The Fedora 4 Object Model
If you're interested in the differences between the object model of Fedora 3 and earlier and the object model of Fedora 4, start here.
(Cribbed heavily from the JCR Repository Model documentation)
This document describes the objects and structures that compose a Fedora repository.
A Fedora 4 repository is composed of one or more workspaces, each consisting of a directed acyclic graph of nodes where edges represent a parent-child relation.
For those without a computer science background, in repository terms:
- A node is an object or datastream.
- An edge is a relationship (in this case, a parent-child relationship) that links two nodes. Edges are represented as RDF properties in a Fedora object.
- A graph is a set of nodes connected by edges. The entire set of objects in the workspace and their relationships to each other form a graph.
- A directed graph is a graph whose edges have direction associated with them: that is, the relations that link the nodes move in a given direction from one node to the next. An example of an edge with a direction is isAChildOf. The relationship expresses a direction from the child to the parent. The relationship isParentOf can also link the same two nodes, but in the opposite direction.
- A directed acyclic graph is a graph in which edges never form loops (i.e., cycles) of nodes. That is, if you start at node A, then follow a set of identical relations from that node to other nodes in a chain, you will never end up back at node A.
- A hierarchical tree, such as the one modeled in the Fedora repository workspace by the relationships provided by the JCR underpinnings of Fedora, is a classic example of a directed acyclic graph of nodes. In this sense, no Fedora object can ever be its own grandpa. Of course, the additional relationships that you, the user, distribute amongst your objects are entirely up to you. You can make them as simple or complex as you like.
Each persistent workspace is identified by a unique name within the repository. There is a default workspace, which has an empty workspace identifier.
Each workspace contains at least one item, the root node. The root node is the only item in the workspace without a parent node; all other items have at least one parent.
Nodes can have zero or more child nodes. Nodes can have zero or more properties. Properties can hold one or more values.
The location of an item in the workspace graph can be described by the path from the root node to that item. The path consists of the name of each interceding node in order from root to target item, much like a file system path.
The name of a item consists of the namespace prefix and a local name component. The namespace prefix must be registered before use.
The local name of a node is limited to the range of valid XML characters (http://www.w3.org/TR/xml/#NT-Char)
#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
except
'/' | ':' | '[' | ']' | '|' | '*'
In addition to a path, every node also has an identifier. The identifier is independent of the path and provides an identity to the node that is stable across moves within the workspace.
Some nodes may possess a "jcr:data" property consisting of a binary payload with a length and digest. These nodes are normally leaf nodes in the tree (meaning they have no child nodes of their own), and they are almost always named "jcr:content". They are the best match in Fedora 4 to the Fedora 3 concept of "datastream".
The content of the jcr:data property is (by default) de-duplicated across the repository. If you store the same binary stream in five jcr:data properties, it will only be persisted once, unless you expressly configure otherwise.
Properties may be single- or multi-valued, and are typed. Some properties are reserved for internal-use under mutation only (e.g. those in the fedora-internal namespace), but can otherwise be used freely by implementors. By default, properties are multivalued string types.
Fedora internal properties:
http://fedora.info/definitions/v4/repository
