Re: Hot Standby query cancellation and Streaming Replication integration

Greg Smith wrote:
> Heikki Linnakangas wrote:
> > One such landmine is that the keepalives need to flow from client to
> > server while the WAL records are flowing from server to client. We'll
> > have to crack that problem for synchronous replication too, but I think
> > that alone is a big enough problem to make this 9.1 material.
> >   
> 
> This seems to be the real sticking point then, given that the 
> xmin/PGPROC side on the master seems logically straightforward.  For 
> some reason I thought the sync rep feature had the reverse message flow 
> already going, and that some other sort of limitation just made it 
> impractical to merge into the main codebase this early.  My hope was 
> that just this particular part could get cherry-picked out of there, and 
> that it might even have been thought about already in that context given 
> the known HS keepalive "serious issue".  If there was a solution or 
> partial solution in progress to that floating around, my thought was 
> that just piggybacking this extra xid info on top of it would be easy 
> enough.
> 
> If there's not already a standby to primary communications backchannel 
> implementation available that can be harvested from that work, your 
> suggestion that this may not be feasible at all for 9.0 seems like a 
> more serious concern than I had thought it was going to be.

I suspect the master could connect to the slave to pull an xid.

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