As a general framework for creating and annotating caGrid service metadata, the MMS can be configured to communicate with nearly any external metadata repository. In order for clients to ascertain the metadata repositories available for a given MMS service, the MMS provides Model Source Metadata which can be retrieved by a client and interrogated for such information.

The Model Source Metadata contains a defaultSourceIdentifier and a collection of supported Model Sources, which describe the requirements and capabilities of the external metadata repositories. Each repository, a Model Source, is described by a SourceDescriptor in the Model Source Metadata. The Model Source is identified by its identifier, which is its unique name used to identify it in the MMS's operations. The Model Source is described by its description attribute, which is used solely for human readable presentation of the source. Each Model Source also has an optional collection of PropertyDescriptors, each of which describe a Property (a name value pair) available for use in UMLProjectIdentifers within the described external metadata repository. That is, the name of descriptor corresponds to the name that would be used in a Property. The PropertyDescriptor has two additional attributes: a description attribute which is used to describe what the Property represents, and a required boolean attribute which indicates whether or not UMLProjectIdentifer's for the given source are required to provide values for this Property.