Re: Libpq support to connect to standby server as priority

On Mon, Feb 8, 2021 at 8:17 PM vignesh C <vignesh21(at)gmail(dot)com> wrote:
>
> >
> > I think what we want to do is mark default_transaction_read_only as
> > GUC_REPORT, instead. That will give a reliable report of what the
> > state of its GUC is, and you can combine it with is_hot_standby
> > to decide whether the session should be considered read-only.
> > If you don't get those two GUC values during connection, then you
> > can fall back on "SHOW transaction_read_only".
> >
>
> I have made a patch for the above with the changes suggested and
> rebased it with the head code.
> Attached v21 patch which has the changes for the same.
> Thoughts?
>

I'm still looking at the patch code, but I noticed that the
documentation update describing how support of read-write transactions
is determined isn't quite right and it isn't clear how the parameters
work.
I'd suggest something like the following (you'd need to fix the line
lengths and line-wrapping appropriately) - please check it for
correctness:

<para>
The support of read-write transactions is determined by the value of the
<varname>default_transaction_read_only</varname> and
<varname>in_hot_standby</varname> configuration parameters,
that, if supported,
are reported by the server upon successful connection. If the
value of either
of these parameters is <literal>on</literal>, it means the
server doesn't support
read-write transactions. If either/both of these parameters
are not reported,
then the support of read-write transactions is determined by
an explicit query,
by sending <literal>SHOW transaction_read_only</literal> after
successful
connection; if it returns <literal>on</literal>, it means the
server doesn't
support read-write transactions. The standby mode state is
determined by either
the value of the <varname>in_hot_standby</varname> configuration
parameter, that is reported by the server (if supported) upon
successful connection, or is otherwise explicitly queried by sending
<literal>SELECT pg_is_in_recovery()</literal> after successful
connection; if it returns <literal>t</literal>, it means the server is
in hot standby mode.
</para>

Regards,
Greg Nancarrow
Fujitsu Australia