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05-stateful-containers.md

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How to deal with data persistence ?

During the previous exercise, you modified a file inside a container, do you think it is something persistent ?

Data persistence

  • First, create a container HTTPD, modify a file, and then restart it.
echo "<h1>It works better with Docker...</h1>" > index.html
docker run --name "my-httpd-server" -p="80:80" -d httpd:2.4 # Maybe you need to stop another instance before
docker cp index.html my-httpd-server:/usr/local/apache2/htdocs/index.html
docker restart my-httpd-server

Check in your browser if restarting a container affected the data inside ?

  • Next, remove this container, and create a new one with the same information.
docker stop my-httpd-server
docker rm my-httpd-server
docker run --name "my-httpd-server" -p="80:80" -d httpd:2.4

And now ? check in your browser what happened.

Containers are stateless

By default, containers are stateless. Their lifetime may be very short. So they don't have any persistence context ! Data AND configurations will be lost at the container removal.

What about databases or others cache system that need to persist some data ? Don't worry, you can tell docker to use (read and/or write) on a disk outside the container ! That is what we call volumes.

Volumes

You can tell docker to share a folder or simply a file between the host and a container. Just like you need a port mapping if you would like to expose a port on your host, you need to add a volume mapping if you would like to share data.

Use option -v, put on the left the folder on the host and put on the right the folder inside the container.

  • Create a folder to persist the index.html file on your host.
mkdir -p $HOME/data/htdocs
echo "<h1>It works better with Docker...</h1>" > $HOME/data/htdocs/index.html
  • Start a new HTTPD container with the volume option and check in your browser. Modify the file on your host and check again in your browser.
docker run --name "my-httpd-server" -p="80:80" -d -v=$HOME/data/htdocs:/usr/local/apache2/htdocs httpd:2.4
# Check in your browser
echo "<h1>It works better with Docker and volumes...</h1>" > $HOME/data/htdocs/index.html
# Check in your browser
  • Stop and remove current container, start a new one with the same volume option to see what happen.
docker stop my-httpd-server
docker rm my-httpd-server
docker run --name "my-httpd-server" -p="80:80" -d -v=$HOME/data/htdocs:/usr/local/apache2/htdocs httpd:2.4
# Check in your browser

Types of volumes

On this exercise, you bind mount a simple volume. But they are others important ways to create and use volumes. If you have a distributed architecture, you will have to mount NFS or other cloud storage, and register these volumes with docker volume command.

Learn more about volumes by reading this page (16 minutes reading.) https://docs.docker.com/storage/volumes/

Permissions

Now you know that a container, easily to start and access, can write some data onto your host ! So be careful of what you do, security of data is very important, you need to manage permission on disk just like if you were not using containers !

There are a lot of security options that you need to use at runtime in order to preserve permissions !

Next Step

That's all for now. Thanks and don't forget to send a feedback !

» (WIP)

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