Before Storage

ZODB storages typically store multiple object revisions to support features such as multi-version concurrency control and undo. In the case of the mod popular storage implementation, old revisions aren't discarded until a pack. This feature has often been exploited to perform time travel, allowing one to look at a database as it existed in at some point in time. In the past, this has been possible with file storage by specifying a time at which to open the file storage. This works fairly well, but is very slow for large databases because existing index files can't easily be used. Time travel is also supported for individual objects through the ZODB history mechanism.

The introduction of multi-version concurrency control provided new opertunities for time travel. Using the storage loadBefore method, one can load transaction records written before a given time. ZODB 3.9 will provide an option to the database open method for opening connections as of a point in time.

Demo storage can be quite useful for testing, and especially staging applications. In a common configuration, they allow for storing changes to a base database without changing the underlying database. Zope functional testing frameworks leverage demo storages to easily roll-back database state after a test to a non-empty state before a test. A significant limitation of demo storages is that they can't be used with base storages that change. This means that they generaly can't be used with ZEO. It isn't enough to have a read-only connections, if the underlying database is still being changed by other clients.

The "before" storage provides another way to leverage the loadBefore method to support time travel and a means to provide an unchanging view into a ZEO server. A before storage is a database adapter that provides a read-only view of an underlying storage as of a particular point in time.