Re: [PATCH v1] Add and report the new "in_hot_standby" GUC pseudo-variable.

On Saturday, March 18, 2017 3:33:16 AM EDT Michael Paquier wrote:
> On Sat, Mar 18, 2017 at 2:56 AM, Elvis Pranskevichus
<elprans(at)gmail(dot)com> wrote:
> > Currently, clients wishing to know when the server exits hot standby
> > have to resort to polling, which is often suboptimal.
> >
> > This adds the new "in_hot_standby" GUC variable that is reported via
> > a>
> > ParameterStatus message. This allows the clients to:
> > (a) know right away that they are connected to a server in hot
> >
> > standby; and
> >
> > (b) know immediately when a server exits hot standby.
> >
> > This change will be most beneficial to various connection pooling
> > systems (pgpool etc.)
>
> Why adding a good chunk of code instead of using pg_is_in_recovery(),
> which switches to false once a server exits recovery?

That requires polling the database continuously, which may not be
possible or desirable.

My main motivation here is to gain the ability to manage a pool of
connections in asyncpg efficiently. A part of the connection release
protocol is "UNLISTEN *;", which the server in Hot Standby would fail to
process. Polling the database for pg_is_in_recovery() is not feasible
in this case, unfortunately.

Elvis