In 9.4 we've the below  block of code to walsender.c as

 * We only send regular messages to the client for full decoded
 * transactions, but a synchronous replication and walsender shutdown
 * possibly are waiting for a later location. So we send pings
 * containing the flush location every now and then.
if (MyWalSnd->flush < sentPtr && !waiting_for_ping_response)
    waiting_for_ping_response = true;

I am finding that my logical replication reader is spending a tremendous amount of time sending feedback to the server because a keep alive reply was requested. My flush pointer is smaller than sendPtr, which I see as the normal case (The client hasn't confirmed all the wal it has been sent). My client queues the records it receives and only confirms when actually processes the record.

So the sequence looks something like

Server Sends LSN 0/1000
Server Sends LSN 0/2000
Server Sends LSN 0/3000
Client confirms LSN 0/2000

I don't see why all these keep alive replies are needed in this case (the timeout value is bumped way up, it's the above block that is triggering the reply request not something related to timeout)


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