config, sysvar: map max-server-connections to max_connections

config/config.go

@@ -117,6 +117,7 @@ var (
 			map[string]string{
 				"check-mb4-value-in-utf8":       "tidb_check_mb4_value_in_utf8",
 				"enable-collect-execution-info": "tidb_enable_collect_execution_info",
+				"max-server-connections":        "max_connections",
 			},
 		},
 		{
@@ -473,6 +474,7 @@ type Instance struct {
 	EnableCollectExecutionInfo bool   `toml:"tidb_enable_collect_execution_info" json:"tidb_enable_collect_execution_info"`
 	PluginDir                  string `toml:"plugin_dir" json:"plugin_dir"`
 	PluginLoad                 string `toml:"plugin_load" json:"plugin_load"`
+	MaxConnections             uint32 `toml:"max_connections" json:"max_connections"`
 }
 
 func (l *Log) getDisableTimestamp() bool {
@@ -824,6 +826,7 @@ var defaultConf = Config{
 		EnableCollectExecutionInfo:  true,
 		PluginDir:                   "/data/deploy/plugin",
 		PluginLoad:                  "",
+		MaxConnections:              0,
 	},
 	Status: Status{
 		ReportStatus:          true,

config/config.toml.example

@@ -89,9 +89,6 @@ repair-mode = false
 # In repair mode, repairing table which is not in repair list will get wrong database or wrong table error.
 repair-table-list = []
 
-# The maximum permitted number of simultaneous client connections. When the value is 0, the number of connections is unlimited.
-max-server-connections = 0
-
 # Whether new collations are enabled, as indicated by its name, this configuration entry take effect ONLY when a TiDB cluster bootstraps for the first time.
 new_collations_enabled_on_first_bootstrap = true
 
@@ -465,3 +462,6 @@ tidb_slow_log_threshold = 300
 # tidb_record_plan_in_slow_log is used to enable record query plan in slow log.
 # 0 is disable. 1 is enable.
 tidb_record_plan_in_slow_log = 1
+
+# The maximum permitted number of simultaneous client connections. When the value is 0, the number of connections is unlimited.
+max_connections = 0

config/config_test.go

@@ -282,9 +282,6 @@ repair-mode = false
 # In repair mode, repairing table which is not in repair list will get wrong database or wrong table error.
 repair-table-list = []
 
-# The maximum permitted number of simultaneous client connections. When the value is 0, the number of connections is unlimited.
-max-server-connections = 0
-
 # Whether new collations are enabled, as indicated by its name, this configuration entry take effect ONLY when a TiDB cluster bootstraps for the first time.
 new_collations_enabled_on_first_bootstrap = true
 
@@ -309,6 +306,11 @@ deprecate-integer-display-length = false
 # See https://dev.mysql.com/doc/refman/8.0/en/string-type-syntax.html for more details.
 enable-enum-length-limit = true
 
+[instance]
+
+# The maximum permitted number of simultaneous client connections. When the value is 0, the number of connections is unlimited.
+max_connections = 0
+
 [log]
 # Log level: debug, info, warn, error, fatal.
 level = "info"
@@ -707,7 +709,7 @@ unrecognized-option-test = true
 	match, err := regexp.Match("(?:.|\n)*invalid configuration option(?:.|\n)*", []byte(err.Error()))
 	require.NoError(t, err)
 	require.True(t, match)
-	require.Equal(t, uint32(0), conf.MaxServerConnections)
+	require.Equal(t, uint32(0), conf.Instance.MaxConnections)
 
 	err = f.Truncate(0)
 	require.NoError(t, err)
@@ -722,7 +724,6 @@ delay-clean-table-lock = 5
 split-region-max-num=10000
 server-version = "test_version"
 repair-mode = true
-max-server-connections = 200
 max-index-length = 3080
 index-limit = 70
 table-column-count-limit = 4000
@@ -768,6 +769,8 @@ grpc-keepalive-timeout = 10
 grpc-concurrent-streams = 2048
 grpc-initial-window-size = 10240
 grpc-max-send-msg-size = 40960
+[instance]
+max_connections = 200
 `)
 
 	require.NoError(t, err)
@@ -797,7 +800,7 @@ grpc-max-send-msg-size = 40960
 	require.Equal(t, uint64(10000), conf.SplitRegionMaxNum)
 	require.True(t, conf.RepairMode)
 	require.Equal(t, uint64(16), conf.TiKVClient.ResolveLockLiteThreshold)
-	require.Equal(t, uint32(200), conf.MaxServerConnections)
+	require.Equal(t, uint32(200), conf.Instance.MaxConnections)
 	require.Equal(t, []string{"tiflash"}, conf.IsolationRead.Engines)
 	require.Equal(t, 3080, conf.MaxIndexLength)
 	require.Equal(t, 70, conf.IndexLimit)

executor/set_test.go

@@ -914,16 +914,17 @@ func TestValidateSetVar(t *testing.T) {
 	result.Check(testkit.Rows("SYSTEM"))
 
 	// The following cases test value out of range and illegal type when setting system variables.
-	// See https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html for more details.
+	// See https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html for more details.
 	tk.MustExec("set @@global.max_connections=100001")
 	tk.MustQuery("show warnings").Check(testkit.RowsWithSep("|", "Warning|1292|Truncated incorrect max_connections value: '100001'"))
 	result = tk.MustQuery("select @@global.max_connections;")
 	result.Check(testkit.Rows("100000"))
 
+	// "max_connections == 0" means there is no limitation on the number of connections.
 	tk.MustExec("set @@global.max_connections=-1")
 	tk.MustQuery("show warnings").Check(testkit.RowsWithSep("|", "Warning|1292|Truncated incorrect max_connections value: '-1'"))
 	result = tk.MustQuery("select @@global.max_connections;")
-	result.Check(testkit.Rows("1"))
+	result.Check(testkit.Rows("0"))
 
 	err = tk.ExecToErr("set @@global.max_connections='hello'")
 	require.True(t, terror.ErrorEqual(err, variable.ErrWrongTypeForVar))
@@ -970,7 +971,7 @@ func TestValidateSetVar(t *testing.T) {
 	tk.MustExec("set @@global.max_connections=-1")
 	tk.MustQuery("show warnings").Check(testkit.RowsWithSep("|", "Warning|1292|Truncated incorrect max_connections value: '-1'"))
 	result = tk.MustQuery("select @@global.max_connections;")
-	result.Check(testkit.Rows("1"))
+	result.Check(testkit.Rows("0"))
 
 	err = tk.ExecToErr("set @@global.max_connections='hello'")
 	require.True(t, terror.ErrorEqual(err, variable.ErrWrongTypeForVar))
@@ -1226,15 +1227,15 @@ func TestSelectGlobalVar(t *testing.T) {
 	defer clean()
 	tk := testkit.NewTestKit(t, store)
 
-	tk.MustQuery("select @@global.max_connections;").Check(testkit.Rows("151"))
-	tk.MustQuery("select @@max_connections;").Check(testkit.Rows("151"))
+	tk.MustQuery("select @@global.max_connections;").Check(testkit.Rows("0"))
+	tk.MustQuery("select @@max_connections;").Check(testkit.Rows("0"))
 
 	tk.MustExec("set @@global.max_connections=100;")
 
 	tk.MustQuery("select @@global.max_connections;").Check(testkit.Rows("100"))
 	tk.MustQuery("select @@max_connections;").Check(testkit.Rows("100"))
 
-	tk.MustExec("set @@global.max_connections=151;")
+	tk.MustExec("set @@global.max_connections=0;")
 
 	// test for unknown variable.
 	err := tk.ExecToErr("select @@invalid")

expression/integration_test.go

@@ -6212,11 +6212,11 @@ func TestGlobalCacheCorrectness(t *testing.T) {
 	defer clean()
 
 	tk := testkit.NewTestKit(t, store)
-	tk.MustQuery("SHOW VARIABLES LIKE 'max_connections'").Check(testkit.Rows("max_connections 151"))
+	tk.MustQuery("SHOW VARIABLES LIKE 'max_connections'").Check(testkit.Rows("max_connections 0"))
 	tk.MustExec("SET GLOBAL max_connections=1234")
 	tk.MustQuery("SHOW VARIABLES LIKE 'max_connections'").Check(testkit.Rows("max_connections 1234"))
 	// restore
-	tk.MustExec("SET GLOBAL max_connections=151")
+	tk.MustExec("SET GLOBAL max_connections=0")
 }
 
 func TestRedundantColumnResolve(t *testing.T) {

server/server.go

@@ -302,6 +302,7 @@ func NewServer(cfg *config.Config, driver IDriver) (*Server, error) {
 	rand.Seed(time.Now().UTC().UnixNano())
 
 	variable.RegisterStatistics(s)
+	variable.SetMaxConnections = s.SetMaxConnections
 
 	return s, nil
 }
@@ -352,7 +353,7 @@ func setTxnScope() {
 // Export config-related metrics
 func (s *Server) reportConfig() {
 	metrics.ConfigStatus.WithLabelValues("token-limit").Set(float64(s.cfg.TokenLimit))
-	metrics.ConfigStatus.WithLabelValues("max-server-connections").Set(float64(s.cfg.MaxServerConnections))
+	metrics.ConfigStatus.WithLabelValues("max_connections").Set(float64(s.cfg.Instance.MaxConnections))
 }
 
 // Run runs the server.
@@ -605,18 +606,37 @@ func (cc *clientConn) connectInfo() *variable.ConnectionInfo {
 }
 
 func (s *Server) checkConnectionCount() error {
-	// When the value of MaxServerConnections is 0, the number of connections is unlimited.
-	if int(s.cfg.MaxServerConnections) == 0 {
+	// When the value of Instance.MaxConnections is 0, the number of connections is unlimited.
+	if int(s.cfg.Instance.MaxConnections) == 0 {
 		return nil
 	}
 
 	s.rwlock.RLock()
 	conns := len(s.clients)
 	s.rwlock.RUnlock()
 
-	if conns >= int(s.cfg.MaxServerConnections) {
+	if conns >= int(s.cfg.Instance.MaxConnections) {
 		logutil.BgLogger().Error("too many connections",
-			zap.Uint32("max connections", s.cfg.MaxServerConnections), zap.Error(errConCount))
+			zap.Uint32("max connections", s.cfg.Instance.MaxConnections), zap.Error(errConCount))
+		return errConCount
+	}
+	return nil
+}
+
+// SetMaxConnections checks and updates the value of max_connections.
+func (s *Server) SetMaxConnections(newMaxConnections uint32) error {
+	// When the value of newMaxConnections is 0, the number of connections is unlimited.
+	if int(newMaxConnections) == 0 {
+		return nil
+	}
+
+	s.rwlock.RLock()
+	conns := len(s.clients)
+	s.rwlock.RUnlock()
+
+	if conns >= int(newMaxConnections) {
+		logutil.BgLogger().Error("Current connections number exceeds the setting value",
+			zap.Uint32("max connections", newMaxConnections), zap.Error(errConCount))
 		return errConCount
 	}
 	return nil

sessionctx/variable/noop.go

@@ -24,7 +24,6 @@ import (
 // but changing them has no effect on behavior.
 
 var noopSysVars = []*SysVar{
-	{Scope: ScopeGlobal, Name: MaxConnections, Value: "151", Type: TypeUnsigned, MinValue: 1, MaxValue: 100000},
 	// It is unsafe to pretend that any variation of "read only" is enabled when the server
 	// does not support it. It is possible that these features will be supported in future,
 	// but until then...

sessionctx/variable/sysvar.go

@@ -415,6 +415,18 @@ var defaultSysVars = []*SysVar{
 	{Scope: ScopeInstance, Name: PluginDir, Value: "/data/deploy/plugin", ReadOnly: true, GetGlobal: func(s *SessionVars) (string, error) {
 		return config.GetGlobalConfig().Instance.PluginDir, nil
 	}},
+	{Scope: ScopeInstance, Name: MaxConnections, Value: strconv.FormatUint(uint64(config.GetGlobalConfig().Instance.MaxConnections), 10), Type: TypeUnsigned, MinValue: 0, MaxValue: 100000, SetGlobal: func(s *SessionVars, val string) error {
+		newVal := uint32(TidbOptInt64(val, 0))
+		if SetMaxConnections != nil {
+			if err := SetMaxConnections(newVal); err != nil {
+				return err
+			}
+		}
+		config.GetGlobalConfig().Instance.MaxConnections = newVal
+		return nil
+	}, GetGlobal: func(s *SessionVars) (string, error) {
+		return strconv.FormatUint(uint64(config.GetGlobalConfig().Instance.MaxConnections), 10), nil
+	}},
 
 	/* The system variables below have GLOBAL scope  */
 	{Scope: ScopeGlobal, Name: MaxPreparedStmtCount, Value: strconv.FormatInt(DefMaxPreparedStmtCount, 10), Type: TypeInt, MinValue: -1, MaxValue: 1048576},

sessionctx/variable/tidb_vars.go

@@ -994,4 +994,6 @@ var (
 	GetMemQuotaAnalyze func() int64 = nil
 	// SetStatsCacheCapacity is the func registered by domain to set statsCache memory quota.
 	SetStatsCacheCapacity atomic.Value
+	// SetMaxConnections is the func registered by derver to set the value of max_connections.
+	SetMaxConnections func(newMaxConnections uint32) error = nil
 )

tidb-server/main.go

@@ -558,6 +558,8 @@ func setGlobalVars() {
 					cfg.Instance.CheckMb4ValueInUTF8.Store(cfg.CheckMb4ValueInUTF8.Load())
 				case "enable-collect-execution-info":
 					cfg.Instance.EnableCollectExecutionInfo = cfg.EnableCollectExecutionInfo
+				case "max-server-connections":
+					cfg.Instance.MaxConnections = cfg.MaxServerConnections
 				}
 			case "log":
 				switch oldName {