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From Jules Gosnell <>
Subject Re: Clustering (long)
Date Tue, 02 Aug 2005 14:21:26 GMT
Andy Piper wrote:

> Hi Jules
> At 05:37 AM 7/27/2005, Jules Gosnell wrote:
>> I agree on the SPoF thing - but I think you misunderstand my 
>> Coordinator arch. I do not have a single static Coordinator node, but 
>> a dynamic Coordinator role, into which a node may be elected. Thus 
>> every node is a potential Coordinator. If the elected Coordinator 
>> dies, another is immediately elected. The election strategy is 
>> pluggable, although it will probably end up being hardwired to 
>> "oldest-cluster-member". The reason behind this is that relaying out 
>> your cluster is much simpler if it is done in a single vm. I 
>> originally tried to do it in multiple vms, each taking responsibility 
>> for pieces of the cluster, but if the vms views are not completely in 
>> sync, things get very hairy, and completely in sync is an expensive 
>> thing to achieve - and would introduce a cluster-wide single point of 
>> contention. So I do it in a single vm, as fast as I can, with fail 
>> over, in case that vm evaporates. Does that sound better than the 
>> scenario that you had in mind ?
> This is exactly the "hard" computer science problem that you shouldn't 
> be trying to solve if at all possible. Its hard because network 
> partitions or hung processes (think GC) make it very easy for your 
> colleagues to think you are dead when you do not share that view. The 
> result is two processes who think they are the coordinator and anarchy 
> can ensue (commonly called split-brain syndrome). I can point you at 
> papers if you want, but I really suggest that you aim for an 
> implementation that is independent of a central coordinator. Note that 
> a central coordinator is necessary if you want to implement a 
> strongly-consistent in-memory database, but this is not usually a 
> requirement for session replication say.
> gives 
> a good introduction to some of these things. I also presented at 
> JavaOne on related issues, you should be able to download the 
> presentation from at some point (not there yet - I 
> just checked).

OK - I will have a look at these papers and reconsider... perhaps I can 
come up with some sort of fractal algorithm which recursively breaks 
down the cluster into subclusters each of which is capable of doing 
likewise to itself and then  layout the buckets recursively via this 
metaphor... - this would be much more robust, as you point out, but, I 
think, a more complicated architecture. I will give it some serious 
thought. Have you any suggestions/papers as to how you might do 
something like this in a distributed manner, bearing in mind that as a 
node joins, some existing nodes will see it as having joined and some 
will not yet have noticed and vice-versa on leaving....

>> The Coordinator is not there to support session replication, but 
>> rather the management of the distributed map (map of which a few 
>> buckets live on each node) which is used by WADI to discover very 
>> efficiently whether a session exists and where it is located. This 
>> map must be rearranged, in the most efficient way possible, each time 
>> a node joins or leaves the cluster.
> Understood. Once you have a fault-tolerant singleton coordinator you 
> can solve lots of interesting problems, its just hard and often not 
> worth the effort or the expense (typical implementations involve HA HW 
> or an HA DB or at least 3 server processes).

Since I am only currently using the singleton coordinator for bucket 
arrangement, I may just live with it for the moment, in order to move 
forward, but make a note to replace it and start background threads on 
how that might be achieved...

>> Replication is NYI - but I'm running a few mental background threads 
>> that suggest that an extension to the index will mean that it 
>> associates the session's id not just to its current location, but 
>> also to the location of a number of replicants. I also have ideas on 
>> how a session might choose nodes into which it will place its 
>> replicants and how I can avoid the primary session copy ever being 
>> colocated with a replicant (potential SPoF - if you only have one 
>> replicant), etc...
> Right definitely something you want to avoid.
>> Yes, I can see that happening - I have an improvement (NYI) to WADI's 
>> evacuation strategy (how sessions are evacuated when a node wishes to 
>> leave). Each session will be evacuated to the node which owns the 
>> bucket into which its id hashes. This is because colocation of the 
>> session with the bucket allows many messages concered with its future 
>> destruction and relocation to be optimised away. Future requests 
>> falling elsewhere but needing this session should, in the most 
>> efficient case, be relocated to this same node, other wise the 
>> session may be relocated, but at a cost...
> How do you relocate the request? Many HW load-balancers do not support 
> this (or else it requires using proprietary APIs), so you probably 
> have to count on
> moving sessions in the normal failover case.

If I can squeeze the behaviour that I require out of the load-balancer, 
then, depending on the request type I may be able to get away with a 
redirection with a changed session cookie or url param, or, failing this 
an http-proxy, across from a filter above the servlet on one side to the 
http-port on the node that owns the session...

The LB-integration object is pluggable and the aim is to supply wadi 
with a good selection of LB integrations - currently I only have a 
ModJK[2] plugin working. This is able to 'restick' clients to their 
session's new location (although messing with the session id is a little 

>> I would be very grateful in any thoughts or feedback that you could 
>> give me. I hope to get much more information about WADI into the wiki 
>> over the next few weeks. That should help generate more discussion, 
>> although I would be more than happy for people to ask me questions 
>> here on Geronimo-dev because this will give me an idea of what 
>> documentation I should write and how existing documentation may be 
>> lacking or misleading.
> I guess my general comment would be that you might find it better to 
> think specifically about the end-user problem you are trying to solve 
> (say session replication) and work towards a solution based on that. 
> Most short-cuts / optimizations that vendors make are specific to the 
> problem domain and do not generally apply to all clustering problems.

The end problem is really clustered web and ejb sessions at the moment, 
although it looks as if by the time we have solved these issues we may 
well have written a fault-tolerant distributed/partitioned index that 
might be very useful as a generic distributed cache building block.

One thing that I do want wadi to do, is to still work when replication 
is switched off. i.e., if a session only exists as a primary copy, even 
if affinity breaks down, wadi will continue to correctly render requests 
for that session unless some form of catastrophic failure causes the 
session to evaporate. This means that I need to ensure the session's 
timely evacuation from a node that chooses to leave the cluster to a 
remaining node, so that it may remain active beyond the lifetime of its 
original node. All of this must work flawlessly under stress, so that an 
admin may add or remove nodes to a running cluster without having to 
worry about the user state that it is managing. Nodes are added by 
simply starting them, and nodes removed via e.g. ctl-c-ing them.

If it is decided that a few more nines are needed in terms of session 
availability and the cluster owner understands the extra cost involved 
in in-vm replication in terms of extra hardware and bandwidth that they 
will have to purchase and is happy to go with in-vm-replication, then it 
should be sufficient to up the number of replicated copies kept by the 
cluster from '0' to e.g. '2' and restart (It might even be possible to 
vary this setting on a node to node basis so that this change does not 
even involve a complete cluster cold start). WADI should deal with the rest.

So, I believe that I have a pretty clear idea of what WADI will do, and 
aside from the replication stuff (phase2) it currently does most of what 
iIhad in mind for phase1, except that it is not yet happy under stress. 
I figure it will probably take one or two more redesign/reimplementation 
iterations to get it to this stage, then I can consider replication.

I have spoken to members of the OpenEJB team about  wadi's ability to 
relocate requests as well as sessions and we came to the conclusion that 
it was just as applicable in the EJB world as the web world. If the node 
an ejb client is talking to leaves the cluster in between calls, the 
client may try to contact it and then failover to another node that it 
hopes holds the session. If, due to other nodes leaving/joining it is 
not always clear which node will contain the session, the ability to 
reply to an RMI and just say "not here - there!" - i.e. an rmi 
redirection - would not be hard to add and would resolve this situation. 
Transactions are another item which I have marked phase2.

So, I am trying hard to stay very focussed on the problem domain, 
otherwise this will never get finished :-)

Right, off to read those papers now - thanks for your posting and your 


> Hope this helps
> andy 

"Open Source is a self-assembling organism. You dangle a piece of
string into a super-saturated solution and a whole operating-system
crystallises out around it."

 * Jules Gosnell
 * Partner
 * Core Developers Network (Europe)
 * Open Source Training & Support.

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