Re: [HACKERS] Transactions involving multiple postgres foreign servers - Mailing list pgsql-hackers

From Masahiko Sawada
Subject Re: [HACKERS] Transactions involving multiple postgres foreign servers
Date
Msg-id CAD21AoCHvaMe6s4VO0SvPqEiC6Tu27LkLOx4A4teZ2vh8RfNVA@mail.gmail.com
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In response to Re: [HACKERS] Transactions involving multiple postgres foreign servers  (Robert Haas <robertmhaas@gmail.com>)
Responses Re: [HACKERS] Transactions involving multiple postgres foreign servers
List pgsql-hackers
On Thu, Jul 27, 2017 at 8:02 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Thu, Jul 27, 2017 at 5:08 AM, Stas Kelvich <s.kelvich@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



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