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From Alex Blewitt <>
Subject Re: [clustering] -- HttpSession and EJB Session
Date Wed, 20 Aug 2003 15:12:30 GMT
So far, the focus has been purely on the implementation of clustring 
WRT WebApps and therefore the HttpSession. However, consideration 
should be equally given to the way that EJB SessionBeans' state are 

It's no good using a web application and for it to fall over to another 
node if that web app was using SessionBeans and it looses its session 

The approach taken by WebSphere is to serialise the SessionBean in 
addition to the HttpSession out to an external system, and then load it 
when required. Not only is this a simple solution, but the same 
techniques can be applied for both HttpSession and SessionBean 

I fear that the HttpSession will work based on very intimate details of 
how the HttpSession works (which is in Geronimo's control), whereas 
this will not be possible with SessionBeans to the same extent.

Have you considered representing the HttpSession as a SessionBean and 
then working on only one solution to the clustering problem from the 
SessionBean end? Then you would have a solution that fits both, and 
whilst the in-memory replication process/buddying system works well for 
HttpSession, I feel that this approach won't be suitable for 

On the other hand, a solution for SessionBeans will implicitly work for 
HttpSession as well.



On Wednesday, Aug 20, 2003, at 13:09 Europe/London, Jules Gosnell wrote:

> I'm going to pick up this thread again :-)
> we have to deal with both dumb and integrated load-balancers...
> (A) undivided cluster, simple approach:
> every node buddies for every other node
> no 'locate/migrate' required since every session is on every node
> replication needs to be synchronous, in order to guarantee that node 
> on which nex request falls will be up-to-date
> problem: unscalable
> (B) subdivided cluster, more complex approach:
> cluster subdivided into buddy teams (possibly only of pairs).
> 'locate/migrate' required since request may fall on node that does not 
> have session to hand
> primary could use asyn and secondary sync replication, provided that 
> 'locate' always talked to primary
> problem: given a cluster of n nodes divided into teams of t nodes: 
> only t/n requests will be able to avoid the 'locate/migrate' step - in 
> a large cluster with small teams, this is not much more efficient than 
> a shared store solution.
> SMART LB (we're assuming it can do pretty much whatever we want it to).
> (A)
> assuming affinity, we can use async replication, because request will 
> always fall on most up to date node.
> if this node fails, the lb MUST pick one to failover to and continue 
> to use that one (or else we have to fall back to sync and assume dumb 
> lb)
> if original node comes back up, it doesn't matter whether lb goes back 
> to it, or remains stuck to fail-over node.
> (B)
> if we can arrange for LB use affinity, with failover limited to our 
> buddy-group, and always stick to the failover node as well we can lose 
> 'locate/migrate' and replicate asych. If we can't get 'always stick to 
> failover node', we replicate synch after failover.
> if we can only arrange affinity, but not failover within group, we can 
> replicate asynch and need 'locate/migrate'. If we can't have 
> lb-remains-stuck-to-failover-node, we are in trouble, because as soon 
> as primary node fails we go back to the situation outlined above where 
> we do a lot of locate/migrate and are not much better off than a 
> shared store.
> The lb-sticks-to-failover-node is not as simple as it sounds - mod_jk 
> doesn't do it.
> it implies
> either :
> you have the ability to change the routing info carried on the session 
> id client side (I've considered this and don't think it practical - I 
> may be wrong ...)
> or :
> the session id needs to carry not just a single piece of routing info 
> (like a mod_jk worker name) but a failover list 
> worker1,worker2,worker3 etc in effect your buddy-team,
> or:
> the lb needs to maintain state, remembering where each session was 
> last serviced and always sticking requests for that session to that 
> node. in a large deployment this requires lbs to replicate this state 
> between them so that they can balance over the same nodes in a 
> coordinated fashion. I think F5 Big-IP is capable of this, but 
> effectively you just shift the state problem from yourself to someone 
> else.
> Note that if your lb can understand extended routing info involving 
> the whole buddy team, then you know that it will always balance 
> requests to members of this team anyway, in which case you can 
> dispense with 'locate/migrate' again.
> Finally - you still need a migrate operation as sessions will need to 
> migrate from buddy-group to buddy-group as buddy-groups are created 
> and destroyed...
> in summary - I think that you can optimise away 'locate' and a lot of 
> 'migrate'-ion - Jetty's current impl has no locate and you can build 
> subdivided clusters with it and mod_jk.... but I don't do automatic 
> repartitioning yet....
> If you are still reading here, then you are doing well :-)
> Jules
> Jeremy Boynes wrote:
>>> I figure that we are talking about two different and orthogonal 
>>> types of
>>> partition here.
>> Agreed.
>>> I'm happy to call the way that nodes are linked into buddy-groups
>>> (groups of peers that store replicated state for each other) 
>>> something
>>> other than 'partition', if we want to reserve that term for some 
>>> sort of
>>> cluster management concept, but you do agree that these structures
>>> exist, do you not ? regardless of what they are called, otherwise 
>>> you do
>>> not scale, as we have all agreed.
>>> As for loadbalancer configuration I think this will draw upon both
>>> 'jeremy-partition' and 'jules-buddy-group' status as :
>>> - you only want to balance requests for a webapp to nodes on which 
>>> it is
>>> deployed
>> Yes
>>> - you only want to fail-over requests to other nodes in the same
>>> buddy-group as the failed node
>> Ideally, yes but this is not essential. See below.
>>> if you can do the latter you can avoid cluster-wide logic for findg 
>>> and
>>> migrating sessions from remote nodes to the one receiving the 
>>> request,
>>> because you can guarantee that the session is already there.
>> The price to pay for this is that you always need to replicate state 
>> to any
>> node to which the request may be directed. If you allow for a locate 
>> phase,
>> then you can minimise the set of nodes to which data is replicated 
>> (the
>> buddy-group) because any node can find it later. In a high-affinity
>> configuration this reduces the overall load.
>> Consider a four node partition A,B,C,D. In the 'replicate-everywhere' 
>> model,
>> A's state is replicated to three other nodes after every request, 
>> incurring
>> the processing cost on three nodes (assuming network multicast). If A 
>> dies,
>> any node can instantly pick up the work. The issue is we have a lot of
>> overhead to reduce the latency in the event of node death (which we 
>> hope is
>> infrequent).
>> The other alternative is that every session has one and only one 
>> buddy. This
>> would result in 1/3 of A's sessions being replicated to B, 1/3 to C 
>> and 1/3
>> to D. Each session is replicated to just one node, allowing unicast 
>> to be
>> used (which has a lower overhead than multicast) and only incurring 
>> the
>> ongoing processing cost on one node.
>> If A dies, then B,C,D pick new buddies for A's sessions and do bulk 
>> state
>> transfer to redistribute, ensuring that the state is always stored on 
>> two
>> nodes. Say B transfers to C, C to D and D to B. Again, unicast 
>> transfer. You
>> can avoid this if you are willing to lose a session if another node 
>> dies
>> (double failure scenario).
>> An A request is now directed to a random node; if this node has the 
>> state,
>> then it becomes the primary and starts replicating to its buddy. If 
>> it does
>> not, then it sends a multicast inquiry to the partition, locates the 
>> state,
>> does a second transfer and starts replicating again.
>> The trade off is lower overhead whilst running but a larger state 
>> transfer
>> in the event of node death. I tend to prefer the latter on the basis 
>> that
>> node deaths are infrequent.
>>> Are we getting closer ?
>> :-)
>> --
>> Jeremy
> -- 
> /**********************************
> * Jules Gosnell
> * Partner
> * Core Developers Network (Europe)
> **********************************/

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