Hi, On Wed, Jun 18, 2025 at 03:22:59PM +0530, shveta malik wrote: > On Wed, Jun 18, 2025 at 2:39 PM Bertrand Drouvot > <bertranddrouvot...@gmail.com> wrote: > > > > IIUC the idea is to "just" increase WAL level to 'logical' so that one > > could then > > be allowed to make use of logical decoding from the standby. The primary > > goal > > of logical decoding from standby is to move some load from the primay to > > the standby i.e we don't expect/want the logical slot to be used on the > > primary. > > > > So what about making sure that if a logical slot is created with > > immediately_reserve > > set to false then no one can use it? (That would ensure that WAL reservation > > will not happen). > > > > Another concern is ensuring that users actually create this slot. If > there is already an active subscription subscribed to the primary, the > effective_wal_level will be set to logical already, allowing logical > decoding on the standby to proceed without issue. In such a case, the > user might not bother to create additional slots (same as problem > described in [1])) and later may unintentionally end up making standby > slots unusable. Any ideas on how to ensure it?
> > That said, we might also want to create another parameter name (than > > immediately_reserve) to better reflect this behavior (if we move that way). > > > > Thoughts? > > Or we could avoid exposing control of immediately_reserve to the user > altogether? Instead, we reserve a specific slot name and ensure that > it never reserves WAL in the future by preventing it from being > consumed under any circumstances (as you suggested). I wonder if a way to address the concerns that we shared above is to use a mixed approach like: - Forget the immediately_reserve idea - If a user creates a logical slot then we automatically switch to wal_level = logical (if not already done): I think that's a nice user experience - *and* provide a new API pg_activate_logical_decoding(), if the user has no need to create a logical slot on the primary (wants to use the standby to offload all the logical decoding) So if the user also uses a logical slot on the primary (for real..) then there is no need to launch pg_activate_logical_decoding(), until....: The user decides to drop the logical slot on the primary, and then: - If the slot is not the last logical slot, that's fine, drop it - If the slot is the last logical one AND the user did not set a new flag "wal_level_action" to "say preserve" or "force downgrade" (in the drop command) then the drop fails with an informative error message. That way: - pg_activate_logical_decoding() is needed only if there is not already a logical slot on the primary - the drop requires the user to think twice if this is the last logical slot - we don't ask the user to create a logical slot if he does not want to use it on the primary Thoughts? Regards, -- Bertrand Drouvot PostgreSQL Contributors Team RDS Open Source Databases Amazon Web Services: https://aws.amazon.com
