[jboss-jira] [JBoss JIRA] (JGRP-1484) SEQUENCER and merge-views broken

[David Hotham (JIRA)]

    [ https://issues.jboss.org/browse/JGRP-1484?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12715693#comment-12715693 ] 

David Hotham commented on JGRP-1484:
------------------------------------

I don't understand your second point.  My proposal massively _reduces_ the number of messages that are sent in the circumstances you describe:

* the merge leader will unicast INSTALL_MERGE_VIEW to all participants anyway.  Neither of us has proposed changing that.
* under the current system, every participant will additionally broadcast this view to his entire sub-group.  (This could become an n-squared flood of messages!)
* whereas under my proposal, every participant will just install the view, without sending further messages.

I agree that it is very hard to guarantee ordering between MergeViews and messages, and that neither of us has managed to do it.  However it seems that my version does a bit better in this respect, and is therefore slightly preferable.  

>From a purely practical point of view, my fix also has the advantage that I've already done lots of testing with it.  That shouldn't be decisive (I can repeat the testing with your fix, it'll just take time) - but it would make my life easier if we could keep mine...

I'm not sure I've understood what you don't like about my proposal:
-  you've ended up making a fix in GMS anyway, so it's not that
-  as above, I think that your worry about sending lots of messages is (if anything) the wrong way round

Sorry to keep bugging you on this one - but could you explain again why you think my fix is not as good?
                
> SEQUENCER and merge-views broken
> --------------------------------
>
>                 Key: JGRP-1484
>                 URL: https://issues.jboss.org/browse/JGRP-1484
>             Project: JGroups
>          Issue Type: Bug
>    Affects Versions: 3.0.10
>            Reporter: David Hotham
>            Assignee: Bela Ban
>             Fix For: 3.2
>
>
> Here's a new way in which putting SEQUENCER below GMS is broken.
> Start with A having view B|4 [B,A], while B, C and D all have view B|7 [B,C,D].
> Now we start a merge, in which B is coordinator.  B creates the view C|8 [C, D, A, B].
> (I've opened a pull request saying that B surely shouldn't issue a view where the ViewID says that C was the creator.  But I think that this is incidental, and not key to the bug that I'm reporting here).
> Now B sends INSTALL_MERGE_VIEW to B (a coordinator) and A (a merge participant, per Util.determineMergeParticipants).
> B gets this first and broadcasts the new view to [B, C, D].  In particular, B is now not a coordinator.
> Then A gets the INSTALL_MERGE_VIEW, and it too tries broadcasting the new view.  SEQUENCER gets involved, and forwards the broadcast to B (as the coordinator in the old view).  B discards this; it's no longer a coordinator.
> So the new view is not installed at A.  All future broadcasts from A are forwarded to B, who discards them.  The group is fractured, and none of A's broadcasts are delivered.
> I'm not sure what the right fix would be.  I wonder whether things should be arranged so that in a merge:
> -  coordinators behave as today, broadcasting the new view to their own sub-groups
> -  but mere participants do not do this: they should just have the new view installed on them.

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