Make heavy loaded optimization configurable

Scylla Go Driver has a capability to avoid sending requests to an
overloaded shard, instead sending the request on a different connection
(at the same node).

This change makes it possible to customize the parameters used to
determine when this behavior would kick in.

cluster.go

@@ -7,6 +7,7 @@ package gocql
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net"
 	"time"
 )
@@ -91,6 +92,24 @@ type ClusterConfig struct {
 	// Default: 128 for older CQL versions
 	MaxRequestsPerConn int
 
+	// Threshold for the number of inflight requests per connection
+	// after which the connection is considered as heavy loaded
+	// Default: 512
+	HeavyLoadedConnectionThreshold int
+
+	// When a connection is considered as heavy loaded, the driver
+	// could switch to the least loaded connection for the same node.
+	// The switch will happen if the other connection is at least
+	// HeavyLoadedSwitchConnectionPercentage percentage less busy
+	// (in terms of inflight requests).
+	//
+	// For the default value of 20%, if the heavy loaded connection
+	// has 100 inflight requests, the switch will happen only if the
+	// least busy connection has less than 80 inflight requests.
+	//
+	// Default: 20%
+	HeavyLoadedSwitchConnectionPercentage int
+
 	// Default consistency level.
 	// Default: Quorum
 	Consistency Consistency
@@ -275,23 +294,25 @@ type Dialer interface {
 // the same host, and will not mark the node being down or up from events.
 func NewCluster(hosts ...string) *ClusterConfig {
 	cfg := &ClusterConfig{
-		Hosts:                  hosts,
-		CQLVersion:             "3.0.0",
-		Timeout:                11 * time.Second,
-		ConnectTimeout:         11 * time.Second,
-		Port:                   9042,
-		NumConns:               2,
-		Consistency:            Quorum,
-		MaxPreparedStmts:       defaultMaxPreparedStmts,
-		MaxRoutingKeyInfo:      1000,
-		PageSize:               5000,
-		DefaultTimestamp:       true,
-		MaxWaitSchemaAgreement: 60 * time.Second,
-		ReconnectInterval:      60 * time.Second,
-		ConvictionPolicy:       &SimpleConvictionPolicy{},
-		ReconnectionPolicy:     &ConstantReconnectionPolicy{MaxRetries: 3, Interval: 1 * time.Second},
-		SocketKeepalive:        15 * time.Second,
-		WriteCoalesceWaitTime:  200 * time.Microsecond,
+		Hosts:                                 hosts,
+		CQLVersion:                            "3.0.0",
+		Timeout:                               11 * time.Second,
+		ConnectTimeout:                        11 * time.Second,
+		Port:                                  9042,
+		NumConns:                              2,
+		Consistency:                           Quorum,
+		MaxPreparedStmts:                      defaultMaxPreparedStmts,
+		MaxRoutingKeyInfo:                     1000,
+		PageSize:                              5000,
+		DefaultTimestamp:                      true,
+		MaxWaitSchemaAgreement:                60 * time.Second,
+		ReconnectInterval:                     60 * time.Second,
+		ConvictionPolicy:                      &SimpleConvictionPolicy{},
+		ReconnectionPolicy:                    &ConstantReconnectionPolicy{MaxRetries: 3, Interval: 1 * time.Second},
+		SocketKeepalive:                       15 * time.Second,
+		WriteCoalesceWaitTime:                 200 * time.Microsecond,
+		HeavyLoadedConnectionThreshold:        512,
+		HeavyLoadedSwitchConnectionPercentage: 20,
 	}
 
 	return cfg
@@ -329,6 +350,26 @@ func (cfg *ClusterConfig) filterHost(host *HostInfo) bool {
 	return !(cfg.HostFilter == nil || cfg.HostFilter.Accept(host))
 }
 
+func (cfg *ClusterConfig) Validate() error {
+	if len(cfg.Hosts) == 0 {
+		return ErrNoHosts
+	}
+
+	if cfg.Authenticator != nil && cfg.AuthProvider != nil {
+		return errors.New("Can't use both Authenticator and AuthProvider in cluster config.")
+	}
+
+	if cfg.HeavyLoadedSwitchConnectionPercentage > 100 || cfg.HeavyLoadedSwitchConnectionPercentage < 0 {
+		return fmt.Errorf("HeavyLoadedSwitchConnectionPercentage must be between 0 and 100, got %d", cfg.HeavyLoadedSwitchConnectionPercentage)
+	}
+
+	if cfg.HeavyLoadedConnectionThreshold < 0 {
+		return fmt.Errorf("HeavyLoadedConnectionThreshold must be greater than or equal to 0, got %d", cfg.HeavyLoadedConnectionThreshold)
+	}
+
+	return nil
+}
+
 var (
 	ErrNoHosts              = errors.New("no hosts provided")
 	ErrNoConnectionsStarted = errors.New("no connections were made when creating the session")

conn.go

@@ -1615,7 +1615,7 @@ func (c *Conn) AvailableStreams() int {
 	return c.streams.Available()
 }
 
-func (c *Conn) InUseStreams() int {
+func (c *Conn) StreamsInUse() int {
 	return c.streams.InUse()
 }
 

scylla.go

@@ -413,15 +413,14 @@ func (p *scyllaConnPicker) maybeReplaceWithLessBusyConnection(c *Conn) *Conn {
 		return c
 	}
 	alternative := p.leastBusyConn()
-	if alternative == nil || alternative.AvailableStreams()*120 > c.AvailableStreams()*100 {
-		return c
-	} else {
+	if alternative != nil && alternative.StreamsInUse()*100 >= c.StreamsInUse()*(100-c.session.cfg.HeavyLoadedSwitchConnectionPercentage) {
 		return alternative
 	}
+	return c
 }
 
 func isHeavyLoaded(c *Conn) bool {
-	return c.streams.NumStreams/2 > c.AvailableStreams()
+	return c.StreamsInUse() > c.session.cfg.HeavyLoadedConnectionThreshold
 }
 
 func (p *scyllaConnPicker) leastBusyConn() *Conn {

session.go

@@ -116,16 +116,9 @@ func addrsToHosts(addrs []string, defaultPort int, logger StdLogger) ([]*HostInf
 
 // NewSession wraps an existing Node.
 func NewSession(cfg ClusterConfig) (*Session, error) {
-	// Check that hosts in the ClusterConfig is not empty
-	if len(cfg.Hosts) < 1 {
-		return nil, ErrNoHosts
+	if err := cfg.Validate(); err != nil {
+		return nil, fmt.Errorf("gocql: unable to create session: cluster config validation failed: %v", err)
 	}
-
-	// Check that either Authenticator is set or AuthProvider, not both
-	if cfg.Authenticator != nil && cfg.AuthProvider != nil {
-		return nil, errors.New("Can't use both Authenticator and AuthProvider in cluster config.")
-	}
-
 	// TODO: we should take a context in here at some point
 	ctx, cancel := context.WithCancel(context.TODO())