, Docker containers are ephemeral, running just as long as it takes for the command issued in the container to complete. Sometimes, however, applications need to share access to data or persist data after a container is deleted. Databases, user-generated content for a web site and log files are just a few examples of data that is impractical or impossible to include in a Docker image but which applications need to access. Persistent access to data is provided with Docker Volumes.

Prerequisites

To follow this article, you will need an Ubuntu 16.04 server with the following:

Note: Even though the Prerequisites give instructions for installing Docker on Ubuntu 16.04, the docker commands for Docker data volumes in this article should work on other operating systems as long as Docker is installed and the sudo user has been added to the docker group.

Docker Volumes can be created and attached in the same command that creates a container, or they can be created independent of any containers and then attached later. In this article, we'll look at four different ways to share data between containers.

1 — Creating an Independent Volume

Introduced in Docker's 1.9 release, the docker volume create command allows you to create a volume without relating it to any particular container. We'll use this command to add a volume named DataVolume1:

  • docker volume create --name DataVolume1

The name is displayed, indicating that the command was successful.

Output
DataVolume1

To make use of the volume, we'll create a new container from the Ubuntu image, using the --rm flag to automatically delete it when we exit. We'll use -v to mount the new volume. -v requires the name of the volume, a colon, then the absolute path to where the volume should appear inside the container. If the directories in the path don't exist as part of the image, they'll be created when the command runs. If they do exist, the mounted volume will hide the existing content.

  • docker run -ti --rm -v DataVolume1:/datavolume1 ubuntu

While we're there, we'll write some data to the volume:

  • echo "Example1" > /datavolume1/Example1.txt

Because we used the --rm flag, our container will be automatically deleted when we exit. Our volume, however, will still be accessible.

  • exit

We can verify the volume is present on the system with docker volume inspect:

  • docker volume inspect DataVolume1
Output
[ { "Name": "DataVolume1", "Driver": "local", "Mountpoint": "/var/lib/docker/volumes/datavolume1/_data", "Labels": null, "Scope": "local" } ]

Note: We can even look at the data on the host at the path listed as the Mountpoint. We should avoid altering it, however, as it can cause data corruption if applications or containers are unaware of changes.

Next, let's start a new container and attach DataVolume1:

  • docker run --rm -ti -v DataVolume1:/datavolume1 ubuntu
  • cat /datavolume1/Example1.txt
Output
Example1

Let's exit the container.

  • exit

In this example, we created a volume, attached it to a container, and verified its persistence.

2 — Creating a Volume that Persists when the Container is Removed

In our next example, we'll create a volume at the same time as the container, delete the container, then attach the volume to a new container.

We'll use the docker run command to create a new container using the base ubuntu image. -t will give us a terminal, and -i will allow us to interact with it. For clarity, we'll use --name to identify the container. Next we'll add the -v flag to create

We'll use -v to create a new volume. We'll name it DataVolume2. We'll use a colon to separate this name from the path where the volume should be mounted in the container. Finally, we will specify the base Ubuntu image and rely on the default command in the Ubuntu base image's Docker file, bash, to drop us into a shell.

  • docker run -ti --name=Container2 -v DataVolume2:/datavolume2 ubuntu

Note: The -v flag is very flexible. It can bindmount or name a volume with just a slight adjustment in syntax. If the first argument begins with a / or ~/ you're creating a bindmount. Remove that, and you're naming the volume. For example:

  • -v /path:/path/in/container mounts the host directory, /path at the /path/in/container
  • -v path:/path/in/container creates a volume named path with no relationship to the host.

For more on bindmounting a directory from the host, see How To Share Data between a Docker Container and the Host

While we're there, we'll write some data to the volume:

  • echo "Example2" > /datavolume2/Example2.txt
  • cat /datavolume2/Example2.txt
Output
Example2

We'll exit the container:

  • exit

When we restart it, the volume will mount automatically.

  • docker start -ai Container2

Let's verify that the volume has indeed mounted and our data is still in place:

  • cat /datavolume2/Example2.txt
Output
Example2

Finally, let's exit and clean up.

  • exit

Docker won't let us remove a volume if it's referenced by a container. Let's see what happens when we try:

  • docker volume rm DataVolume2

The message tells us that the volume is still in use and supplies the long version of the container ID:

Output
Error response from daemon: Unable to remove volume, volume still in use: remove DataVolume2: volume is in use - [719f98391ecf1d6f0f053ffea1bbd84cd2dc9cf6d31d5a4f348c60d98392804c]

We can use the ID to remove the container:

  • docker rm 719f98391ecf1d6f0f053ffea1bbd84cd2dc9cf6d31d5a4f348c60d98392804c
Output
719f98391ecf1d6f0f053ffea1bbd84cd2dc9cf6d31d5a4f348c60d98392804c

Removing the container won't affect the volume. We can see it's still present on the system by listing the volumes with docker volume ls:

  • docker volume ls
Output
DRIVER VOLUME NAME local DataVolume2

And we can use docker volume rm to remove it.

  • docker volume rm DataVolume2

In this example, we created an empty data volume at the same time that we created a container. In our next example, we'll explore what happens when we create a volume with a container directory that already contains data.

3 — Creating a Volume from an Existing Directory with Data

Generally, creating a volume independently with docker volume create and creating one while creating a container are equivalent, with one exception. If we create a volume at the same time as we create a container and we provide the path to a directory that contains data in the base image, that data will be copied into the volume.

As an example, we’ll create a container and add the data volume at /var, a directory which contains data in the base image:

  • docker run -ti --rm -v DataVolume3:/var ubuntu

All the content from the base image's /var directory is copied into the volume, and we can mount that volume in a new container. This time, rather than relying on the base image's default bash command, we'll issue our own ls command, which will show the contents of the volume without entering the shell:

  • docker run --rm -v DataVolume3:/datavolume3 ubuntu ls DataVolume3

DataVolume3 has a copy of the contents of the base image's /var directory:

Output
backups cache lib local lock log mail opt run spool tmp

It's unlikely that we would want to mount /var/ in this way, but this can be helpful if we've crafted our own image and want an easy way to preserve data. In our next example, we'll demonstrate how a volume can be shared between multiple containers.

4 — Sharing Data between Multiple Docker Containers

So far, we've attached a volume to one container at a time. But often, we'll want multiple containers to attach to the same data volume. This is relatively straightforward to accomplish, but there's one critical caveat: at this time, Docker doesn't handle file locking. If you need multiple containers writing to the volume, the applications running in those containers must be designed to write to shared data stores in order to prevent data corruption.

Create Container4 and DataVolume4

Use docker run to create a new container named Container4 with a data volume attached.

  • docker run -ti --name=Container4 -v DataVolume4:/datavolume4 ubuntu

Next we'll create a file and add some text:

  • echo "This file is shared between containers" > /datavolume4/Example4.txt

Then, we'll exit the container.

  • exit

This returns us to the host command prompt, where we'll make a new container that mounts the data volume from Container4

Create Container5 and Mount Volumes from Container4

We're going to create Container5, and mount the volumes from Container4:

  • docker run -ti --name=Container5 --volumes-from Container4 ubuntu
  • cat /datavolume4/Example4.txt
Output
This file is shared between containers

Now let's append some text from the second container:

  • echo "Both containers can write to DataVolume4" >> /datavolume4/Example4.txt

Finally, we'll exit the container:

  • exit

Next, we'll check that our data is still present to Container4.

View Changes Made in Container5

Let's check for the changes that were written to the data volume by Container5 by restarting Container4:

  • docker start -ai Container4
  • cat /datavolume4/Example4.txt
  • cat /DataVolume4/Example4.txt
Output
This file is shared between containers Both containers can write to DataVolume4

Now that we've verified that both containers were able to read and write from the data volume, we'll exit the container:

  • exit

Again, Docker doesn't handle any file locking, so applications must account for the file locking themselves. It is possible to mount a Docker volume as read-only to ensure that data corruption won't happen by accident when a container requires read-only access by adding :ro. Let's look at how this works.

Start Container 6 and Mount the Volume Read-Only

Once a volume has been mounted in a container, rather than unmounting it like we would with a typical Linux file system, we instead create a new container mounted the way we want it and, if needed, remove the previous container. To make the volume read-only, we append :ro to the end of the container name:

  • docker run -ti --name=Container6 --volumes-from Container4:ro ubuntu

We'll check the read-only status by trying to remove our example file:

  • rm /datavolume4/Example4.txt
Output
rm: cannot remove '/datavolume4/Example4.txt': Read-only file system

Finally, we'll exit the container and clean up our test containers and volumes:

  • exit

Now that we’re done, let’s clean up our containers and volume:

  • docker rm Container4 Container5 Container6
  • docker volume rm DataVolume4

In this example, we've shown how to share data between two containers using a data volume and how to mount a data volume as read-only.

Conclusion

In this tutorial, we've created a data volume which allowed data to persist through the deletion of a container. We've shared data volumes between containers, with the caveat that applications will need to be designed to handle the file locking to prevent data corruption. Finally, we've shown how to mount a shared volume in read-only mode. If you're interested learning about sharing data between containers and the host system, see How To Share Data between the Docker Container and the Host.

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