Split Alpha into AlphaPreference and AlphaConfidence (#2125)

config/config.go

@@ -89,9 +89,10 @@ var (
 )
 
 func getConsensusConfig(v *viper.Viper) snowball.Parameters {
-	return snowball.Parameters{
-		K:     v.GetInt(SnowSampleSizeKey),
-		Alpha: v.GetInt(SnowQuorumSizeKey),
+	p := snowball.Parameters{
+		K:               v.GetInt(SnowSampleSizeKey),
+		AlphaPreference: v.GetInt(SnowPreferenceQuorumSizeKey),
+		AlphaConfidence: v.GetInt(SnowConfidenceQuorumSizeKey),
 		// During the X-chain linearization we require BetaVirtuous and
 		// BetaRogue to be equal. Therefore we use the more conservative
 		// BetaRogue value for both BetaVirtuous and BetaRogue.
@@ -105,6 +106,11 @@ func getConsensusConfig(v *viper.Viper) snowball.Parameters {
 		MaxOutstandingItems:   v.GetInt(SnowMaxProcessingKey),
 		MaxItemProcessingTime: v.GetDuration(SnowMaxTimeProcessingKey),
 	}
+	if v.IsSet(SnowQuorumSizeKey) {
+		p.AlphaPreference = v.GetInt(SnowQuorumSizeKey)
+		p.AlphaConfidence = p.AlphaPreference
+	}
+	return p
 }
 
 func getLoggingConfig(v *viper.Viper) (logging.Config, error) {
@@ -447,7 +453,10 @@ func getNetworkConfig(
 }
 
 func getBenchlistConfig(v *viper.Viper, consensusParameters snowball.Parameters) (benchlist.Config, error) {
-	alpha := consensusParameters.Alpha
+	// AlphaConfidence is used here to ensure that benching can't cause a
+	// liveness failure. If AlphaPreference were used, the benchlist may grow to
+	// a point that committing would be extremely unlikely to happen.
+	alpha := consensusParameters.AlphaConfidence
 	k := consensusParameters.K
 	config := benchlist.Config{
 		Threshold:              v.GetInt(BenchlistFailThresholdKey),
@@ -1102,6 +1111,11 @@ func getSubnetConfigsFromFlags(v *viper.Viper, subnetIDs []ids.ID) (map[ids.ID]s
 				return nil, err
 			}
 
+			if config.ConsensusParameters.Alpha != nil {
+				config.ConsensusParameters.AlphaPreference = *config.ConsensusParameters.Alpha
+				config.ConsensusParameters.AlphaConfidence = config.ConsensusParameters.AlphaPreference
+			}
+
 			if err := config.Valid(); err != nil {
 				return nil, err
 			}
@@ -1150,6 +1164,11 @@ func getSubnetConfigsFromDir(v *viper.Viper, subnetIDs []ids.ID) (map[ids.ID]sub
 			return nil, fmt.Errorf("%w: %w", errUnmarshalling, err)
 		}
 
+		if config.ConsensusParameters.Alpha != nil {
+			config.ConsensusParameters.AlphaPreference = *config.ConsensusParameters.Alpha
+			config.ConsensusParameters.AlphaConfidence = config.ConsensusParameters.AlphaPreference
+		}
+
 		if err := config.Valid(); err != nil {
 			return nil, err
 		}

config/config_test.go

@@ -399,22 +399,22 @@ func TestGetSubnetConfigsFromFile(t *testing.T) {
 		},
 		"invalid consensus parameters": {
 			fileName:  "2Ctt6eGAeo4MLqTmGa7AdRecuVMPGWEX9wSsCLBYrLhX4a394i.json",
-			givenJSON: `{"consensusParameters":{"k": 111, "alpha":1234} }`,
+			givenJSON: `{"consensusParameters":{"k": 111, "alphaPreference":1234} }`,
 			testF: func(require *require.Assertions, given map[ids.ID]subnets.Config) {
 				require.Nil(given)
 			},
 			expectedErr: snowball.ErrParametersInvalid,
 		},
 		"correct config": {
 			fileName:  "2Ctt6eGAeo4MLqTmGa7AdRecuVMPGWEX9wSsCLBYrLhX4a394i.json",
-			givenJSON: `{"validatorOnly": true, "consensusParameters":{"alpha":16} }`,
+			givenJSON: `{"validatorOnly": true, "consensusParameters":{"alphaConfidence":16} }`,
 			testF: func(require *require.Assertions, given map[ids.ID]subnets.Config) {
 				id, _ := ids.FromString("2Ctt6eGAeo4MLqTmGa7AdRecuVMPGWEX9wSsCLBYrLhX4a394i")
 				config, ok := given[id]
 				require.True(ok)
 
 				require.Equal(true, config.ValidatorOnly)
-				require.Equal(16, config.ConsensusParameters.Alpha)
+				require.Equal(16, config.ConsensusParameters.AlphaConfidence)
 				// must still respect defaults
 				require.Equal(20, config.ConsensusParameters.K)
 			},
@@ -499,7 +499,7 @@ func TestGetSubnetConfigsFromFlags(t *testing.T) {
 				"2Ctt6eGAeo4MLqTmGa7AdRecuVMPGWEX9wSsCLBYrLhX4a394i": {
 					"consensusParameters": {
 						"k": 111,
-						"alpha": 1234
+						"alphaPreference": 1234
 					}
 				}
 			}`,
@@ -513,7 +513,8 @@ func TestGetSubnetConfigsFromFlags(t *testing.T) {
 				"2Ctt6eGAeo4MLqTmGa7AdRecuVMPGWEX9wSsCLBYrLhX4a394i": {
 					"consensusParameters": {
 						"k": 30,
-						"alpha": 20
+						"alphaPreference": 16,
+						"alphaConfidence": 20
 					},
 					"validatorOnly": true
 				}
@@ -523,7 +524,8 @@ func TestGetSubnetConfigsFromFlags(t *testing.T) {
 				config, ok := given[id]
 				require.True(ok)
 				require.Equal(true, config.ValidatorOnly)
-				require.Equal(20, config.ConsensusParameters.Alpha)
+				require.Equal(16, config.ConsensusParameters.AlphaPreference)
+				require.Equal(20, config.ConsensusParameters.AlphaConfidence)
 				require.Equal(30, config.ConsensusParameters.K)
 				// must still respect defaults
 				require.Equal(uint(10), config.GossipConfig.AppGossipValidatorSize)

config/flags.go

@@ -299,7 +299,9 @@ func addNodeFlags(fs *pflag.FlagSet) {
 
 	// Consensus
 	fs.Int(SnowSampleSizeKey, snowball.DefaultParameters.K, "Number of nodes to query for each network poll")
-	fs.Int(SnowQuorumSizeKey, snowball.DefaultParameters.Alpha, "Alpha value to use for required number positive results")
+	fs.Int(SnowQuorumSizeKey, snowball.DefaultParameters.AlphaConfidence, "Threshold of nodes required to update this node's preference and increase its confidence in a network poll")
+	fs.Int(SnowPreferenceQuorumSizeKey, snowball.DefaultParameters.AlphaPreference, fmt.Sprintf("Threshold of nodes required to update this node's preference in a network poll. Ignored if %s is provided", SnowQuorumSizeKey))
+	fs.Int(SnowConfidenceQuorumSizeKey, snowball.DefaultParameters.AlphaConfidence, fmt.Sprintf("Threshold of nodes required to increase this node's confidence in a network poll. Ignored if %s is provided", SnowQuorumSizeKey))
 	// TODO: Replace this temporary flag description after the X-chain
 	// linearization with "Beta value to use for virtuous transactions"
 	fs.Int(SnowVirtuousCommitThresholdKey, snowball.DefaultParameters.BetaVirtuous, "This flag is temporarily ignored due to the X-chain linearization")

config/keys.go

@@ -123,6 +123,8 @@ const (
 	LogDisableDisplayPluginLogsKey                     = "log-disable-display-plugin-logs"
 	SnowSampleSizeKey                                  = "snow-sample-size"
 	SnowQuorumSizeKey                                  = "snow-quorum-size"
+	SnowPreferenceQuorumSizeKey                        = "snow-preference-quorum-size"
+	SnowConfidenceQuorumSizeKey                        = "snow-confidence-quorum-size"
 	SnowVirtuousCommitThresholdKey                     = "snow-virtuous-commit-threshold"
 	SnowRogueCommitThresholdKey                        = "snow-rogue-commit-threshold"
 	SnowConcurrentRepollsKey                           = "snow-concurrent-repolls"

snow/consensus/snowball/binary_snowball.go

@@ -19,13 +19,13 @@ type binarySnowball struct {
 	// wrap the binary snowflake logic
 	binarySnowflake
 
-	// preference is the choice with the largest number of successful polls.
-	// Ties are broken by switching choice lazily
+	// preference is the choice with the largest number of polls which preferred
+	// the color. Ties are broken by switching choice lazily
 	preference int
 
-	// numSuccessfulPolls tracks the total number of successful network polls of
-	// the 0 and 1 choices
-	numSuccessfulPolls [2]int
+	// preferenceStrength tracks the total number of network polls which
+	// preferred each choice
+	preferenceStrength [2]int
 }
 
 func (sb *binarySnowball) Preference() int {
@@ -40,18 +40,27 @@ func (sb *binarySnowball) Preference() int {
 }
 
 func (sb *binarySnowball) RecordSuccessfulPoll(choice int) {
-	sb.numSuccessfulPolls[choice]++
-	if sb.numSuccessfulPolls[choice] > sb.numSuccessfulPolls[1-choice] {
-		sb.preference = choice
-	}
+	sb.increasePreferenceStrength(choice)
 	sb.binarySnowflake.RecordSuccessfulPoll(choice)
 }
 
+func (sb *binarySnowball) RecordPollPreference(choice int) {
+	sb.increasePreferenceStrength(choice)
+	sb.binarySnowflake.RecordPollPreference(choice)
+}
+
 func (sb *binarySnowball) String() string {
 	return fmt.Sprintf(
-		"SB(Preference = %d, NumSuccessfulPolls[0] = %d, NumSuccessfulPolls[1] = %d, %s)",
+		"SB(Preference = %d, PreferenceStrength[0] = %d, PreferenceStrength[1] = %d, %s)",
 		sb.preference,
-		sb.numSuccessfulPolls[0],
-		sb.numSuccessfulPolls[1],
+		sb.preferenceStrength[0],
+		sb.preferenceStrength[1],
 		&sb.binarySnowflake)
 }
+
+func (sb *binarySnowball) increasePreferenceStrength(choice int) {
+	sb.preferenceStrength[choice]++
+	if sb.preferenceStrength[choice] > sb.preferenceStrength[1-choice] {
+		sb.preference = choice
+	}
+}

snow/consensus/snowball/binary_snowball_test.go

@@ -38,6 +38,42 @@ func TestBinarySnowball(t *testing.T) {
 	require.True(sb.Finalized())
 }
 
+func TestBinarySnowballRecordPollPreference(t *testing.T) {
+	require := require.New(t)
+
+	red := 0
+	blue := 1
+
+	beta := 2
+
+	sb := newBinarySnowball(beta, red)
+	require.Equal(red, sb.Preference())
+	require.False(sb.Finalized())
+
+	sb.RecordSuccessfulPoll(blue)
+	require.Equal(blue, sb.Preference())
+	require.False(sb.Finalized())
+
+	sb.RecordSuccessfulPoll(red)
+	require.Equal(blue, sb.Preference())
+	require.False(sb.Finalized())
+
+	sb.RecordPollPreference(red)
+	require.Equal(red, sb.Preference())
+	require.False(sb.Finalized())
+
+	sb.RecordSuccessfulPoll(red)
+	require.Equal(red, sb.Preference())
+	require.False(sb.Finalized())
+
+	sb.RecordSuccessfulPoll(red)
+	require.Equal(red, sb.Preference())
+	require.True(sb.Finalized())
+
+	expected := "SB(Preference = 0, PreferenceStrength[0] = 4, PreferenceStrength[1] = 1, SF(Confidence = 2, Finalized = true, SL(Preference = 0)))"
+	require.Equal(expected, sb.String())
+}
+
 func TestBinarySnowballRecordUnsuccessfulPoll(t *testing.T) {
 	require := require.New(t)
 
@@ -64,7 +100,7 @@ func TestBinarySnowballRecordUnsuccessfulPoll(t *testing.T) {
 	require.Equal(blue, sb.Preference())
 	require.True(sb.Finalized())
 
-	expected := "SB(Preference = 1, NumSuccessfulPolls[0] = 0, NumSuccessfulPolls[1] = 3, SF(Confidence = 2, Finalized = true, SL(Preference = 1)))"
+	expected := "SB(Preference = 1, PreferenceStrength[0] = 0, PreferenceStrength[1] = 3, SF(Confidence = 2, Finalized = true, SL(Preference = 1)))"
 	require.Equal(expected, sb.String())
 }
 
@@ -104,7 +140,7 @@ func TestBinarySnowballAcceptWeirdColor(t *testing.T) {
 	require.Equal(blue, sb.Preference())
 	require.True(sb.Finalized())
 
-	expected := "SB(Preference = 1, NumSuccessfulPolls[0] = 2, NumSuccessfulPolls[1] = 2, SF(Confidence = 2, Finalized = true, SL(Preference = 0)))"
+	expected := "SB(Preference = 1, PreferenceStrength[0] = 2, PreferenceStrength[1] = 2, SF(Confidence = 2, Finalized = true, SL(Preference = 0)))"
 	require.Equal(expected, sb.String())
 }
 
@@ -128,11 +164,12 @@ func TestBinarySnowballLockColor(t *testing.T) {
 	require.Equal(red, sb.Preference())
 	require.True(sb.Finalized())
 
+	sb.RecordPollPreference(blue)
 	sb.RecordSuccessfulPoll(blue)
 
 	require.Equal(red, sb.Preference())
 	require.True(sb.Finalized())
 
-	expected := "SB(Preference = 1, NumSuccessfulPolls[0] = 1, NumSuccessfulPolls[1] = 2, SF(Confidence = 1, Finalized = true, SL(Preference = 0)))"
+	expected := "SB(Preference = 1, PreferenceStrength[0] = 1, PreferenceStrength[1] = 3, SF(Confidence = 1, Finalized = true, SL(Preference = 0)))"
 	require.Equal(expected, sb.String())
 }

snow/consensus/snowball/binary_snowflake.go

@@ -49,6 +49,15 @@ func (sf *binarySnowflake) RecordSuccessfulPoll(choice int) {
 	sf.binarySlush.RecordSuccessfulPoll(choice)
 }
 
+func (sf *binarySnowflake) RecordPollPreference(choice int) {
+	if sf.finalized {
+		return // This instance is already decided.
+	}
+
+	sf.confidence = 0
+	sf.binarySlush.RecordSuccessfulPoll(choice)
+}
+
 func (sf *binarySnowflake) RecordUnsuccessfulPoll() {
 	sf.confidence = 0
 }

snow/consensus/snowball/binary_snowflake_test.go

@@ -37,8 +37,15 @@ func TestBinarySnowflake(t *testing.T) {
 	require.Equal(blue, sf.Preference())
 	require.False(sf.Finalized())
 
+	sf.RecordPollPreference(red)
+	require.Equal(red, sf.Preference())
+	require.False(sf.Finalized())
+
 	sf.RecordSuccessfulPoll(blue)
+	require.Equal(blue, sf.Preference())
+	require.False(sf.Finalized())
 
+	sf.RecordSuccessfulPoll(blue)
 	require.Equal(blue, sf.Preference())
 	require.True(sf.Finalized())
 }

snow/consensus/snowball/consensus.go

@@ -61,6 +61,11 @@ type NnarySnowflake interface {
 	// specified choice. Assumes the choice was previously added.
 	RecordSuccessfulPoll(choice ids.ID)
 
+	// RecordPollPreference records a poll that preferred the specified choice
+	// but did not contribute towards finalizing the specified choice. Assumes
+	// the choice was previously added.
+	RecordPollPreference(choice ids.ID)
+
 	// RecordUnsuccessfulPoll resets the snowflake counter of this instance
 	RecordUnsuccessfulPoll()
 
@@ -100,6 +105,10 @@ type BinarySnowflake interface {
 	// specified choice
 	RecordSuccessfulPoll(choice int)
 
+	// RecordPollPreference records a poll that preferred the specified choice
+	// but did not contribute towards finalizing the specified choice
+	RecordPollPreference(choice int)
+
 	// RecordUnsuccessfulPoll resets the snowflake counter of this instance
 	RecordUnsuccessfulPoll()
 
@@ -130,6 +139,10 @@ type UnarySnowball interface {
 	// RecordSuccessfulPoll records a successful poll towards finalizing
 	RecordSuccessfulPoll()
 
+	// RecordPollPreference records a poll that strengthens the preference but
+	// did not contribute towards finalizing
+	RecordPollPreference()
+
 	// RecordUnsuccessfulPoll resets the snowflake counter of this instance
 	RecordUnsuccessfulPoll()