Re: when the startup process doesn't

On Mon, Apr 19, 2021 at 8:44 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Hmm ... +1 for progress messages in the log, but I'm suspicious about
> the complexity-and-fragility-versus-value tradeoff for the other thing.

All right, so it's nice to hear that nobody so far is opposed to the
log-based solution, and I think it's sensible to think about building
that one first and doing anything else later.

But, if we did want to invent something to allow monitoring via libpq
even at this early stage, how would we make it work? Magnus pointed
out that we can hardly read pg_authid during crash recovery, which
means that accepting logins in the usual sense at that stage is not
feasible. But, what if we picked a fixed, hard-coded role name for
this? I would suggest pg_monitor, but that's already taken for
something else, so maybe pg_console or some better thing someone else
can suggest. Without a pg_authid row, you couldn't use password, md5,
or scram authentication, unless we provided some other place to store
the verifier, like a flatfile. I'm not sure we want to go there, but
that still leaves a lot of workable authentication methods.

I think Álvaro is right to see this kind of work as an extension of
pg_isready, but the problem with pg_isready is that we don't want to
expose a lot of information to the whole Internet, or however much of
it can reach the postgres port. But with this approach, you can lock
down access via pg_hba.conf, which means that it's OK to expose
information that we don't want to make available to everyone. I think
we're still limited to exposing what can be observed from shared
memory here, because the whole idea is to have something that can be
used even before consistency is reached, so we shouldn't really be
doing anything that would look at the contents of data files. But that
still leaves a bunch of things that we could show here, the progress
of the startup process being one of them.

-- 
Robert Haas
EDB: http://www.enterprisedb.com