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From James Strachan <>
Subject Re: Web State Replication... (long)
Date Thu, 30 Oct 2003 13:03:32 GMT
n Thursday, October 30, 2003, at 12:19  pm, gianny DAMOUR wrote:
> Hello,
> Just a couple of questions regarding this design:
> - Is it possible to configure the weight of a node? If yes, is the 
> same auto-partitioning policy applicable? My concern is that a 
> "clockwise" policy may add a significant load on nodes hosted by low 
> spec hosts.

This is partly a problem for the sticky load balancer to deal with i.e. 
it should load requests to primary machines based on spec/power.

If we partitioned the session data into buckets (rather than one big 
lump), then the buckets of session data can be distributed evenly 
around the cluster so that each session bucket has N buddies (replicas) 
but that a load-balancing algorithm could be used to distribute the 
buckets based on (say) a host spec weighting or whatnot. e.g. nodes in 
the cluster could limit how many buckets to accept due to their lack of 
resources etc.

Imagine having 1 massive box and 2 small ones in a cluster - you'd 
probably want to give the big box more buckets than the smaller ones. 
The previous model Jules described still holds (that was a view of 1 
session bucket) - its just that the total session state for a machine 
might be spread over many buckets.

Having multiple buckets could also help spread the load of recovering 
from a node failure in larger clusters.

> - I have the feeling that one can not configure a preferred 
> replication group for primary sessions of a specific node: if four 
> nodes are available, I would like to configure that sessions of the 
> first node should be replicated by the third node, if available, or 
> the fourth one.
> - Is it not an overhead to have b-1 replica? AFAIK, a single secondary 
> should be enough.

It all depends on your risk profile I suppose. I backup is usually 
enough but you may want 2 for extra resilience - especially as one of 
those could be in a separate DR zone for really serious fail-over 


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