On Thu, Jul 27, 2017 at 8:02 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Thu, Jul 27, 2017 at 5:08 AM, Stas Kelvich <s.kelv...@postgrespro.ru> > wrote: >> As far as I understand any solution that provides proper isolation for >> distributed >> transactions in postgres will require distributed 2PC somewhere under the >> hood. >> That is just consequence of parallelism that database allows — transactions >> can >> abort due concurrent operations. So dichotomy is simple: either we need 2PC >> or >> restrict write transactions to be physically serial. >> >> In particular both Postgres-XL/XC and postgrespro multimaster are using 2PC >> to >> commit distributed transaction. > > Ah, OK. I was imagining that a transaction manager might be > responsible for managing both snapshots and distributed commit. But > if the transaction manager only handles the snapshots (how?) and the > commit has to be done using 2PC, then we need this.
One way to provide snapshots to participant nodes is giving a snapshot data to them using libpq protocol with the query when coordinator nodes starts transaction on a remote node (or we now can use exporting snapshot infrastructure?). IIUC Postgres-XL/XC uses this approach. That also requires to share the same XID space with all remote nodes. Perhaps the CSN based snapshot can make this more simple. >> Also I see the quite a big value in this patch even without >> tm/snapshots/whatever. >> Right now fdw doesn’t guarantee neither isolation nor atomicity. And if one >> isn’t >> doing cross-node analytical transactions it will be safe to live without >> isolation. >> But living without atomicity means that some parts of data can be lost >> without simple >> way to detect and fix that. > > OK, thanks for weighing in. > Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers