Re: Synchronizing slots from primary to standby

Hi,

On 10/20/23 5:27 AM, shveta malik wrote:
> On Wed, Oct 18, 2023 at 4:24 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> PFA v25 patch set. The changes are:
> 
> 1) 'enable_failover' is changed to 'failover'
> 2) Alter subscription changes to support 'failover'
> 3) Fixes a bug in patch001 wherein any change in standby_slot_names
> was not considered in the flow where logical walsenders wait for
> standby's confirmation. Now during the wait, if standby_slot_names is
> changed, wait is restarted using new standby_slot_names.
> 4) Addresses comments by Bertrand and Amit in [1],[2],[3]
> 
> The changes are mostly in patch001 and a very few in patch002.
> 
> Thank You Ajin for working on alter-subscription changes and adding
> more TAP-tests for 'failover'
> 

Thanks for updating the patch!

Looking at 0001 and doing some experiment:

Creating a logical slot with failover = true and then launching
pg_logical_slot_get_changes() or pg_recvlogical() on it results
to setting failover back to false.

It occurs while creating the decoding context here:

@@ -602,6 +602,9 @@ CreateDecodingContext(XLogRecPtr start_lsn,
         SnapBuildSetTwoPhaseAt(ctx->snapshot_builder, start_lsn);
     }

+   /* set failover in the slot, as requested */
+   slot->data.failover = ctx->failover;
+

I think we can get rid of this change in CreateDecodingContext().

Looking at 0002:

     /* Enter main loop */
     for (;;)
     {
         int         rc;
         long        wait_time = DEFAULT_NAPTIME_PER_CYCLE;

         CHECK_FOR_INTERRUPTS();

         /*
          * If it is Hot standby, then try to launch slot-sync workers else
          * launch apply workers.
          */
         if (RecoveryInProgress())
         {
             /* Launch only if we have succesfully made the connection */
             if (wrconn)
                 LaunchSlotSyncWorkers(&wait_time, wrconn);
         }

We are waiting for DEFAULT_NAPTIME_PER_CYCLE (3 minutes) before checking if there
is new synced slot(s) to be created on the standby. Do we want to keep this behavior
for V1?

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com