Rackspace Cloud Files is a cloud storage system that is great for storing large amounts of information. A common misconception is that this storage system behaves like a traditional file system, complete with byte-level manipulation and nested folders. It is the second of these that I want to talk about: how to simulate a nested directory (or folder) structure in Rackspace Cloud Files.
Cloud Files is better understood as a storage system, not a file system with three basic parts: accounts, containers, and objects. These three parts can be easily seen in the URL referencing an object. The URL one uses for the ReST API is of the form . Containers are large-scale groupings of objects, operating at a higher level, conceptually, than folders. If objects were books, containers may be genres. Containers cannot be nested. That is, one cannot put a container inside of another container.
However, it is fairly easy to simulate a directory structure with objects. These “virtual directories” are not directories, per se, but object name prefixes over which one can iterate. An example should make this concept easy to understand. Suppose I wanted to store books in Cloud Files. From my analogy above, I can use the genre of the book as my container name. The object name will be of the form “author/title.” This way, I can list all books by a particular author (within a genre).
Let’s load the following books into Cloud Files:
• The Pit and the Pendulum, Poe, Horror
• The Masque of the Red Death, Poe, Horror
• Pride and Prejudice and Zombies, Grahame-Smith, Horror
• The Far Side Gallery, Larson, Comics
• Something Under the Bed Is Drooling, Watterson, Comics
• It’s A Magical World, Watterson, Comics
First, I will create two containers, horror and comics. Next I will name my files according to the pattern I laid out above. I will have the files “poe/the_pit_and_the_pendulum”, “poe/the_masque_of_the_red_death”, “larson/the_far_side_gallery”, etc. Then I will upload these files to their appropriate container. As a final step, I need to upload “directory marker” files. These are empty (zero-sized) files with a content-type of “application/directory.”
[NOTE: The following gets technical. For those wishing to use this feature of Cloud Files and not wanting to program, I recommend using a third-party tool like Cyberduck (if you are using a Mac). Cyberduck handles virtual nested directories completely transparently.]
Now to take advantage of these “virtual directories”, I can do container listings and give an appropriate path value. In the Python language bindings, this would look similar to the following:
1 container = cf_connection.get_container(‘horror’)
2 books_by_poe = container.get_objects(path=’poe’)
The path parameter on the get_objects call returns all objects in the given value. In this case, it returns the two books in the virtual “poe” directory. Similarly, if I had given the value “grahame-smith,” I would have found his adaptation of the classic love story.
In my example, I’ve used two genre containers and virtual directories only one level deep. I could just as easily put everything into one container and nested the authors under a genre virtual directory. An object name would then be like “comics/larson/the_far_side_gallery.” The only limitation to using this feature in Cloud Files is keeping the length of the object name (including all virtual directories) under the maximum allowed (1024 characters).
For more detailed information on how to implement virtual directories, see the Cloud Files developer guide. The relevant information is found in the “Pseudo hierarchical folders/directories” section.