The Hierarchy OSID is an auxiliary service providing a means for accessing and managing hierarchical relationships among OSID Ids.

An OSID Id may have onr or more parents or children and the hierarchy itself represents a directed acyclic graph. The hierarchy service defines a set of interfaces used among other OSIDs that utilize hierarchies and can also be used to abstract hierarchical data into a standalone service.

Hierarchical queries may be performed using the HierarchyTraversalSession. A set of methods exist to query parents, children, ancestors, and decendants. A Node structure may be retrieved to access a portion of a hierarchy in bulk. The Node provides methods to get parents and children of the node directly.

Hierarchies are federateable by combining nodes. There is no hierarchy service for the hierarchy catalog.