Re: WIP - syslogger infrastructure changes

On Fri, Sep 25, 2009 at 4:13 PM, Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> On Fri, Sep 25, 2009 at 05:18, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Mon, Sep 14, 2009 at 4:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Magnus Hagander <magnus(at)hagander(dot)net> writes:
>>>> First, the patch removes the logging_collector parameter and basically
>>>> assumes that logging_collector is always on.
>>>
>>> I don't find that to be a good idea, and you certainly have not made
>>> a case why we should change it.  I can't see any reason why pushing
>>> functionality out of backends and downstream to the syslogger process
>>> is an improvement.  What it's more likely to do is create a processing
>>> bottleneck and a single point of failure.
>>
>> Hmm.  I think the justification was supposed to be this part here:
>>
>> $ With that, it's no longer necessary to restart your server just to
>> $ reconfigure the logging, and it also takes away a confusing parameter
>> $ (really "log_destination=stderr, logging_collector=on" is *not* a logical
>> $ way to say "log to file"). Instead, it adds a log_destination of "file" that
>> $ is the standard log to file."
>>
>> Do we have any positive or negative experience with logging_collector
>> as a performance bottleneck?  Are there people running with
>> logging_collector=off to avert disaster?
>
> I've never heard of that, so I'd be very interested in hearing if somebody did.
>
> Actually, it's not starting the logging collector that's the issue.
> It's the redirection.
>
> So we could always start the logging collector, but not necessarily
> redirect stderr. For those who then set logging to "file", it gets
> passed to the logging collector. Those who set it to "stderr" just
> have to deal with stderr somehow (pg_ctl -l for example). It's still
> going to require a restart to change stderr, but that will only hit
> those people who are actually having performance issues from it  (and
> want to switch to using stderr instead of logging collector).
>
> Then we just send the data to the syslogger over a separate pipe. If
> we keep the "endpoint" of this pipe in the postmaster, a new syslogger
> could just "pick that up" after a crash, no? Data that goes on stderr
> will still go to stderr and not be captured in this case, but normally
> that would be no data at all, and if it did happen it'd be caught by
> wherever you point pg_ctl -l at (which is the same place the early
> startup messages goes, before the logger is running).

That doesn't sound horribly robust. I thought part of the point of
the design of the logging collector was that if random libraries
linked into the backend emit messages on stderr, we'll still capture
them and send them on to the appropriate place, even though we don't
really know what they are. Having such messages go off to some
probably-much-less-carefully-monitored logfile somewhere doesn't seem
good.

Could we handle this by allow the setting to be changed while the
system is up, but just have it take effect for any NEW backends? As
long as we keep the original stderr around in the postmaster, we ought
to be able to change how new backends are set up after the fact.
Anyone who really needs to make sure the setting gets changed, but
without shutting down the system, will just have to make sure to kill
off all their old backends after making the change.

...Robert