On Thu, Jan 05, 2006 at 07:30:40AM -0700, David Boreham wrote:
> This can be a problem for replication mechanisms if the propagation of 
> update
> records between nodes is not pipelined. In that case there will be a 
> stall with
> 1 x RTT duration between each update. With a typical WAN latency of 50ms or
> so this will limit replication performance to only 10 updates per second.
> I have no idea if slony suffers from this problem, but seeing the post 
> made me
> think of this issue, which I have seen in other replication products.

That's not a problem.  Slony is _asyc_ replication precisely because
we needed wide-area replication as one of the things it could do. 
The trade-off is that the replicas are not always perfect copies of
the origin (that is, there is some lag, and the lag is unpredictable
within some upper bound for a given network throughput and workload). 
That "not always perfect copies" issue is what makes automatic
failover mostly dangerous.

It seems that what Rod is talking about is either heavy workload or
an unreliable network or both; and there will indeed be a measurable
effect on the origin node if the write sets are very large (or the
network is slow, or both) -- i.e. large enough to cause you to need
more transfer speed than you have.  We have experienced nodes being
as much as a day behind without any serious effects, but our write
traffic wasn't that heavy.  If the network were very unstable,
though, you'd be in a different situation.

You also need to have pretty siginificant security around all this. 
Remember that Slony _requires_ superuser acces on all active nodes in
a cluster.  So you'd better be using a VPN of some sort.

A

A


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Andrew Sullivan  | ajs at crankycanuck.ca
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