Re: what does this mean: "running xacts with xcnt == 0" - Mailing list pgsql-hackers

On 2014-11-12 12:40:41 -0500, Robert Haas wrote:
> On Wed, Nov 12, 2014 at 10:59 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> > On 2014-11-12 11:56:01 -0300, Alvaro Herrera wrote:
> >> Andres Freund wrote:
> >> > Hi,
> >> >
> >> > On 2014-11-12 09:03:40 -0500, Peter Eisentraut wrote:
> >> > > Could someone translate this detail message to English:
> >> > >
> >> > >         ereport(LOG,
> >> > >                 (errmsg("logical decoding found consistent point at %X/%X",
> >> > >                         (uint32) (lsn >> 32), (uint32) lsn),
> >> > >                  errdetail("running xacts with xcnt == 0")));
> >> >
> >> > It means there a xl_running_xacts record was encountered that had xcnt =
> >> > 0 - allowing logical decoding to find a consistent start point
> >> >
> >> > > (or downgrade to debug message, if appropriate)?
> >> >
> >> > The message generally is quite relevant, as the process of finding a
> >> > consistent start point can take quite a while. we don't really have a
> >> > nice way to make errdetail() only be logged on a certain severity level
> >> > as far as I am aware off.
> >>
> >> Can we do just the errmsg() and remove with the errdetail?
> >
> > No, I really don't want to do that. When trying to see whether logical
> > replication started that's imo quite an importantdetail. Especially when
> > first seing
> >                 ereport(LOG,
> >                         (errmsg("logical decoding found initial starting point at %X/%X",
> >                                         (uint32) (lsn >> 32), (uint32) lsn),
> >                          errdetail_plural("%u transaction needs to finish.",
> >                                                           "%u transactions need to finish.",
> >                                                           builder->running.xcnt,
> >                                                           (uint32) builder->running.xcnt)));
> >
> > Btw, Peter, why did you add a (uint32) to one, but not both,
> > builder->running.xcnt references?
> >
> >> > So maybe 'Encountered xl_running_xacts record with xcnt = 0.'?
> >>
> >> That's not very user-facing, is it -- I mean, why bother the user with
> >> the names of structs and members thereof?  It seems better to describe
> >> what the condition is; something like "found point in time with no
> >> running transaction".  Maybe "point in time" should be "WAL record"
> >> instead.
> >
> > Is that really a win in clarity? When analyzing a problem I'd much
> > rather have a concrete hint than something fuzzy.
> 
> You can't phrase error messages in terms of internal concepts that 99%
> of users won't understand or care about.  Like Peter says, user-facing
> error messages need to be written in English, not C.

That's not the actual message, but an errdetail() - and lots of those
refer to internals? And it's not an error, but a log message? E.g. we
add error contexts for wal replay errors that print the internals
literaly?  And it's *really* helpful?

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
Services