Persistent storage

As you remember from Chapter 1, Introduction to Docker, the Docker container filesystem is kind of temporary by default. If you start up a Docker image (that is, run the container), you'll end up with a read-write layer on top of the layers stack. You can create, modify, and delete files as you wish; if you commit the changes back into the image, they will become persisted. This is a great feature if you want to create a complete setup of your application in the image, altogether with all its environment. But, this is not very convenient when it comes to storing and retrieving data. The best option would be to separate the container life cycle and your application from the data. Ideally, you would probably want to keep these separate, so that the data generated (or being used) by your application is not destroyed or tied to the container life cycle and can thus be reused.

The perfect example would be a web application server: the Docker image contains web server software...