raft: send commit index only if necessary

This commit fixes one case of unnecessary MsgApp sends. The leader now
checks that the follower's commit index is behind, and only then sends a
commit index update.

Signed-off-by: Pavel Kalinnikov <[email protected]>

Author: pav-kv

raft.go

@@ -1519,11 +1519,8 @@ func stepLeader(r *raft, m pb.Message) error {
 					// to respond to pending read index requests
 					releasePendingReadIndexMessages(r)
 					r.bcastAppend()
-				} else if oldPaused {
-					// If we were paused before, this node may be missing the
-					// latest commit index, so send it.
-					// TODO(pav-kv): remove this branch, and decide on sending the commit
-					// index update based on pr.Commit.
+				} else if oldPaused && r.id != m.From && pr.Commit < r.raftLog.committed {
+					// The node is potentially missing the latest commit index. Send it.
 					r.sendAppend(m.From)
 				}
 				// We've updated flow control information above, which may

raft_paper_test.go

@@ -633,7 +633,7 @@ func TestFollowerCheckMsgApp(t *testing.T) {
 
 		msgs := r.readMessages()
 		wmsgs := []pb.Message{
-			{From: 1, To: 2, Type: pb.MsgAppResp, Term: 2, Index: tt.windex, Reject: tt.wreject, RejectHint: tt.wrejectHint, LogTerm: tt.wlogterm},
+			{From: 1, To: 2, Type: pb.MsgAppResp, Term: 2, Index: tt.windex, Commit: tt.wcommit, Reject: tt.wreject, RejectHint: tt.wrejectHint, LogTerm: tt.wlogterm},
 		}
 		if !reflect.DeepEqual(msgs, wmsgs) {
 			t.Errorf("#%d: msgs = %+v, want %+v", i, msgs, wmsgs)

testdata/confchange_v2_replace_leader.txt

@@ -284,12 +284,10 @@ stabilize
   CommittedEntries:
   2/5 EntryNormal ""
   Messages:
-  4->1 MsgApp Term:2 Log:2/5 Commit:4
   4->1 MsgApp Term:2 Log:2/5 Commit:5
   4->2 MsgApp Term:2 Log:2/5 Commit:5
   4->3 MsgApp Term:2 Log:2/5 Commit:5
 > 1 receiving messages
-  4->1 MsgApp Term:2 Log:2/5 Commit:4
   4->1 MsgApp Term:2 Log:2/5 Commit:5
 > 2 receiving messages
   4->2 MsgApp Term:2 Log:2/5 Commit:5
@@ -301,7 +299,6 @@ stabilize
   CommittedEntries:
   2/5 EntryNormal ""
   Messages:
-  1->4 MsgAppResp Term:2 Log:0/5 Commit:4
   1->4 MsgAppResp Term:2 Log:0/5 Commit:5
 > 2 handling Ready
   Ready MustSync=false:
@@ -318,7 +315,6 @@ stabilize
   Messages:
   3->4 MsgAppResp Term:2 Log:0/5 Commit:5
 > 4 receiving messages
-  1->4 MsgAppResp Term:2 Log:0/5 Commit:4
   1->4 MsgAppResp Term:2 Log:0/5 Commit:5
   2->4 MsgAppResp Term:2 Log:0/5 Commit:5
   3->4 MsgAppResp Term:2 Log:0/5 Commit:5

testdata/probe_and_replicate.txt

@@ -513,18 +513,6 @@ stabilize 1 2
   2->1 MsgAppResp Term:8 Log:0/21 Commit:18
 > 1 receiving messages
   2->1 MsgAppResp Term:8 Log:0/21 Commit:18
-> 1 handling Ready
-  Ready MustSync=false:
-  Messages:
-  1->2 MsgApp Term:8 Log:8/21 Commit:18
-> 2 receiving messages
-  1->2 MsgApp Term:8 Log:8/21 Commit:18
-> 2 handling Ready
-  Ready MustSync=false:
-  Messages:
-  2->1 MsgAppResp Term:8 Log:0/21 Commit:18
-> 1 receiving messages
-  2->1 MsgAppResp Term:8 Log:0/21 Commit:18
 
 stabilize 1 3
 ----
@@ -579,18 +567,6 @@ stabilize 1 3
   3->1 MsgAppResp Term:8 Log:0/21 Commit:18
 > 1 receiving messages
   3->1 MsgAppResp Term:8 Log:0/21 Commit:18
-> 1 handling Ready
-  Ready MustSync=false:
-  Messages:
-  1->3 MsgApp Term:8 Log:8/21 Commit:18
-> 3 receiving messages
-  1->3 MsgApp Term:8 Log:8/21 Commit:18
-> 3 handling Ready
-  Ready MustSync=false:
-  Messages:
-  3->1 MsgAppResp Term:8 Log:0/21 Commit:18
-> 1 receiving messages
-  3->1 MsgAppResp Term:8 Log:0/21 Commit:18
 
 stabilize 1 4
 ----
@@ -674,18 +650,6 @@ stabilize 1 5
   5->1 MsgAppResp Term:8 Log:0/21 Commit:21
 > 1 receiving messages
   5->1 MsgAppResp Term:8 Log:0/21 Commit:21
-> 1 handling Ready
-  Ready MustSync=false:
-  Messages:
-  1->5 MsgApp Term:8 Log:8/21 Commit:21
-> 5 receiving messages
-  1->5 MsgApp Term:8 Log:8/21 Commit:21
-> 5 handling Ready
-  Ready MustSync=false:
-  Messages:
-  5->1 MsgAppResp Term:8 Log:0/21 Commit:21
-> 1 receiving messages
-  5->1 MsgAppResp Term:8 Log:0/21 Commit:21
 
 stabilize 1 6
 ----
@@ -741,18 +705,6 @@ stabilize 1 6
   6->1 MsgAppResp Term:8 Log:0/21 Commit:21
 > 1 receiving messages
   6->1 MsgAppResp Term:8 Log:0/21 Commit:21
-> 1 handling Ready
-  Ready MustSync=false:
-  Messages:
-  1->6 MsgApp Term:8 Log:8/21 Commit:21
-> 6 receiving messages
-  1->6 MsgApp Term:8 Log:8/21 Commit:21
-> 6 handling Ready
-  Ready MustSync=false:
-  Messages:
-  6->1 MsgAppResp Term:8 Log:0/21 Commit:21
-> 1 receiving messages
-  6->1 MsgAppResp Term:8 Log:0/21 Commit:21
 
 stabilize 1 7
 ----
@@ -816,15 +768,3 @@ stabilize 1 7
   7->1 MsgAppResp Term:8 Log:0/21 Commit:21
 > 1 receiving messages
   7->1 MsgAppResp Term:8 Log:0/21 Commit:21
-> 1 handling Ready
-  Ready MustSync=false:
-  Messages:
-  1->7 MsgApp Term:8 Log:8/21 Commit:21
-> 7 receiving messages
-  1->7 MsgApp Term:8 Log:8/21 Commit:21
-> 7 handling Ready
-  Ready MustSync=false:
-  Messages:
-  7->1 MsgAppResp Term:8 Log:0/21 Commit:21
-> 1 receiving messages
-  7->1 MsgAppResp Term:8 Log:0/21 Commit:21