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Figure 2: The Metadata Collection API

There are four five groups of methods on Metadata Collection:

1. Retrieving information about the metadata repository
2. Retrieving information about the types supported by the metadata repository
3. Retrieving information about the metadata instances stored in the metadata repository
4. Updating the metadata instances stored in the metadata repository.
5. Repairing the metadata collection when anomalies are detected.

The POJO objects returned for metadata instances are represented as Entities and Relationships.  Entities are the concepts and assets that the metadata represents and the Relationships between them show how they are related.  Entities and Relationship can contain properties.   Entities may have complex structures as properties (called Structs) and they may be classified using a Classification.  Structs and Classification can contain properties and are linked to the Entity.

This is illustrated in figure 3:

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Figure 3: Structure of metadata instances

The TypeDefs define the properties for Structs, Classifications, Entities and Relationships and how they relate to one another.  These are defined in StructDefs, ClassificationDefs, EntityDefs and RelationshipDefs respectively.  Each property attribute is described by an AttributeDef.  It is also possible to define enumeration types for properties using EnumDefs and structs (complex types) using StructDefs (although StructDefs are only allowed in AttributeDefs for entities).