Re: Support for N synchronous standby servers - take 2

From: Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Thom Brown <thom(at)linux(dot)com>, Beena Emerson <memissemerson(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Support for N synchronous standby servers - take 2
Date: 2016-03-30 14:43:36
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On Tue, Mar 29, 2016 at 5:36 PM, Kyotaro HORIGUCHI
<horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> I personally don't think it needs such a survive measure. It is
> very small syntax and the parser reads very short text. If the
> parser failes in such mode, something more serious should have
> occurred.
> At Tue, 29 Mar 2016 16:51:02 +0900, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote in <CAHGQGwFth8pnYhaLBx0nF8o4qmwctdzEOcWRqEu7HOwgdJGa3g(at)mail(dot)gmail(dot)com>
>> On Tue, Mar 29, 2016 at 4:23 PM, Kyotaro HORIGUCHI
>> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> > Hello,
>> >
>> > At Mon, 28 Mar 2016 18:38:22 +0900, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote in <CAD21AoAJMDV1EUKMfeyaV24arx4pzUjGHYbY4ZxzKpkiCUvh0Q(at)mail(dot)gmail(dot)com>
>> > sawada.mshk> On Mon, Mar 28, 2016 at 5:50 PM, Kyotaro HORIGUCHI
>> >> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>> > As mentioned in my comment, SQL parser converts yy_fatal_error
>> > into ereport(ERROR), which can be caught by the upper PG_TRY (by
>> > #define'ing fprintf). So it is doable if you mind exit().
>> I'm afraid that your idea doesn't work in postmaster. Because ereport(ERROR) is
>> implicitly promoted to ereport(FATAL) in postmaster. IOW, when an internal
>> flex fatal error occurs, postmaster just exits instead of jumping out of parser.
> If The ERROR may be LOG or DEBUG2 either, if we think the parser
> fatal erros are recoverable. guc-file.l is doing so.
>> ISTM that, when an internal flex fatal error occurs, it's
>> better to elog(FATAL) and terminate the problematic
>> process. This might lead to the server crash (e.g., if
>> postmaster emits a FATAL error, it and its all child processes
>> will exit soon). But probably we can live with this because the
>> fatal error basically rarely happens.
> I agree to this
>> OTOH, if we make the process keep running even after it gets an internal
>> fatal error (like Sawada's patch or your idea do), this might cause more
>> serious problem. Please imagine the case where one walsender gets the fatal
>> error (e.g., because of OOM), abandon new setting value of
>> synchronous_standby_names, and keep running with the previous setting value.
>> OTOH, the other walsender processes successfully parse the setting and
>> keep running with new setting. In this case, the inconsistency of the setting
>> which each walsender is based on happens. This completely will mess up the
>> synchronous replication.
> On the other hand, guc-file.l seems ignoring parser errors under
> normal operation, even though it may cause similar inconsistency,
> if any..
> | LOG: received SIGHUP, reloading configuration files
> | LOG: input in flex scanner failed at file "/home/horiguti/data/data_work/postgresql.conf" line 1
> | LOG: configuration file "/home/horiguti/data/data_work/postgresql.conf" contains errors; no changes were applied
>> Therefore, I think that it's better to make the problematic process exit
>> with FATAL error rather than ignore the error and keep it running.
> +1. Restarting walsender would be far less harmful than keeping
> it running in doubtful state.
> Sould I wait for the next version or have a look on the latest?

Attached latest patch incorporate some review comments so far, and is
rebased against current HEAD.


Masahiko Sawada

Attachment Content-Type Size
multi_sync_replication_v21.patch application/octet-stream 41.8 KB

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