Re: [SyncEvolution] [os-libsynthesis] super data store: trying to read to check for existance
Hello Patrick,
On Oct 24, 2011, at 16:55 , Patrick Ohly wrote:
Why is it necessary to read before trying to delete? If the item
exists,
then reading it is a fairly expensive test.
Unfortunately, with some some backends, this is the only test that reliably works. For
example, some SQL databases (or their ODBC driver) can't return a correct "number
of affected rows" for DELETE statements. So reading before deleting was the only way
to detect if any of the subdatastores really had that item and correctly return 404 or
200.
So far, my backends were written with the expectation that they have
to
cope with delete requests for items which are already deleted. This
follows from the inherent race condition between syncing and some other
process which might delete items while a sync runs.
Are all backends expected to return 404 when an item doesn't exist?
Not all backends (see above, built-in ODBC can't), but yes, for plugin backends,
returning proper error codes is specified, also for delete. Still, if a plugin would not
conform to that in its implementation of delete, that would probably have gone unnoticed
so far.
Of course, the test is a bit expensive - if that's a concern, one could differentiate
between plugin datastores and others (all the builtin ones, including SQL/ODBC), and use
the expensive read test only for the latter. Like a virtual dsDeleteDetectsItemPresence()
which returns false by default, but can be overridden in plugin datastores to return
true.
Best Regards,
Lukas Zeller, plan44.ch
luz(a)plan44.ch - www.plan44.ch