Re: shared-memory based stats collector

Hi,

On 2020-03-09 15:04:23 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2020-03-09 15:37:05 -0300, Alvaro Herrera wrote:
> >> I'm worried that we're causing all processes to terminate when an
> >> archiver dies in some ugly way; but in the current coding, it's pretty
> >> harmless and we'd just start a new one. I think this needs to be
> >> reconsidered. As far as I know, pgarchiver remains unconnected to
> >> shared memory so a crash-restart cycle is not necessary. We should
> >> continue to just log the error message and move on.
>
> > Why is it worth having the archiver be "robust" that way?
>
> I'd ask a different question: what the heck is this patchset doing
> touching the archiver in the first place? I can see no plausible
> reason for that doing anything related to stats collection.

As of a release or two back, it sends stats messages for archiving
events:

if (pgarch_archiveXlog(xlog))
{
/* successful */
pgarch_archiveDone(xlog);

/*
* Tell the collector about the WAL file that we successfully
* archived
*/
pgstat_send_archiver(xlog, false);

break; /* out of inner retry loop */
}
else
{
/*
* Tell the collector about the WAL file that we failed to
* archive
*/
pgstat_send_archiver(xlog, true);

> If we now need some new background processing for stats, let's make a
> new postmaster child process to do that, not overload the archiver
> with unrelated responsibilities.

I don't think that's what's going on. It's just changing the archiver so
it can report stats via shared memory - which subsequently means that it
needs to deal differently with errors than when it wasn't connected.

Greetings,

Andres Freund