On Sat, Feb 19, 2022 at 10:35 PM David G. Johnston <david.g.johnston@gmail.com> wrote: > > On Sat, Feb 19, 2022 at 9:37 AM Andres Freund <andres@anarazel.de> wrote: >> >> IMO the type of information you'd want for apply failures is substantially >> >> different enough from worker failures that I don't really see the temptation >> to put them in the same table. >> > > It's an error message and a transaction LSN in both cases right now, along with knowledge of whether said transaction only affects a single table (relid is not null) or not (relid is null). Do you have a concrete idea in mind that would make this separation need more obvious? >
I would also like to mention that in some cases, sync workers also behaves like apply worker (after initial sync till it catches up with the apply worker) and try to stream and apply changes similar to apply worker. The error during that phase will be no different than the apply worker. One idea to make the separation more obvious is to introduce 'worker_type' column similar to backend_type in pg_stat_activity which will tell whether it is an apply worker or a table sync worker.
The point isn't to make the separation more obvious by specifying which worker type is doing the work. It is to make the concept of worker type (and identity) irrelevant. The end user cannot (and should not be able to) address individual workers - only the subscription.
Even while a sync worker is in synchronization mode (as opposed to whatever mode comes before synchronization mode) it still only affects a single table. To the end user the distinction between the two modes is immaterial.
The statement "will be no different than the apply worker" doesn't make sense to me given that in a multiple-table subscription (the only kind where this matters...) you will have multiple table sync workers in synchronization mode and they both cannot behave identically to an apply worker otherwise they would step on each other's toes. That two different table-specific updates produce the same error shouldn't be a problem if that is indeed what happens (though if the error is on tableA having the worker for tableB report the tableA error would be odd - but not problematic).
I'll admit I don't fully understand the details of this particular synchronization interaction but I'm not see how the discussion of "errors during table-specific updates" vs "errors during whole transaction application" can be affected by it.
