On Thu, Dec 22, 2022 at 11:16 AM Hayato Kuroda (Fujitsu) <kuroda.hay...@fujitsu.com> wrote: > > Dear hackers, > (I added Amit as CC because we discussed in another thread) > > This is a fork thread from time-delayed logical replication [1]. > While discussing, we thought that we could extend the condition of walsender > shutdown[2][3]. > > Currently, walsenders delay the shutdown request until confirming all sent > data > are flushed on remote side. This condition was added in 985bd7[4], which is > for > supporting clean switchover. Supposing that there is a primary-secondary > physical replication system, and do following steps. If any changes are come > while step 2 but the walsender does not confirm the remote flush, the reboot > in > step 3 may be failed. > > 1. Stops primary server. > 2. Promotes secondary to new primary. > 3. Reboot (old)primary as new secondary. > > In case of logical replication, however, we cannot support the use-case that > switches the role publisher <-> subscriber. Suppose same case as above, > additional > transactions are committed while doing step2. To catch up such changes > subscriber > must receive WALs related with trans, but it cannot be done because subscriber > cannot request WALs from the specific position. In the case, we must truncate > all > data in new subscriber once, and then create new subscription with copy_data > = true. > > Therefore, I think that we can ignore the condition for shutting down the > walsender in logical replication. > > This change may be useful for time-delayed logical replication. The walsender > waits the shutdown until all changes are applied on subscriber, even if it is > delayed. This causes that publisher cannot be stopped if large delay-time is > specified.
I think the current behaviour is an artifact of using the same WAL sender code for both logical and physical replication. I agree with you that the logical WAL sender need not wait for all the WAL to be replayed downstream. I have not reviewed the patch though. -- Best Wishes, Ashutosh Bapat
