The model extension registry needs to be persisted in the extension plugin's state location under the .metadata folder. Using an Eclipse linked resource for the exact location should be investigated as one day this location could exist outside the workspace. The ExtensionPlugin should persist the registry at shutdown and restore the registry at startup. Only user-defined (non-built-in) model extension definitions (MEDs) need to be persisted as the built-in MEDs can continue to be loaded into the registry using the existing extension point. Two possible ways of persisting the registry would be (1) create/save *.mxd file for each registered MED, or (2) create one *.xml file that contains entries for each registered MED. Advantage of (1) is that at startup the existing way of reading and writing MEDs could be used. Advantage of (2) would be only one file would be needed. The existing MED writer could potentially be modified for use in (2). See the DqpPlugin as it is solving a similar problem.