[jboss-jira] [JBoss JIRA] (JGRP-1486) Merge failure when dead instances remain in view

[David Hotham (JIRA)]

David Hotham created JGRP-1486:
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             Summary: Merge failure when dead instances remain in view
                 Key: JGRP-1486
                 URL: https://issues.jboss.org/browse/JGRP-1486
             Project: JGroups
          Issue Type: Bug
    Affects Versions: 3.0.10
            Reporter: David Hotham
            Assignee: Bela Ban


I've hit this testing my JGRP-1485 fix, but I think it's a logically independent issue.

So, I've reached a point where:
-  A, B and C all have view {C,A,B}
-  D has view {B', D', D, A', C}, in which B', D' and A' are all dead instances

As in JGRP-1485, an optimal fix would surely be to allow D to recover all by itself, but it's not clear to me how to do that.  However, my expectation was that a merge should sort things out; and I think that if it did then that ought to be good enough.

But what's actually happening is this:

-  C becomes merge leader
-  determines that merge participants are C, D', D, A'
-  sends MERGE_REQ to those members
-  the MERGE_REQ to D' reaches D (and that to A' reaches A)
-  D sends a positive response for the MERGE_REQ that was meant for it, but after 2.5 seconds also sends a negative response to the MERGE_REQ meant for D'.  (I think that the negative response is because it can't fetch the digest from D')
-  likewise A sends a negative response to the MERGE_REQ meant for A'

So what C sees is:
-  good responses from C and D, followed by merge_rejected responses from A and D
-  so it removes A' and D' from the merge (it didn't get responses from them)
-  then it removes D from the merge (because the most recent response from D said merge_rejected)
-  so it is left only with itself, and comes up with a consolidated view that is identical to its original view

in short: the merge doesn't do anything useful after all.

I think that the key here is the confusion between D and D'.  Possibly the fix is as simple as: ignore MERGE_REQs where the destination address on the message is not the local address.

I'll try this out and, if it looks good, submit a pull request.

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