Re: [HACKERS] Transactions involving multiple postgres foreign servers

Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:

> On Mon, Oct 30, 2017 at 5:48 PM, Ashutosh Bapat
> <ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:
> > On Thu, Oct 26, 2017 at 7:41 PM, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >>
> >> Because I don't want to break the current user semantics. that is,
> >> currently it's guaranteed that the subsequent reads can see the
> >> committed result of previous writes even if the previous transactions
> >> were distributed transactions. And it's ensured by writer side. If we
> >> can make the reader side ensure it, the backend process don't need to
> >> wait for the resolver process.
> >>
> >> The waiting backend process are released by resolver process after the
> >> resolver process tried to resolve foreign transactions. Even if
> >> resolver process failed to either connect to foreign server or to
> >> resolve foreign transaction the backend process will be released and
> >> the foreign transactions are leaved as dangling transaction in that
> >> case, which are processed later. Also if resolver process takes a long
> >> time to resolve foreign transactions for whatever reason the user can
> >> cancel it by Ctl-c anytime.
> >>
> >
> > So, there's no guarantee that the next command issued from the
> > connection *will* see the committed data, since the foreign
> > transaction might not have committed because of a network glitch
> > (say). If we go this route of making backends wait for resolver to
> > resolve the foreign transaction, we will have add complexity to make
> > sure that the waiting backends are woken up in problematic events like
> > crash of the resolver process OR if the resolver process hangs in a
> > connection to a foreign server etc. I am not sure that the complexity
> > is worth the half-guarantee.
> >
>
> Hmm, maybe I was wrong. I now think that the waiting backends can be
> woken up only in following cases;
> - The resolver process succeeded to resolve all foreign transactions.
> - The user did the cancel (e.g. ctl-c).
> - The resolver process failed to resolve foreign transaction for a
> reason of there is no such prepared transaction on foreign server.
>
> In other cases the resolver process should not release the waiters.

I'm not sure I see consensus here. What Ashutosh says seems to be: "Special
effort is needed to ensure that backend does not keep waiting if the resolver
can't finish it's work in forseable future. But this effort is not worth
because by waking the backend up you might prevent the next transaction from
seeing the changes the previous one tried to make."

On the other hand, your last comments indicate that you try to be even more
stringent in letting the backend wait. However even this stringent approach
does not guarantee that the next transaction will see the data changes made by
the previous one.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
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Web: http://www.postgresql-support.de, http://www.cybertec.at