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

On Wednesday, May 24, 2017 9:38:36 PM EDT Tsunakawa, Takayuki wrote:
> The clients will know the change of session_read_only when they do
> something that calls RecoveryInProgress().  Currently,
> RecoveryInProgress() seems to be the only place where the sessions
> notice the promotion, so I set session_read_only to the value of
> default_transaction_read_only there.  I think that there is room for
> discussion here.  It would be ideal for the sessions to notice the
> server promotion promptly and notify the clients of the change.  I
> have no idea to do that well.

My original patch did that via the new SendSignalToAllBackends() helper,
which is called whenever the postmaster exits hot stanby.

I incorporated those bits into your patch and rebased in onto master.  
Please see attached.

FWIW, I think that mixing the standby status and the default 
transaction writability is suboptimal.  They are related, yes, but not 
the same thing.  It is possible to have a master cluster in the 
read-only mode, and with this patch it would be impossible to 
distinguish from a hot-standby replica without also polling 
pg_is_in_recovery(), which defeats the purpose of having to do no
database roundtrips.  

                                 Elvis

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