BackportAdd dry-run mode to the TxThrottler

This is a backport of upstreamed vitessio#13604

Signed-off-by: Eduardo J. Ortega U <5791035+ejortegau@users.noreply.github.com>

go/flags/endtoend/vttablet.txt

@@ -939,6 +939,8 @@ max_rate_approach_threshold: 0.9
 )
   --tx-throttler-default-priority int
 	Default priority assigned to queries that lack priority information.
+  --tx-throttler-dry-run bool
+    If present, the transaction throttler only records metrics about requests received and throttled, but does not actually throttle any requests.
   --tx-throttler-healthcheck-cells value
 	Synonym to -tx_throttler_healthcheck_cells
   --tx-throttler-tablet-types value

go/vt/vttablet/tabletserver/tabletenv/config.go

@@ -141,6 +141,7 @@ func init() {
 	flagutil.DualFormatStringListVar(&currentConfig.TxThrottlerHealthCheckCells, "tx_throttler_healthcheck_cells", defaultConfig.TxThrottlerHealthCheckCells, "A comma-separated list of cells. Only tabletservers running in these cells will be monitored for replication lag by the transaction throttler.")
 	flag.IntVar(&currentConfig.TxThrottlerDefaultPriority, "tx-throttler-default-priority", defaultConfig.TxThrottlerDefaultPriority, "Default priority assigned to queries that lack priority information.")
 	topoproto.TabletTypeListVar(&currentConfig.TxThrottlerTabletTypes, "tx-throttler-tablet-types", "A comma-separated list of tablet types. Only tablets of this type are monitored for replication lag by the transaction throttler. Supported types are replica and/or rdonly. (default replica)")
+	flag.BoolVar(&currentConfig.TxThrottlerDryRun, "tx-throttler-dry-run", defaultConfig.TxThrottlerDryRun, "If present, the transaction throttler only records metrics about requests received and throttled, but does not actually throttle any requests.")
 
 	flag.BoolVar(&enableHotRowProtection, "enable_hot_row_protection", false, "If true, incoming transactions for the same row (range) will be queued and cannot consume all txpool slots.")
 	flag.BoolVar(&enableHotRowProtectionDryRun, "enable_hot_row_protection_dry_run", false, "If true, hot row protection is not enforced but logs if transactions would have been queued.")
@@ -306,6 +307,7 @@ type TabletConfig struct {
 	TxThrottlerHealthCheckCells []string                `json:"-"`
 	TxThrottlerDefaultPriority  int                     `json:"-"`
 	TxThrottlerTabletTypes      []topodatapb.TabletType `json:"-"`
+	TxThrottlerDryRun           bool                    `json:"-"`
 
 	EnableLagThrottler bool `json:"-"`
 
@@ -538,6 +540,7 @@ var defaultConfig = TabletConfig{
 	TxThrottlerConfig:           defaultTxThrottlerConfig(),
 	TxThrottlerHealthCheckCells: []string{},
 	TxThrottlerDefaultPriority:  0, // This leads to all queries being candidates to throttle
+	TxThrottlerDryRun:           false,
 
 	EnableLagThrottler: false, // Feature flag; to switch to 'true' at some stage in the future
 

go/vt/vttablet/tabletserver/txthrottler/tx_throttler.go

@@ -135,7 +135,7 @@ type txThrottler struct {
 
 	// state holds an open transaction throttler state. It is nil
 	// if the TransactionThrottler is closed.
-	state *txThrottlerState
+	state txThrottlerState
 
 	target     *querypb.Target
 	topoServer *topo.Server
@@ -154,6 +154,10 @@ type txThrottlerConfig struct {
 	// returns false.
 	enabled bool
 
+	// if dryRun is true, the txThrottler will run only on monitoring mode, meaning that it will increase counters for
+	// total and actually throttled requests, but it will not actually return that a transaction should be throttled.
+	dryRun bool
+
 	throttlerConfig *throttlerdatapb.Configuration
 	// healthCheckCells stores the cell names in which running vttablets will be monitored for
 	// replication lag.
@@ -162,8 +166,14 @@ type txThrottlerConfig struct {
 	tabletTypes []topodatapb.TabletType
 }
 
-// txThrottlerState holds the state of an open TxThrottler object.
-type txThrottlerState struct {
+type txThrottlerState interface {
+	deallocateResources()
+	StatsUpdate(tabletStats *discovery.LegacyTabletStats)
+	throttle() bool
+}
+
+// txThrottlerStateImpl holds the state of an open TxThrottler object.
+type txThrottlerStateImpl struct {
 	config *txThrottlerConfig
 
 	// throttleMu serializes calls to throttler.Throttler.Throttle(threadId).
@@ -187,7 +197,10 @@ func NewTxThrottler(env tabletenv.Env, topoServer *topo.Server) TxThrottler {
 		log.Errorf("Error creating transaction throttler. Transaction throttling will"+
 			" be disabled. Error: %v", err)
 		// newTxThrottler with disabled config never returns an error
-		txThrottler, _ = newTxThrottler(env, topoServer, &txThrottlerConfig{enabled: false})
+		txThrottler, _ = newTxThrottler(env, topoServer, &txThrottlerConfig{
+			enabled: false,
+			dryRun:  env.Config().TxThrottlerDryRun,
+		})
 	} else {
 		log.Infof("Initialized transaction throttler with config: %+v", txThrottler.config)
 	}
@@ -201,7 +214,7 @@ func (t *txThrottler) InitDBConfig(target *querypb.Target) {
 
 func tryCreateTxThrottler(env tabletenv.Env, topoServer *topo.Server) (*txThrottler, error) {
 	if !env.Config().EnableTxThrottler {
-		return newTxThrottler(env, topoServer, &txThrottlerConfig{enabled: false})
+		return newTxThrottler(env, topoServer, &txThrottlerConfig{enabled: false, dryRun: env.Config().TxThrottlerDryRun})
 	}
 
 	var throttlerConfig throttlerdatapb.Configuration
@@ -216,6 +229,7 @@ func tryCreateTxThrottler(env tabletenv.Env, topoServer *topo.Server) (*txThrott
 
 	return newTxThrottler(env, topoServer, &txThrottlerConfig{
 		enabled:          true,
+		dryRun:           env.Config().TxThrottlerDryRun,
 		tabletTypes:      env.Config().TxThrottlerTabletTypes,
 		throttlerConfig:  &throttlerConfig,
 		healthCheckCells: healthCheckCells,
@@ -294,10 +308,10 @@ func (t *txThrottler) Throttle(priority int, workload string) (result bool) {
 		t.requestsThrottled.Add(workload, 1)
 	}
 
-	return result
+	return result && !t.config.dryRun
 }
 
-func newTxThrottlerState(topoServer *topo.Server, config *txThrottlerConfig, target *querypb.Target) (*txThrottlerState, error) {
+func newTxThrottlerState(topoServer *topo.Server, config *txThrottlerConfig, target *querypb.Target) (txThrottlerState, error) {
 	maxReplicationLagModuleConfig := throttler.MaxReplicationLagModuleConfig{Configuration: config.throttlerConfig}
 
 	t, err := throttlerFactory(
@@ -314,7 +328,7 @@ func newTxThrottlerState(topoServer *topo.Server, config *txThrottlerConfig, tar
 		t.Close()
 		return nil, err
 	}
-	result := &txThrottlerState{
+	result := &txThrottlerStateImpl{
 		config:    config,
 		throttler: t,
 	}
@@ -337,7 +351,7 @@ func newTxThrottlerState(topoServer *topo.Server, config *txThrottlerConfig, tar
 	return result, nil
 }
 
-func (ts *txThrottlerState) throttle() bool {
+func (ts *txThrottlerStateImpl) throttle() bool {
 	if ts.throttler == nil {
 		log.Error("throttle called after deallocateResources was called")
 		return false
@@ -348,7 +362,7 @@ func (ts *txThrottlerState) throttle() bool {
 	return ts.throttler.Throttle(0 /* threadId */) > 0
 }
 
-func (ts *txThrottlerState) deallocateResources() {
+func (ts *txThrottlerStateImpl) deallocateResources() {
 	// We don't really need to nil out the fields here
 	// as deallocateResources is not expected to be called
 	// more than once, but it doesn't hurt to do so.
@@ -360,14 +374,14 @@ func (ts *txThrottlerState) deallocateResources() {
 	ts.healthCheck.Close()
 	ts.healthCheck = nil
 
-	// After ts.healthCheck is closed txThrottlerState.StatsUpdate() is guaranteed not
+	// After ts.healthCheck is closed txThrottlerStateImpl.StatsUpdate() is guaranteed not
 	// to be executing, so we can safely close the throttler.
 	ts.throttler.Close()
 	ts.throttler = nil
 }
 
 // StatsUpdate updates the health of a tablet with the given healthcheck.
-func (ts *txThrottlerState) StatsUpdate(tabletStats *discovery.LegacyTabletStats) {
+func (ts *txThrottlerStateImpl) StatsUpdate(tabletStats *discovery.LegacyTabletStats) {
 	if ts.config.tabletTypes == nil {
 		return
 	}

go/vt/vttablet/tabletserver/txthrottler/tx_throttler_test.go

@@ -181,3 +181,54 @@ func TestNewTxThrottler(t *testing.T) {
 		assert.NotNil(t, throttler)
 	}
 }
+
+func TestDryRunThrottler(t *testing.T) {
+	config := tabletenv.NewDefaultConfig()
+	env := tabletenv.NewEnv(config, t.Name())
+
+	testCases := []struct {
+		Name                           string
+		txThrottlerStateShouldThrottle bool
+		throttlerDryRun                bool
+		expectedResult                 bool
+	}{
+		{Name: "Real run throttles when txThrottlerStateImpl says it should", txThrottlerStateShouldThrottle: true, throttlerDryRun: false, expectedResult: true},
+		{Name: "Real run does not throttle when txThrottlerStateImpl says it should not", txThrottlerStateShouldThrottle: false, throttlerDryRun: false, expectedResult: false},
+		{Name: "Dry run does not throttle when txThrottlerStateImpl says it should", txThrottlerStateShouldThrottle: true, throttlerDryRun: true, expectedResult: false},
+		{Name: "Dry run does not throttle when txThrottlerStateImpl says it should not", txThrottlerStateShouldThrottle: false, throttlerDryRun: true, expectedResult: false},
+	}
+
+	for _, aTestCase := range testCases {
+		theTestCase := aTestCase
+
+		t.Run(theTestCase.Name, func(t *testing.T) {
+			aTxThrottler := &txThrottler{
+				config: &txThrottlerConfig{
+					enabled: true,
+					dryRun:  theTestCase.throttlerDryRun,
+				},
+				state:             &mockTxThrottlerState{shouldThrottle: theTestCase.txThrottlerStateShouldThrottle},
+				throttlerRunning:  env.Exporter().NewGauge("TransactionThrottlerRunning", "transaction throttler running state"),
+				requestsTotal:     env.Exporter().NewCountersWithSingleLabel("TransactionThrottlerRequests", "transaction throttler requests", "workload"),
+				requestsThrottled: env.Exporter().NewCountersWithSingleLabel("TransactionThrottlerThrottled", "transaction throttler requests throttled", "workload"),
+			}
+
+			assert.Equal(t, theTestCase.expectedResult, aTxThrottler.Throttle(100, "some-workload"))
+		})
+	}
+}
+
+type mockTxThrottlerState struct {
+	shouldThrottle bool
+}
+
+func (t *mockTxThrottlerState) deallocateResources() {
+
+}
+func (t *mockTxThrottlerState) StatsUpdate(*discovery.LegacyTabletStats) {
+
+}
+
+func (t *mockTxThrottlerState) throttle() bool {
+	return t.shouldThrottle
+}