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A PCR is a type of Object Database tailored to the storage and retrieval of hierarchical data. The PCR API grew out of the needs of content management systems, which require storage of documents and other binary objects with associated metadata. However the API is applicable to many types of applications.

The data in a PCR consists of a tree of Nodes with associated Properties. Data is stored in the Properties, which may hold simple values such as numbers and strings or binary data of arbitrary length.

Hierarchical modeling is much more simple than relational modeling, commonly used when building applications with RDBMSs.

For example, one may model a simple guest book application as such:

/apps/guestbook/signatures/<user entries>

Where user entries would have unique node names such as their email address - e.g. 'gbush@whitehouse.gov'.

Then, one would simply add properties to the node 'gbush@whitehouse.gov', such as:

Title = President of the United States
Nickname = POTUS
Age = 59
Gender = male

RULES:

Sibling nodes must have a unique name
'pcr:root' must be the root node
All nodes, other than root, will be descendants of the 'pcr:root' node