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OwnCloud: Making Your Own DB Backend
OwnCloud's flexible architecture makes it easy to integrate existing databases. Here's an informal document with tips on how to do it.
First, keep in mind that there are 2 backends that are common implemented. A User Backend (implements \OCP\UserInterface) and a Storage Backend (extends OC_Filestorage_Common)
NOTE: For a discussion on "implements" and "extends", take a look at this link: http://stackoverflow.com/questions/4961906/php-when-to-implement-and-extend
For an idea of what a backend looks like, take a look at the LDAP owncloud/htdocs/apps/user_ldap/user_ldap.php file. This shows what a basic backend looks like.
Another way is to look at the storage backend: owncloud\htdocs\lib\filestorage\common.php
Also, you should look at the local filesystem: local.php
About Extending Backends...
http://mail.kde.org/pipermail/owncloud/2012-July/004516.html