Re: Global snapshots

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Stas Kelvich <s(dot)kelvich(at)postgrespro(dot)ru>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Global snapshots
Date: 2018-05-01 19:43:43
Message-ID: CA+TgmoazjBL1D-hSV-pMYK=g9L-Zc3v0Gh7TiAhMYRzo6nXdSg@mail.gmail.com
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On Tue, May 1, 2018 at 12:27 PM, Stas Kelvich <s(dot)kelvich(at)postgrespro(dot)ru> wrote:
> Here proposed a set of patches that allow achieving proper snapshot isolation
> semantics in the case of cross-node transactions. Provided infrastructure to
> synchronize snapshots between different Postgres instances and a way to
> atomically commit such transaction with respect to other concurrent global and
> local transactions. Such global transactions can be coordinated outside of
> Postgres by using provided SQL functions or through postgres_fdw, which make use
> of this functions on remote nodes transparently.

I'm concerned about the provisioning aspect of this problem. Suppose
I have two existing database systems with, perhaps, wildly different
XID counters. On a certain date, I want to start using this system.
Or conversely, I have two systems that are bonded together using this
system from the beginning, and then, as of a certain date, I want to
break them apart into two standalone systems. In your proposed
design, are things like this possible? Can you describe the setup
involved?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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