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Better ZODB handling of broken interfaces and components

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README.rst

Experimental ZODB Broken Object Handling

WARNING! This package is highly experimental and should not be used by anyone but experts in the inner workings of the ZCA and the ZODB and it should never be used in production.

The handling of broken objects by the ZODB can make an application with add-ons that use zope.interface far too fragile. If marker interfaces from an add-on are used on objects in the ZODB, removing that add-on can make any zope.interface operation on that object fail. Even worse, if an add-on registers any components in a registry in the ZODB, that entire registry will become unusable for any ZCA operations.

Since the interfaces and the ZCA are often core parts of an application using the ZODB, it may be appropriate to add special handling for broken objects to those services.The patches included in this package attempt to do just that.

For objects in the ZODB which directly provide a marker interface, these patches allow that object to behave as without the application of the marker interface if the interface is no longer available. If the interface is made available again, the full behavior of that interface is restored. Similarly, if a component whose class, provided interface, or required interfaces are missing, these patches allow the registry to perform lookups it would have been able to do without the broken component registration. If the component class, provided interface, and required interfaces are restored, then the component registration is fully restored. If an object or registry in the ZODB is committed to the ZODB with broken interfaces or components, the commit will succeed and it is still possible to fully restore previous behavior if the missing classes and interfaces are restored.

The one exception is when the class of a non-persistent component registered in a persistent registry is removed, the registry cannot be successfully committed back to the ZODB. IOW, if your persistent registry contains a registration for a component whose base classes do not include persistent.Persistent, then you'll be able to use the registry but you won't be able to make any changes to the registry without first removing the broken component registration.

The intention of this package is to see if the implementation of broken object handling is correct and robust enough to merge into zope.interface and zope.component themselves. IOW, when this code is found to be robust enough to use more widely, this package won't be the home for it. So once again, unless you are a senior Zope developer, do not use these patches.

If you want to try this package out, work only on a copy of your ZODB, never on the live data, and simply add this package to your instance's eggs. It uses z3c.autoinclude to automatically apply the patches under a Plone instance.

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