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From Jules Gosnell <ju...@coredevelopers.net>
Subject Re: Clustering (long)
Date Tue, 02 Aug 2005 23:03:48 GMT
I've had a look at the Lampson paper, but didn't take it all in on the 
first pass - I think it will need some serious concentration. The Paxos 
algorithm looks interesting, I will definitely pursue this avenue.

I've also given a little thought to exactly why I need a Coordinator and 
how Paxos might be used to replace it. My use of a Coordinator and plans 
for its future do not actually seem that far from Paxos, on a 
preliminary reading.

Given that WADI currently uses a distributed map of 
sessionId:sessionLocation, that this distribution is achieved by sharing 
out responsibility for the set number of buckets that comprise the map 
roughly evenly between the cluster members and that this is currently my 
most satisfying design, I can break my problem space (for bucket 
arrangement) down into 3 basic cases :

1) Node joins
2) Node leaves in controlled fashion
3) Node dies

If the node under discussion is the only cluster member, then no bucket 
rearrangement is necessary - this node will either create or destroy the 
full set of buckets. I'll leave this set of subcases as trivial.

1)  The joining node will need to assume responsibility for a number of 
buckets. If buckets-per-node is to be kept roughly the same for every 
node, it is likely that the joining node will require transfer of a 
small number of buckets from every current cluster member i.e. we are 
starting a bucket rearrangement that will involve every cluster member 
and only need be done if the join is successful. So, although we wish to 
avoid an SPoF, if that SPoF turns out to be the joining node, then I 
don't see it as a problem, If the node joining dies, then we no longer 
have to worry about rearranging our buckets (unless we have lost some 
that had already been transferred - see (3)). Thus the joining node may 
be used as a single Coordinator/Leader for this negotiation without fear 
of the SPoF problem. Are we on the same page here ?

2) The same argument may be applied in reverse to a node leaving in a 
controlled fashion. It will wish to evacuate its buckets roughly equally 
to all remaining cluster members. If it shuts down cleanly, this would 
form part of its shutdown protocol. If it dies before or during the 
execution of this protocol then we are back at (3), if not, then the 
SPoF issue may again be put to one side.

3) This is where things get tricky :-) Currently WADI has, for the sake 
of simplicity, one single algorithm / thread / point-of-failure which 
recalculates a complete bucket arrangement if it detects (1), (2) or 
(3). It would be simple enough to offload the work done for (1) and (2) 
to the node joining/leaving and this should reduce wadi's current 
vulnerability, but we still need to deal with catastrophic failure. 
Currently WADI rebuilds the missing buckets by querying the cluster for 
the locations of any sessions that fall within them, but it could 
equally carry a replicated backup and dust it off as part of this 
procedure. It's just a trade-off between work done up front and work 
done in exceptional circumstance... This is the place where the Paxos 
algorithm may come in handy - bucet recomposition and rearrangement. I 
need to give this further thought. For the immediate future, however, I 
think WADI will stay with a single Coordinator in this situation, which 
fails-over if http://activecluster.codehaus.org says it should - I'm 
delegating the really thorny problem to James :-). I agree with you that 
this is an SPoF and that WADI's ability to recover from failure here 
depends directly on how we decide if a node is alive or dead - a very 
tricky thing to do.

In conclusion then, I think that we have usefully identified a weakness 
that will become more relevant as the rest of WADI's features mature. 
The Lampson paper mentioned describes an algorithm for allowing nodes to 
reach a consensus on actions to be performed, in a redundant manner with 
no SPoF and I shall consider how this might replace WADI's currently 
single Coordintor, whilst also looking at performing other Coordination 
on joining/leaving nodes where its failure, coinciding with that of its 
host node, will be irrelevant, since the very condition that it was 
intended to resolve has ceased to exist.

How does that sound, Andy ? Do you agree with my thoughts on (1) & (2) ? 
This is great input - thanks,


Jules


Jules Gosnell wrote:

> 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.
>>
>> http://research.microsoft.com/Lampson/58-Consensus/Abstract.html 
>> 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 dev2dev.bea.com 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 dodgy...).
>
>>
>>> 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 
> interest,
>
> Jules
>
>>
>> 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)
 *
 *    www.coredevelopers.net
 *
 * Open Source Training & Support.
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