Storage

Being a media publishing platform, storage is a big deal in MediaGoblin. As such there are a few systems that are storage-related that you may encounter while doing some MediaGoblin hacking.

MediaGoblin also comes with an extensible storage interface and several implementations mapping to it: basic local file storage, OpenStack "swift" style storage... and a few more plus the ability to write your own.

= The storage systems attached to your app =

Dynamic content: queue_store and public_store
Two instances of the StorageInterface come attached to your app. These are:


 * queue_store: When a user submits a fresh piece of media for their gallery, before the Processing stage, that piece of media sits here in the queue_store. (It's possible that we'll rename this to "private_store" and start storing more non-publicly-stored stuff in the future...).  This is a StorageInterface implementation instance.  Visitors to your site probably cannot see it... it isn't designed to be seen, anyway.
 * public_store: After your media goes through processing it gets moved to the public store. This is also a StorageInterface implelementation, and is for stuff that's intended to be seen by site visitors.

The workbench
In addition, there's a "workbench" used during processing... it's just for temporary files during processing, and also for making local copies of stuff that might be on remote storage interfaces while transitionally moving/converting from the queue_store to the public store. See the workbench module documentation for more.

Static assets / staticdirect
On top of all that, there is some static media that comes bundled with your application. This stuff is kept in:

mediagoblin/static/

These files are for mediagoblin base assets. Things like the CSS files, logos, etc. You can mount these at whatever location is appropriate to you (see the direct_remote_path option in the config file) so if your users are keeping their static assets at http://static.mgoblin.example.org/ but their actual site is at http://mgoblin.example.org/, you need to be able to get your static files in a where-it's-mounted agnostic way. There's a "staticdirector" attached to the request object. It's pretty easy to use; just look at this bit taken from the mediagoblin/templates/mediagoblin/base.html main template:



see? Not too hard. As expected, if you configured direct_remote_path to be http://static.mgoblin.example.org/ you'll get back http://static.mgoblin.example.org/css/extlib/text.css just as you'd probably expect.

= StorageInterface and implementations =

The guts of StorageInterface and friends
So, the StorageInterface!

So, the public and queue stores both use StorageInterface implementations... but what does that mean? It's not too hard.

Open up: mediagoblin/storage/__init__.py

In here you'll see a couple of things. First of all, there's the StorageInterface class. What you'll see is that this is just a very simple python class. A few of the methods actually implement things, but for the most part, they don't. What really matters about this class is the docstrings. Each expected method is documented as to how it should be constructed. Want to make a new StorageInterface? Simply subclass it. Want to know how to use the methods of your storage system? Read these docs, they span all implementations.

There are a couple of implementations of these classes bundled in storage.py as well. The most simple of these is BasicFileStorage, which is also the default storage system used. As expected, this stores files locally on your machine.

There's also a CloudFileStorage system. This provides a mapping to OpenStack's swift storage system (used by RackSpace Cloud files and etc).

Between these two examples you should be able to get a pretty good idea of how to write your own storage systems, for storing data across your beowulf cluster of radioactive monkey brains, whatever.

Writing code to store stuff
So what does coding for StorageInterface implementations actually look like? It's pretty simple, really. For one thing, the design is fairly inspired by Django's file storage API... with some differences.

Basically, you access files on "file paths", which aren't exactly like unix file paths, but are close. If you wanted to store a file on a path like dir1/dir2/filename.jpg you'd actually write that file path like:

['dir1', 'dir2', 'filename.jpg']

This way we can be *sure* that each component is actually a component of the path that's expected... we do some filename cleaning on each component.

Your StorageInterface should pass in and out "file like objects". In other words, they should provide .read and .write at minimum, and probably also .seek and .close.