INDY-1290: Add analysis of backup instances

Signed-off-by: Sergey Khoroshavin <sergey.khoroshavin@dsr-corporation.com>

design/view-change.md

@@ -180,6 +180,28 @@ we don't place any upper limit on this time.
   complex request selection algorithm can be simplified and view change
   algorithm will still work.
 
+- All above is applied to just one instance of PBFT protocol, but RBFT is
+  basically multiple PBFT instances running in parallel, with only master
+  altering state. RBFT states that all instances on given node should start
+  view change simultaneously and each instance should elect new primary
+  independently. Since indy plenum implementation has simple round robin
+  election it should be ok to just run view change on master with each backup
+  just waiting for view change completion on master.
+
+  This should not affect performance measurements (triggering another view
+  change) because, given non-faulty primary on master replica:
+  - it is guaranteed that primary on master replica will be the first to end
+    view change and enter normal ordering state
+  - therefore master instance will be the first one where new PRE-PREPARE
+    messages will be generated
+  - no COMMIT quorum could be gathered by any instance until _n-f_ nodes
+    finish view change
+  - so ordering (and performance measurements) by any instance will start only
+    when _n-f_ nodes finish view change, and master instance will have at least
+    as many pre-prepared requests as any other backup
+  - therefore master instance performance will be not less than any backup
+    instance in the beginning of new view
+
 ## Summary of main deviations from PBFT
 
 1. Indy plenum resets ppSeqNo after each view change. This should either
@@ -200,6 +222,11 @@ we don't place any upper limit on this time.
    as possible (even if they were not commited in previous view) improves
    overall performance of system.
 
+4. Indy plenum implements RBFT which basically is multiple PBFT instances,
+   with only one (master) changing state. For simplicity view change protocol
+   can be applied only to master instance, with backups synced to master
+   during view change.
+
 ## Rough roadmap
 
 1. Implement PBFT view change as pluggable strategy that can cope with