On Tue, Jun 16, 2015 at 5:17 PM, Kevin Benton <blak...@gmail.com> wrote: > There seems to be confusion on what causes deadlocks. Can one of you explain > to me how an optimistic locking strategy (a.k.a. compare-and-swap) results > in deadlocks? > > Take the following example where two workers want to update a record: > > Worker1: "UPDATE items set value=newvalue1 where value=oldvalue" > Worker2: "UPDATE items set value=newvalue2 where value=oldvalue" > > Then each worker checks the count of rows affected by the query. The one > that modified 1 gets to proceed, the one that modified 0 must retry.
Here's my understanding: In a Galera cluster, if the two are run in parallel on different masters, then the second one gets a write certification failure after believing that it had succeeded *and* reading that 1 row was modified. The transaction -- when it was all prepared for commit -- is aborted because the server finds out from the other masters that it doesn't really work. This failure is manifested as a deadlock error from the server that lost. The code must catch this "deadlock" error and retry the entire thing. I just learned about Mike Bayer's DBFacade from this thread which will apparently make the db behave as an active/passive for writes which should clear this up. This is new information to me. I hope my understanding is sound and that it makes sense. Carl __________________________________________________________________________ OpenStack Development Mailing List (not for usage questions) Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev
