I just pinged the JGroups folks again. You can also try the jgroups user mail
list at https://lists.sourceforge.net/lists/listinfo/javagroups-users .
Re: HASingletonController, first thing to understand is that who the master is
is a function of what nodes in the cluster have that HASingletonController
deployed. There is the overall cluster view, which more people are familiar
with, and then something that I call the "service view", which reflects what
nodes in the cluster have a particular service deployed.
So, say you have a 4 node cluster and the view for the cluster is {A, B, C, D}.
But the "Foo" service's HASingletonController is only deployed on 2 nodes. So
it's service view is {C, D}. The "Bar" service's HASingletonController is
deployed on a different 2 nodes, so it's service view is {A, B}.
In this example the master for the "Foo" service is C and the master for "Bar"
is A.
A service view is always a subset of the overall cluster view. If a node
crashes, it's removed from all service views.
How all this works behind the scenes is an HASingletonController uses a
component of the HAPartition known as the DistributedReplicantManager. The DRM
basically maintains the service views. When for example the "Foo" service
HASingletonController is deployed on node D, it registers itself with the DRM
by invoking the DRM add method. DRM in turn invokes a method on the
HAPartition which does a group RPC telling the other DRMs in the cluster that
"Foo" is now available on D. Those DRMs in turn issue a notification to the
"Foo" HASingletonController on their nodes telling them that the service view
for the "Foo" service has changed, and now includes "D". From that they can
determine who the master is.
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