MERGE4
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Author: Bela Ban
JIRA: https://jira.jboss.org/jira/browse/JGRP-937

Goal
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To merge asymmetric partitions, e.g. A: {A,B,C}, B: {A,B}, C: {A,C}. The current merge algorithm wouldn't merge these
partitions because B and C are not coordinators and therefore don't participate in the merge.

The implementation involves creating a new protocol (MERGE4) and modifying GMS and CoordGmsImpl.

MERGE4
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- Periodically runs a discovery
- The Discovery protocol is changed such that each participant additionally returns its view
- If the consolidated discovery responses result in more than 1 view, send up a MERGE event with a
  list of all (different) views

GMS / CoordGmsImpl
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- On MERGE(V1,V2,V3,...):
  - Determine all coordinators, e.g. A for V1 and V2, D for V3
  - Determine the membership for each coord, e.g. {A,B,C} for A and {D,E,F} for D
  - Send a MERGE-REQ to A and D
    - The MERGE-REQ for A includes {A,B,C}, the MERGE-REQ for D includes {D,E,F}
  - A and D fetch digest information from {A,B,C} (A) and {D,E,F} (D) respectively
  - This information is consolidated in A (merge leader) and installed in both partitions

- Example:
  - A: V3 {A,B,C}
  - B: V1 {A,B}
  - C: V2 {A,C}
  - D: V7 {D,E,F}
  - E: V6 {D,E}
  - F: V5 {D,F}

  - MERGE4 sends up a MERGE(V1,V2,V3,V5,V6,V7)
  - CoordGmsImpl determines that the coords are A and D and the merge leader is A
  - A sends a MERGE-REQ(A,B,C} to A and a MERGE-REQ(D,E,F} to D
  - A fetches the digest and view for A,B,C and returns it to A
  - D fetches the digest and view for D,E,F and returns it to A
  - A consolidates the digests and views (into a MergeView) and tells A and D to install the new MergeView plus digests