[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=12715697#comment-12715697 ] 

David Hotham edited comment on JGRP-1484 at 9/4/12 7:45 AM:
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I wonder if we're having terminology confusion.  You say "usually, we have only a few merge coordinators, but many participants".  But I think that usually we have only a few coordinators, and (if all is well) even fewer participants.

To be clear, by participant I mean a member who has reported himself to the merge leader; but for whom no coordinator claims responsibility.  Eg if A, B, C report {A,B}, {A, B}, {A, B, C} then C is a participant because his supposed coordinator A does not have C in view.

Then I'm proposing that the merge leader sends INSTALL_MERGE_VIEW
* to A, who broadcasts it to A and B
* to C, who installs it directly

(whereas today C will attempt to broadcast this to A,B,C - which I claim is pointless).
                
      was (Author: dimbleby):
    I wonder if we're having terminology confusion.  You say "usually, we have only a few merge coordinators, but many participants".  But I think that usually we have only a few coordinators, and (if all is well) even fewer participants.

To be clear, by participant I mean a member who has reported himself to the merge leader; but for whom no coordinator claims responsibility.  Eg if A, B, C report {A,B}, {A, B}, {A, B, C} then C is a participant because his supposed coordinator A does not have C in view.

Then I'm proposing that the merge leader sends INSTALL_MERGE_VIEW
* to A, who broadcasts it to A and B
* to C, who installs it directly

(whereas today C will attempt to broadcast this to A,B,C - which is pointless).
                  
> 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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