On Thu, Oct 6, 2016 at 1:41 PM, Ashutosh Bapat <ashutosh.ba...@enterprisedb.com> wrote: >>> >>> No, the COMMIT returns after the first phase. It can not wait for all >>> the foreign servers to complete their second phase >> >> Hm, it sounds like it's same as normal commit (not 2PC). >> What's the difference? >> >> My understanding is that basically the local server can not return >> COMMIT to the client until 2nd phase is completed. > > > If we do that, the local server may not return to the client at all, > if the foreign server crashes and never comes up. Practically, it may > take much longer to finish a COMMIT, depending upon how long it takes > for the foreign server to reply to a COMMIT message.
Yes, I think 2PC behaves so, please refer to [1]. To prevent local server stops forever due to communication failure., we could provide the timeout on coordinator side or on participant side. > >> Otherwise the next transaction can see data that is not committed yet >> on remote server. > > 2PC doesn't guarantee transactional consistency all by itself. It only > guarantees that all legs of a distributed transaction are either all > rolled back or all committed. IOW, it guarantees that a distributed > transaction is not rolled back on some nodes and committed on the > other node. > Providing a transactionally consistent view is a very hard problem. > Trying to solve all those problems in a single patch would be very > difficult and the amount of changes required may be really huge. Then > there are many possible consistency definitions when it comes to > consistency of distributed system. I have not seen a consensus on what > kind of consistency model/s we want to support in PostgreSQL. That's > another large debate. We have had previous attempts where people have > tried to complete everything in one go and nothing has been completed > yet. Yes, providing a atomic visibility is hard problem, and it's a separated issue[2]. > 2PC implementation OR guaranteeing that all the legs of a transaction > commit or roll back, is an essential block of any kind of distributed > transaction manager. So, we should at least support that one, before > attacking further problems. I agree. [1]https://en.wikipedia.org/wiki/Two-phase_commit_protocol [2]http://www.bailis.org/papers/ramp-sigmod2014.pdf Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
