This class implements a Cacheable for a DataDictionary cache of table descriptors. It is an abstract class - there is more than one cache of table descriptors per data dictionary, and this class provides the implementation that's common to all of them. The lookup key for the cache (the "identity" of the cache item) is provided by the subclass. Another design alternative was to make the table descriptors themselves the cacheable objects. This was rejected because: we would have only one way of caching table descriptors, and we need at least two (by UUID and by name); the contents of a table descriptor would have to be split out into a separate class, so it could be used as the createParameter to the createIdentity() method; the releasing of the Cacheable would have to be done when at the end of compilation by traversing the tree - by creating a separate Cacheable object, we can release the object within the getTableDescriptor() method after getting the table descriptor out of it.