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The Tigris site will receive a major upgrade the evening of Monday, December 1, beginning at 8:30 pm PST. Downtime is projected to be about ten hours. Further details in the announcement

The Consistent Snapshot Problem

Because Ubik loads many objects on demand, it is possible that an inconsistent subset of the object model might be created on the client. Two related objects with a particular constraint on their relationship may have this constraint broken if one is loaded, and the other is changed significantly before it is loaded itself.

This issue is also present in most relational databases under their default isolation settings, which are either 'Read Committed' or 'Repeatable Read.' Databases, however, provide the 'Serialisable' isolation level for transactions that absolutely must be isolated. This situation is probably rare enough that it doesn't warrant major attention, however it would be a worthwhile addition to Ubik.

Without access to the underlying database connection, it is difficult for Ubik to provide this isolation level.

One solution is to allow clients to instigate long-running server-side transactions, which could be applied only when absolutely necessary. There are many issues with this although they can be worked around to a reasonable degree. Added complexity to what is currently a very simple system is the main drawback.

Another solution is to provide a command interface to the server, through which serialised command objects can be executed in a server-side application domain, within a serialisable transaction. This approach places more design constraints on the application but given that it is for one-off cases this may be acceptable.

As more alternatives develop they'll be listed here.