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).
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).
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.
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.
Hope this helps
andy
