ZenPacks can provide Python classes for objects that will be stored in the object database. The most frequent example of this is DataSource subclasses. When a ZenPack is removed those classes are no longer accessible so the objects in the database are broken. (Zeo needs to have the appropriate Python class in order to unpickle an object from the database.) In previous versions of Zenoss there was not an easy way to associate instances of a ZenPack-provided class with the ZenPack that provided the class. As a result ZenPack removal could easily cause broken objects to remain in the database. If Zope had already loaded a class into the interpreter the objects in question might continue to function until Zope was restarted, making diagnosis of such problems even more difficult.

In Zenoss 2.2 the ZenPackPersistance class aims to remedy this problem. Any Python class provided by a ZenPack should subclass ZenModel.ZenPackPersistence.ZenPackPersistence. Zenoss maintains a catalog of all ZenPackPersistence instances in the database. When a ZenPack is removed, the catalog is queried to determine which objects need to be deleted. Any ZenPack-provided Python class that might be instantiaed in the object database should subclass ZenPackPersistence and define ZENPACKID in the class as the name of the ZenPack providing the class. For an example of this see ZenPacks.zenoss.MySqlMonitor.datasources.MySqlMonitorDataSource.