Hi, On 2022-12-27 14:37:11 -0800, Nathan Bossart wrote: > On Tue, Dec 27, 2022 at 02:11:11PM -0800, Andres Freund wrote: > > On 2022-12-27 11:24:49 -0800, Nathan Bossart wrote: > >> I've attached a patch set that adds the restore_library, > >> archive_cleanup_library, and recovery_end_library parameters to allow > >> archive recovery via loadable modules. This is a follow-up to the > >> archive_library parameter added in v15 [0] [1]. > > > > Why do we need N parameters for this? To me it seems more sensible to have > > one > > parameter that then allows a library to implement all these (potentially > > optionally). > > The main reason is flexibility. Separate parameters allow using a library > for one thing and a command for another, or different libraries for > different things. If that isn't a use-case we wish to support, I don't > mind combining all three into a single recovery_library parameter.
I think the configuration complexity is a sufficient concern to not go that direction... > >> * Unlike archive modules, recovery libraries cannot be changed at runtime. > >> There isn't a safe way to unload a library, and archive libraries work > >> around this restriction by restarting the archiver process. Since recovery > >> libraries are loaded via the startup and checkpointer processes (which > >> cannot be trivially restarted like the archiver), the same workaround is > >> not feasible. > > > > I don't think that's a convincing reason to not support configuration > > changes. Sure, libraries cannot be unloaded, but an unnecessarily loaded > > library is cheap. All that's needed is to redirect the relevant function > > calls. > > This might leave some stuff around (e.g., GUCs, background workers), but if > that isn't a concern, I can adjust it to work as you describe. You can still have a shutdown hook re background workers. I don't think the GUCs matter, given that it's the startup/checkpointer processes. > >> * pg_rewind uses restore_command, but there isn't a straightforward path to > >> support restore_library. I haven't addressed this in the attached patches, > >> but perhaps this is a reason to allow specifying both restore_command and > >> restore_library at the same time. pg_rewind would use restore_command, and > >> the server would use restore_library. > > > > That seems problematic, leading to situations where one might not be able to > > use restore_command anymore, because it's not feasible to do > > segment-by-segment restoration. > > I'm not following why this would make segment-by-segment restoration > infeasible. Would you mind elaborating? Latency effects for example can make it infeasible to do segment-by-segment restoration infeasible performance wise. On the most extreme end, imagine WAL archived to tape or such. Greetings, Andres Freund
