The architecture of the PHPCR-ODM is similar to that of Doctrine 2 ORM. Please read the ORM architecture chapter to get a basic understanding of the Doctrine architecture. We will focus on some notable differences here.
PHPCR-ODM is built on top of Doctrine Common. However, it does not need a custom database abstraction layer (DBAL). The PHPCR standard already is an implementation independent. PHPCR-ODM does not rely on implementation specific features of the content repository but fully operate through the standard API.
The stack thus looks like this
- PHPCR (and an implementation like Jackalope or Midgard2 - like you need pdo_mysql or similar with DBAL)
This manual mainly covers the PHPCR-ODM package, sometimes touching parts of the underlying PHPCR and Common packages.
The main difference between a relational database and the PHP content repository is that in PHPCR, content is stored in one tree, not split over tables. You can think of this like a file system, or of an XML document with elements and attributes.
As long as you do not use special node types, documents can be put anywhere into the tree, regardless of the document class, like in a NoSQL database. The difference is that the tree must be complete, meaning you always need a parent to add child documents to. This is bootstrapped by adding documents to the root document.
If you are not yet familiar with PHPCR, you can just go on with PHPCR-ODM, but you will get better understanding when you also take a look at the PHPCR documentation.