Hi hackers, Building on bf279ddd1c, this patch introduces a GUC 'standby_slot_names_from_syncrep' which allows logical failover slots to wait for changes to have been synchronously replicated before sending the decoded changes to logical subscribers.
The existing 'standby_slot_names' isn't great for users who are running clusters with quorum-based synchronous replicas. For instance, if the user has synchronous_standby_names = 'ANY 3 (A,B,C,D,E)' it's a bit tedious to have to reconfigure the standby_slot_names to set it to the most updated 3 sync replicas whenever different sync replicas start lagging. In the event that both GUCs are set, 'standby_slot_names' takes precedence. I did some very brief pgbench runs to compare the latency. Client instance was running pgbench and 10 logical clients while the Postgres box hosted the writer and 5 synchronous replicas. There's a hit to TPS, which I'm thinking is due to more contention on the SyncRepLock, and that scales with the number of logical walsenders. I'm guessing we can avoid this if we introduce another set of lsn[NUM_SYNC_REP_WAIT_MODE] and have the logical walsenders check and wait on that instead but I wasn't sure if that's the right approach. pgbench numbers: // Empty standby_slot_names_from_syncrep query mode: simple number of clients: 8 number of threads: 8 maximum number of tries: 1 duration: 1800 s number of transactions actually processed: 1720173 number of failed transactions: 0 (0.000%) latency average = 8.371 ms initial connection time = 7.963 ms tps = 955.651025 (without initial connection time) // standby_slot_names_from_syncrep = 'true' scaling factor: 200 query mode: simple number of clients: 8 number of threads: 8 maximum number of tries: 1 duration: 1800 s number of transactions actually processed: 1630105 number of failed transactions: 0 (0.000%) latency average = 8.834 ms initial connection time = 7.670 ms tps = 905.610230 (without initial connection time) Thanks, -- John Hsu - Amazon Web Services
0001-Introduce-a-new-guc-standby_slot_names_from_syncrep.patch
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