On Tue, Jun 8, 2010 at 1:00 PM, Laurence Rowe <l@lrowe.co.uk> wrote:
On 8 June 2010 14:38, Jim Fulton <jim@zope.com> wrote:
This is intended as a broad response to the thread, rather than a response to any specific post. :)
I've been thinking of expanding the data manager API to add an optional tpc_rollback method. If tpc_finish returns a value and a data manager provided tpc_rollback and some other data manager fails in tpc_finish, then tpc_rollback would be used to *try* to recover from the other data managers failure. Note that even if tpc_rollback is implemented, it might fail if the transaction can't be rolled back (due, typically, to subsequent conflicting transactions).
While I can imagine a ZODB implementation of tpc_rollback that could work in some circumstances for some storages, even then it seems it would be quite complex and perhaps unlikely to succeed - as soon as another connection read anything from the database you would be unable to tpc_rollback, unless you deferred truly committing the transaction to a tpc_truly_finished which would just bring you back where you started.
No. It would behave exactly like (probably built on) undo, which generates suitable invalidations. (Of course, undo itself weakens consistency to some degree.)
For other systems I can't think how it might be implemented - you can't unsend a mail or uncommit a committed transaction in a relational database.
Of course not. Those wouldn't implement this method. But it would provide a saner way to deal with *some* failures in 2pc. For example, if you had a ZODB database and a relational db and the rdb raised an error in tpc_finish, you could perhaps roll back the zodb transaction. Jim -- Jim Fulton