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From Jules Gosnell <>
Subject Re: Web State Replication... (long)
Date Thu, 30 Oct 2003 17:43:23 GMT
James Strachan wrote:

> 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.

James, I have given this quite a bit of thought... and whilst it was 
initially appealing and seemed a sensible extension of my train of 
thought, I have not been able to find any advantage in splitting one 
nodes state into mutiple buckets....

If a node joins or leaves, you still have exactly the same amount of 
state to shift around the cluster.

If you back up your sessions off-node, then whether these are all on one 
backup node, or spread over 10 makes no difference, since in the first 
case if you lose the backup node you have to shift 100% x 1 nodes state. 
In the second case you have to shift 10% x 10 nodes state (since the 
backup node will be carrying 10% of the state of another 9 nodes as well 
as your own). Initially it looks more resilient but...

So I am sticking, by virtue of Occam's razor, to the simpler approach 
for them moment, until someone can draw attention to a situation where 
the extra complexity of a higher granularity replication strategy is 
worth the gain.

Thinking about it, my current design is probably hybrid - since whilst a 
nodes state is all held in a single bucket, individual sessions may be 
migrated out of that bucket and into another one on another node. So it 
is replication granularity that is set to node-level, but migration 
granularity is at session level. I guess you are suggesting that a 
bucket is somewhere between the two of these and is the level at which 
both are replicated and migrated ? I'll give it some more thought :-)


>> - 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 
> scenarios.
> James
> -------

 * Jules Gosnell
 * Partner
 * Core Developers Network (Europe)

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