Re: Libpq support to connect to standby server as priority

On Thu, Feb 28, 2019 at 1:00 PM Tsunakawa, Takayuki <
tsunakawa.takay@jp.fujitsu.com> wrote:

> From: Haribabu Kommi [mailto:kommi.haribabu@gmail.com]
> > Attached are the updated patches.
>
> Thanks, all look fixed.
>
>
> > The target_server_type option yet to be implemented.
>
> Please let me review once more and proceed to testing when the above is
> added, to make sure the final code looks good.  I'd like to see how complex
> the if conditions in multiple places would be after adding
> target_server_type, and consider whether we can simplify them together with
> you.  Even now, the if conditions seem complicated to me... that's probably
> due to the existence of read_write_host_index.
>

Yes, if checks are little bit complex because of additional checks to
identify, I will check if there is
any easier way to update them without introducing code duplication.

While working on implementation of target_server_type new connection option
for the libpq
to specify master, slave and etc, there is no problem when the newly added
target_server_type
option is used separate, but when it is combined with the existing
target_session_attrs, there
may be some combinations that are not valid or such servers doesn't exist.

Target_session_attrs      Target_server_type

read-write                       prefer-slave, slave
prefer-read                     master, slave
read-only                        master, prefer-slave

I know that some of the cases above is possible, like master server with by
default accepts
read-only sessions. Instead of we put a check to validate what is right
combination, how
about allowing the combinations and in case if such combination is not
possible, means
there shouldn't be any server the supports the requirement, and connection
fails.

comments?

And also as we need to support the new option to connect to servers < 12
also, this option
sends the command "select pg_is_in_recovery()" to the server to find out
whether the server
is recovery mode or not?

And also regarding the implementation point of view, the new
target_server_type option
validation is separately handled, means the check for the required server
is handled in a separate
switch case, when both options are given, first find out the required
server for target_session_attrs
and validate the same again with target_server_type?

Regards,
Haribabu Kommi
Fujitsu Australia