| 摘要 |
A new schema implementation enables reversible changes to be made to definitions in a schema. The definitions of attributes and classes are in the form of schema objects. Each schema object has an internal ID in addition to its unique name and object ID (DID), and a deactivation flag for indicating whether the schema object has been deactivated. To modify an attribute or class, the corresponding schema object is deactivated by setting its deactivation flag to "TRUE," and a new schema object is created to contain the changes. The new schema object has the same unique name and DID as the old schema object but is given a different internal ID. When a client wants to access instances of the schema object by its unique name or DID, a translation is made to provide the internal ID of the new schema object such that instances of the new schema object are located instead of the old schema object. The changes to the schema can be reversed by reactivating the first schema object and deactivating the second schema object.
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