Re: Transactions involving multiple postgres foreign servers, take 2

On Mon, Jun 15, 2020 at 7:06 PM Masahiko Sawada
<masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:
>
> On Mon, 15 Jun 2020 at 15:20, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> >
> > > > Even the local
> > > > server is not modified, since a resolver process commits prepared
> > > > foreign transactions one by one another user could see an inconsistent
> > > > result. Providing globally consistent snapshots to transactions
> > > > involving foreign servers is one of the solutions.
> >
> > How would it be able to do that? Say, when it decides to take a
> > snapshot the transaction on the foreign server appears to be committed
> > but the transaction on the local server won't appear to be committed,
> > so the consistent data visibility problem as mentioned above could
> > still arise.
>
> There are many solutions. For instance, in Postgres-XC/X2 (and maybe
> XL), there is a GTM node that is responsible for providing global
> transaction IDs (GXID) and globally consistent snapshots. All
> transactions need to access GTM when checking the distributed
> transaction status as well as starting transactions and ending
> transactions. IIUC if a global transaction accesses a tuple whose GXID
> is included in its global snapshot it waits for that transaction to be
> committed or rolled back.
>

Is there some mapping between GXID and XIDs allocated for each node or
will each node use the GXID as XID to modify the data? Are we fine
with parking the work for global snapshots and atomic visibility to a
separate patch and just proceed with the design proposed by this
patch? I am asking because I thought there might be some impact on
the design of this patch based on what we decide for that work.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com