Re: [jgroups-users] Unexpected Merge, Coordinator in multiple partitions

Forgot to reply to your actual email, see below...

> On 04/04/17 16:05, Questions/problems related to using JGroups wrote:
>> Hi JGroups Users,
>>
>> We have seen an unexpected Merge View in one of our deployments, where the coordinator is in multiple partitions.
>> And I am not really sure if this is a bug or a feature (TM).
>>
>> The situation is like this:
>> Members: [A,B,C]
>> Subgroup 1: [A,B,C]
>> Subgroup 2: [A,B]
>>
>> We would have expected to see:
>> Members: [A,B,C]
>> Subgroup 1: [A,B]
>> Subgroup 2: [C]

No, this won't happen as C's FD protocol has msg_counts_as_heartbeat set 
to true by default. So whenver C gets a message from A or B, it'll reset 
the heartbeat counter for these members and thus won't exclude them.

In contrast, A and B's DISCARD protocol drops messages from C, and 
therefore A and B will exclude C from their view.

This is expected behavior and is unit tested in OverlappingMergeTest.

>> It also looks like Node C has been isolated from A,B but the isolation ended before Node C changed its View.

Node C will _not_ change its view!

>> With a View like this is it hard on Node C to detect that there has been a certain change in the topology,
>> especially if the coordinator (A) is in the major and in the minor partition.
>>
>> Has anyone seen similar behaviour or an idea how to prevent a Merge situation like this ?
>>
>> A small example project which reproduces the mentioned behaviour can be downloaded here:
>> https://www.dc-square.de/wp-content/uploads/jgroups-test.zip
>>
>>
>> Regards,
>> Christoph
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>

-- 
Bela Ban, JGroups lead (http://www.jgroups.org)


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