> I think we need to clarify that suppose the standby has a slot with > failover=true and synced=false and the primary has the slot with the > same name, failover=true, and synced=true... I'm not sure to understand the semantics related to the `synced` flag but why `synced` flag can be true on a primary instance? AFAICS if `synced=true` then it means taht the slot is inactive and it is synchronized with a slot on a remote instance. On a primary, what is the meaning of having the flag synced set to true? There's already an open thread dealing with this issue [1]. The problem I see is being able to distinguish between 2 situations: 1) A failover slot has been created on a standby (failover=true and synced=false) in a context of cascading standby. In this case the slot must not be deleted. 2) A former primary has a slot (failover=true and synced=false) that must be resynchronized and that can be overwritten. Why not to use a slot's metadata (allow_overwrite) to treat these two situations separately.
Regards, Fabrice [1]: PostgreSQL: Clear logical slot's 'synced' flag on promotion of standby <https://www.postgresql.org/message-id/flat/CAJpy0uBJqTQdq+UDW55oK2bhxMig=FaEYFV=+zakmtmgnww...@mail.gmail.com> On Wed, Nov 19, 2025 at 10:44 PM Masahiko Sawada <[email protected]> wrote: > On Tue, Nov 18, 2025 at 10:27 PM Amit Kapila <[email protected]> > wrote: > > > > On Wed, Nov 19, 2025 at 6:26 AM Masahiko Sawada <[email protected]> > wrote: > > > > > > On Tue, Nov 18, 2025 at 1:30 AM Amit Kapila <[email protected]> > wrote: > > > > > > > > On Sat, Nov 15, 2025 at 4:02 AM Masahiko Sawada < > [email protected]> wrote: > > > > > > > > > > On Fri, Nov 14, 2025 at 2:39 AM Amit Kapila < > [email protected]> wrote: > > > > > > > > > > > > > > > > > > The point is quite fundamental, do you think we can sync to a > > > > > > pre-existing slot with the same name and failover marked as true > after > > > > > > the first time the node joins a new primary? > > > > > > > > > > Given the current behavior that we cannot create a logical slot > with > > > > > failover=true on the standby, it makes sense to me that we > overwrite > > > > > the pre-existing slot (with synced=false and failover=true) on the > old > > > > > primary by the slot (with synced=true and failover=true) on the new > > > > > primary if their names, plugin and other properties matches and the > > > > > pre-existing slot has lesser LSNs and XIDs than the one on the new > > > > > primary. But at the same time, we need to consider the possible > future > > > > > changes that allow users to create a slot with failover=true also > on > > > > > the standby. > > > > > > > > > > Alexander pointed out[1] that allowing to create a slot with > > > > > failover=true on the standby won't work with the current > > > > > implementation. I agree with his analysis, and I guess we would > need > > > > > more changes than simply allowing it, regardless of accepting the > > > > > proposed change. We might need to introduce a replication slot > origin > > > > > or a generation. > > > > > > > > > > > > > AFAICS, the email you pointed out wrote about use cases, not the > > > > actual code implementation. We can discuss use cases if we want to > > > > pursue that implementation, but the reason why we decided not to > allow > > > > it was for the cases where users try to configure cascaded standbys > to > > > > also try to sync slots from the first standby that are already being > > > > synced from the primary. There are quite a few technical challenges > in > > > > supporting that, like how to make sure primary waits even for > cascaded > > > > standbys before sending the changes to logical subscribers. > > > > > > Right. My point is that these are two independent issues. The fact > > > that creating a slot with failover=true directly on a standby is > > > difficult (due to the cascaded-standby cases you mentioned) does not, > > > by itself, justify allowing us to overwrite an existing slot with > > > failover=true and synced=false during slot synchronization. > > > > > > > OTOH, for the cases where there is a totally different logical slot > on > > > > standby (not present on primary) with failover=true, we can allow it > > > > to be synced from standby-1 to a cascaded standby, though we need > some > > > > way to distinguish those cases. For example, during sync on cascaded > > > > standby, we can ensure that the slot being synced is not a sync-slot > > > > (failover=true and sync=true). > > > > > > Yes. We need some way to distinguish those slots, otherwise if users > > > create a slot with the same name on the primary, the slot on standby-1 > > > (a cascading standby) could be overwritten. I think we would need some > > > additional metadata per slot to support that safely. > > > > > > > It shouldn't be overwritten as of current HEAD because we give ERROR > > if such a slot exists on standby but if we allow overwriting it as > > being discussed here then we would require additional metadata per > > slot to distinguish this case. > > I think we need to clarify that suppose the standby has a slot with > failover=true and synced=false and the primary has the slot with the > same name, failover=true, and synced=true, what problem could occur if > we allow overwriting the slot on the standby by the one on the > primary. I see there are some cases where the old primary needs to > delete pre-existing slots before joining the new primary, but should > we make such operation mandatory in switchover cases? It might be > worth considering whether such additional metadata per slot is > required before allowing overwriting the slot. > > Regards, > > -- > Masahiko Sawada > Amazon Web Services: https://aws.amazon.com >
