[jboss-jira] [JBoss JIRA] Created: (JBAS-4229) HASingletonController doesn't handle "split brain" correctly.

[Adrian Brock (JIRA)]

HASingletonController doesn't handle "split brain" correctly.
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                 Key: JBAS-4229
                 URL: http://jira.jboss.com/jira/browse/JBAS-4229
             Project: JBoss Application Server
          Issue Type: Bug
      Security Level: Public (Everyone can see)
          Components: Clustering
    Affects Versions: JBossAS-4.0.3 SP1
            Reporter: Adrian Brock
         Assigned To: Brian Stansberry


The HASingletonController doesn't understand the "split brain" problem.

Take for example a JBossMQ destination (queue) which needs to be "restored" from the database
when a new singleton is elected.

The scenario goes as follows:

STEP1 (original state):
cluster=server1, server2
master=server1

STEP2 (unplug server1 from the network):
cluster=server2
master=server2
server2 will now restore the queue from the database

BUT! server1 has the view
cluster=server1
master=server1

STEP3 (plug server1 back into the network)
cluster=server1, server2
master=server1

We are back to the original state, but since server1 thinks it never left the cluster,
it doesn't restore the changes (from the database) that server2 made to the queue between STEP2 and STEP3.
This is because it doesn't restart the HASingleton on server1.

If there was a third server (server3) to abitrate this wouldn't be a problem.
Both server2 and server3 would agree that server2 is the master and that server1 has left and must rejoin.

There needs to be some extra processing in the "merge" when two masters agree to form a cluster
that ensures that whoever they elect as the new master gets its HASingleton(s) restarted.
Otherwise, it won't have the up-to-date state from the other master.

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