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From Alex Blewitt <>
Subject Re: Web Clustering
Date Sun, 17 Aug 2003 16:19:14 GMT
On Sunday, Aug 17, 2003, at 16:35 Europe/London, Jules Gosnell wrote:

> Alex,
> we seem to be using the same terms to mean very different things and 
> this is leading to confusion.
> I'll go first:
> shared-store - many nodes storing their state in a common service.

Seems like a fairly good definition; I was using the term 'store' to 
imply something like a database or other external product at the back 

Since I'm presuming that in order for this to occur, there has to be a 
shared-store (and will all implementations will be a shared-store) then 
can we come up with a name for sessions-stored-in-a-backend-system? Or 
is that what you mean by shared-store? (I.e. do you consider your 
'replication' model to be a 'shared store' because to me, it reads like 
it does.)

> replication - each node storing copies of it's state somewhere 
> off-node (probably not in the same place as all the other nodes). I am 
> suggesting that this 'other-place' is on the back of another 
> web-container.

There's a difference between having the current state in memory, and 
replicating it off-node, to only loading the state into memory 
dynamically from a backend database. Further, the term replication is 
used in databases as well which could add to the confusion.

Replicated sessions sounds like a good name for this concept, though.

> The implementation of the storage in either situation is not specified 
> - I'm doing a logical design.
> Affinity is load-balancing policy, which allows you to optimise 
> caching (and possibly forget about session distribution entirely, if 
> you can live with SPoFs), because you can predict where subsequent 
> requests for a session will fall.

Yes, that's the general idea I had of affinity.

> How do these compare to your definitions ?

Pretty well, I think; the only thing that I'm not sure is whether 
'shared-store' covers your concept of 'replication' or not (since that 
is, by definition, a common service across nodes ...)

> Once we talk the same language, we can go forward :-)

I tried to put those definititions on Wiki (or at least, what I thought 
they were). I'm pretty sure we've got the same ideas of concepts; if 
you want to edit the wiki and put '.. or Xxxx ' in the section titles 
then once we've got the ideas mapped to what we call them, we're in a 
much better position :-)


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