On 2017-08-02 16:35:17 -0400, Robert Haas wrote: > I actually think failover slots are quite desirable, especially now > that we've got logical replication in core. In a review of this > thread I don't see anyone saying otherwise. The debate has really > been about the right way of implementing that.
Given that I presumably was one of the people pushing back more strongly: I agree with that. Besides disagreeing with the proposed implementation our disagreements solely seem to have been about prioritization. I still think we should have a halfway agreed upon *design* for logical failover, before we introduce a concept that's quite possibly going to be incompatible with that, however. But that doesn't mean it has to submitted/merged to core. > - When a standby connects to a master, it can optionally supply a list > of slot names that it cares about. > - The master responds by periodically notifying the standby of changes > to the slot contents using some new replication sub-protocol message. > - The standby applies those updates to its local copies of the slots. > So, you could create a slot on a standby with an "uplink this" flag of > some kind, and it would then try to keep it up to date using the > method described above. It's not quite clear to me how to handle the > case where the corresponding slot doesn't exist on the master, or > initially does but then it's later dropped, or it initially doesn't > but it's later created. I think there's a couple design goals we need to agree upon, before going into the weeds of how exactly we want this to work. Some of the axis I can think of are: - How do we want to deal with cascaded setups, do slots have to be available everywhere, or not? - What kind of PITR integration do we want? Note that simple WAL based slots do *NOT* provide proper PITR support, there's not enough interlock easily available (you'd have to save slots at the end, then increment minRecoveryLSN to a point later than the slot saving) - How much divergence are we going to accept between logical decoding on standbys, and failover slots. I'm probably a lot closer to closer than than Craig is. - How much divergence are we going to accept between infrastructure for logical failover, and logical failover via failover slots (or however we're naming this)? Again, I'm probably a lot closer to zero than craig is. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers