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Re: Hot Standby query cancellation and Streaming Replication integration

From: Greg Smith <greg(at)2ndquadrant(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hot Standby query cancellation and Streaming Replication integration
Date: 2010-02-27 02:52:12
Message-ID: 4B8888DC.5060401@2ndquadrant.com (view raw or flat)
Thread:
Lists: pgsql-hackers
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.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com   www.2ndQuadrant.us


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