On Thu, Jan 12, 2023 at 11:46 PM Andres Freund <and...@anarazel.de> wrote: > > Hi, > > On 2023-01-12 20:08:55 +0530, Ashutosh Sharma wrote: > > I previously participated in the discussion on "Synchronizing the > > logical replication slots from Primary to Standby" and one of the > > purposes of that project was to synchronize logical slots from primary > > to standby so that if failover occurs, it will not affect the logical > > subscribers of the old primary much. Can someone help me understand > > how we are going to solve this problem with this patch? Are we going > > to encourage users to do LR from standby instead of primary to get rid > > of such problems during failover? > > It only provides a building block towards that. The "Synchronizing the logical > replication slots from Primary to Standby" project IMO needs all of the > infrastructure in this patch. With the patch, a logical rep solution can > e.g. maintain one slot on the primary and one on the standby, and occasionally > forward the slot on the standby to the position of the slot on the primary. In > case of a failover it can just start consuming changes from the former > standby, all the necessary changes are guaranteed to be present. > > > > Also, one small observation: > > > > I just played around with the latest (v38) patch a bit and found that > > when a new logical subscriber of standby is created, it actually > > creates two logical replication slots for it on the standby server. > > May I know the reason for creating an extra replication slot other > > than the one created by create subscription command? See below: > > That's unrelated to this patch. There's no changes to the "higher level" > logical replication code dealing with pubs and subs, it's all on the "logical > decoding" level. > > I think this because logical rep wants to be able to concurrently perform > ongoing replication, and synchronize tables added to the replication set. The > pg_16399_sync_16392_7187728548042694423 slot should vanish after the initial > synchronization. >
Thanks Andres. I have one more query (both for you and Bertrand). I don't know if this has already been answered somewhere in this mail thread, if yes, please let me know the mail that answers this query. Will there be a problem if we mandate the use of physical replication slots and hot_standby_feedback to support minimum LD on standby. I know people can do a physical replication setup without a replication slot or even with hot_standby_feedback turned off, but are we going to have any issue if we ask them to use a physical replication slot and turn on hot_standby_feedback for LD on standby. This will reduce the code changes required to do conflict handling for logical slots on standby which is being done by v50-0001 and v50-0002* patches currently. IMHO even in normal scenarios i.e. when we are not doing LD on standby, we should mandate the use of a physical replication slot. -- With Regards, Ashutosh Sharma.
