Re: [HACKERS] [PATCH v1] Add and report the new "in_hot_standby" GUC pseudo-variable. - Mailing list pgsql-hackers

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@gmail.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