Re: Replication & recovery_min_apply_delay

On 15.11.2019 5:52, Michael Paquier wrote:
> On Tue, Sep 10, 2019 at 03:23:25PM +0900, Michael Paquier wrote:
>> Yes, I suspect that it could be very costly in some configurations if
>> there is a large gap between the last replayed LSN and the last LSN
>> the WAL receiver has flushed.
>>
>> There is an extra thing, which has not been mentioned yet on this
>> thread, that we need to be very careful about:
>>     <para>
>>         When the standby is started and <varname>primary_conninfo</varname> is set
>>         correctly, the standby will connect to the primary after replaying all
>>         WAL files available in the archive. If the connection is established
>>         successfully, you will see a walreceiver process in the standby, and
>>         a corresponding walsender process in the primary.
>>     </para>
>> This is a long-standing behavior, and based on the first patch
>> proposed we would start a WAL receiver once consistency has been
>> reached if there is any delay defined even if restore_command is
>> enabled.  We cannot assume either that everybody will want to start a
>> WAL receiver in this configuration if there is archiving behind with a
>> lot of segments which allow for a larger catchup window..
> Two months later, we still have a patch registered in the CF which is
> incorrect on a couple of aspects, and with scenarios which are
> documented and getting broken.  Shouldn't we actually have a GUC to
> control the startup of the WAL receiver instead?
> --
> Michael

Attached pleased find rebased version of the patch with 
"wal_receiver_start_condition" GUC added (preserving by default original 
behavior).

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company