[FAB-18378] Log warning when peer is lagging behind and cannot catch up

Fabric 2.2 made gossip state transfer be disabled by default.

When a peer is a gossip follower, it does not pull blocks from the ordering service,
and relies on the leader peer of its organization to forward blocks directly or indirectly.

However, if a peer misses a block and the rest of the peers in its organization advance too much,
it will never catch up because its state transfer is disabled.

This commit makes the peer log a warning if it is lagging behind, and its configuration
might prevent it from obtaining blocks via state transfer and the orderer.

Change-Id: Ie7bc67cb13cb2d8e7ffab638a49e0365535865c2
Signed-off-by: Yacov Manevich <yacovm@il.ibm.com>
(cherry picked from commit 5f61ff0)

gossip/state/config.go

@@ -30,6 +30,8 @@ type StateConfig struct {
 	StateBlockBufferSize int
 	StateChannelSize     int
 	StateEnabled         bool
+	UseLeaderElection    bool
+	OrgLeader            bool
 }
 
 func GlobalConfig() *StateConfig {
@@ -68,4 +70,8 @@ func (c *StateConfig) loadStateConfig() {
 	if viper.IsSet("peer.gossip.state.enabled") {
 		c.StateEnabled = viper.GetBool("peer.gossip.state.enabled")
 	}
+	// The below two configuration parameters are used for straggler() which warns
+	// if our peer is lagging behind the rest and has no way to catch up.
+	c.UseLeaderElection = viper.GetBool("peer.gossip.useLeaderElection")
+	c.OrgLeader = viper.GetBool("peer.gossip.orgLeader")
 }

gossip/state/state.go

@@ -41,9 +41,9 @@ type GossipStateProvider interface {
 }
 
 const (
-	defAntiEntropyBatchSize = 10
-
-	defMaxBlockDistance = 20
+	stragglerWarningThreshold = 100
+	defAntiEntropyBatchSize   = 10
+	defMaxBlockDistance       = 20
 
 	blocking    = true
 	nonBlocking = false
@@ -761,6 +761,13 @@ func (s *GossipStateProviderImpl) addPayload(payload *proto.Payload, blockingMod
 	}
 
 	if !blockingMode && payload.SeqNum-height >= uint64(s.config.StateBlockBufferSize) {
+		if s.straggler(height, payload) {
+			s.logger.Warningf("[%s] Current block height (%d) is too far behind other peers at height (%d) to be able to receive blocks "+
+				"without state transfer which is disabled in the configuration "+
+				"(peer.gossip.state.enabled = false). Consider enabling it or setting the peer explicitly to be a leader (peer.gossip.orgLeader = true) "+
+				"in order to pull blocks directly from the ordering service.",
+				s.chainID, height, payload.SeqNum+1)
+		}
 		return errors.Errorf("Ledger height is at %d, cannot enqueue block with sequence of %d", height, payload.SeqNum)
 	}
 
@@ -773,6 +780,16 @@ func (s *GossipStateProviderImpl) addPayload(payload *proto.Payload, blockingMod
 	return nil
 }
 
+func (s *GossipStateProviderImpl) straggler(currHeight uint64, receivedPayload *proto.Payload) bool {
+	// If state transfer is disabled, there is no way to request blocks from peers that their ledger has advanced too far.
+	stateDisabled := !s.config.StateEnabled
+	// We are too far behind if we received a block with a sequence number more than stragglerWarningThreshold ahead of our height.
+	tooFarBehind := currHeight+stragglerWarningThreshold < receivedPayload.SeqNum
+	// We depend on other peers for blocks if we use leader election, or we are not explicitly configured to be an org leader.
+	peerDependent := s.config.UseLeaderElection || !s.config.OrgLeader
+	return stateDisabled && tooFarBehind && peerDependent
+}
+
 func (s *GossipStateProviderImpl) commitBlock(block *common.Block, pvtData util.PvtDataCollections) error {
 
 	t1 := time.Now()

gossip/state/state_test.go

@@ -484,6 +484,61 @@ func newBootNode(id int, committer committer.Committer, acceptor peerIdentityAcc
 	return newPeerNodeWithGossipWithValidatorWithMetrics(logger, id, committer, acceptor, nil, v, gossipMetrics)
 }
 
+func TestStraggler(t *testing.T) {
+	for _, testCase := range []struct {
+		stateEnabled   bool
+		orgLeader      bool
+		leaderElection bool
+		height         uint64
+		receivedSeq    uint64
+		expected       bool
+	}{
+		{
+			height:         100,
+			receivedSeq:    300,
+			leaderElection: true,
+			expected:       true,
+		},
+		{
+			height:      100,
+			receivedSeq: 300,
+			expected:    true,
+		},
+		{
+			height:      100,
+			receivedSeq: 300,
+			orgLeader:   true,
+		},
+		{
+			height:         100,
+			receivedSeq:    105,
+			leaderElection: true,
+		},
+		{
+			height:         100,
+			receivedSeq:    300,
+			leaderElection: true,
+			stateEnabled:   true,
+		},
+	} {
+		description := fmt.Sprintf("%+v", testCase)
+		t.Run(description, func(t *testing.T) {
+			s := &GossipStateProviderImpl{
+				config: &StateConfig{
+					StateEnabled:      testCase.stateEnabled,
+					OrgLeader:         testCase.orgLeader,
+					UseLeaderElection: testCase.leaderElection,
+				},
+			}
+
+			s.straggler(testCase.height, &proto.Payload{
+				SeqNum: testCase.receivedSeq,
+			})
+		})
+	}
+
+}
+
 func TestNilDirectMsg(t *testing.T) {
 	mc := &mockCommitter{Mock: &mock.Mock{}}
 	mc.On("LedgerHeight", mock.Anything).Return(uint64(1), nil)

sampleconfig/core.yaml

@@ -230,7 +230,13 @@ peer:
             # indicates whenever state transfer is enabled or not
             # default value is true, i.e. state transfer is active
             # and takes care to sync up missing blocks allowing
-            # lagging peer to catch up to speed with rest network
+            # lagging peer to catch up to speed with rest network.
+            # Keep in mind that when peer.gossip.useLeaderElection is true
+            # and there are several peers in the organization,
+            # or peer.gossip.useLeaderElection is false alongside with
+            # peer.gossip.orgleader being false, the peer's ledger may lag behind
+            # the rest of the peers and will never catch up due to state transfer
+            # being disabled.
             enabled: false
             # checkInterval interval to check whether peer is lagging behind enough to
             # request blocks via state transfer from another peer.