We have two sub-groups, [B, C, A] and [D].

(1) D sends a MERGE_REQ to B.

2012-02-18 22:15:03.888 [MergeTask,Clumpy Test Cluster,Member-D] TRACE org.jgroups.protocols.pbcast.Merger - Member-B: sending MERGE_REQ to [Member-D, Member-B]

(2) B receives this. There's some delay in processing it (I think because there's another merge or flush going on; but the exact reason doesn't matter for this issue). When processing does start, B begins a flush.

2012-02-18 22:15:03.889 [OOB-2,Clumpy Test Cluster,Member-B] TRACE org.jgroups.protocols.TCP - received [dst: Member-B, src: Member-D (3 headers), size=0 bytes, flags=OOB], headers are GMS: GmsHeader[MERGE_REQ]: merge_id=Member-D::1, mbrs=[Member-B, Member-C, Member-A, Member-D], UNICAST2: DATA, seqno=1, conn_id=2, first, TCP: [channel_name=Clumpy Test Cluster]

2012-02-18 22:15:08.811 [OOB-2,Clumpy Test Cluster,Member-B] TRACE org.jgroups.protocols.pbcast.Merger - Member-B: got merge request from Member-D, merge_id=Member-D::1, mbrs=[Member-B, Member-A, Member-C, Member-D]

(3) D times out waiting for the MERGE_RSP from B:

2012-02-18 22:15:08.889 [MergeTask,Clumpy Test Cluster,Member-D] TRACE org.jgroups.protocols.pbcast.Merger - Member-D: collected 1 merge response(s) in 5001 ms
2012-02-18 22:15:08.889 [MergeTask,Clumpy Test Cluster,Member-D] DEBUG org.jgroups.protocols.pbcast.Merger - merge leader Member-D did not get responses from all 2 partition coordinators; missing responses from 1 members, removing them from the merge

(4) D completes the (failed) merge and broadcasts STOP_FLUSH:

2012-02-18 22:15:08.896 [MergeTask,Clumpy Test Cluster,Member-D] TRACE org.jgroups.protocols.pbcast.Merger - Member-D: received all ACKs (1) for merge view MergeView::[Member-D|1] [Member-D], subgroups=[Member-D|0] [Member-D] in 7ms
2012-02-18 22:15:08.896 [MergeTask,Clumpy Test Cluster,Member-D] TRACE org.jgroups.protocols.pbcast.GMS - Member-D: sending RESUME event
2012-02-18 22:15:08.897 [MergeTask,Clumpy Test Cluster,Member-D] DEBUG org.jgroups.protocols.pbcast.FLUSH - Member-D: received RESUME, sending STOP_FLUSH to all

But, since B is not a member of D's view, B does not receive this message.

(5) Now all future merge attempts fail, because B is stuck in a flush:

2012-02-18 22:15:31.186 [OOB-1,Clumpy Test Cluster,Member-B] WARN org.jgroups.protocols.pbcast.GMS - Member-B: GMS flush by coordinator failed
2012-02-18 22:15:54.380 [OOB-2,Clumpy Test Cluster,Member-B] WARN org.jgroups.protocols.pbcast.GMS - Member-B: GMS flush by coordinator failed
2012-02-18 22:16:13.705 [OOB-1,Clumpy Test Cluster,Member-B] WARN org.jgroups.protocols.pbcast.GMS - Member-B: GMS flush by coordinator failed

Note that I have implemented a workaround in my application where I:

• start a long-ish timer in the block() callback; and stop that timer in unblock()
• if the timer is allowed to pop, call channel.stopFlush()

This seems to be allowing the group to recover. Any comments on whether this is a good or bad idea would be appreciated.