Auto-set 5s on-demand heartbeat if --enable_heartbeat is disabled

go/flags/endtoend/vtcombo.txt

@@ -169,7 +169,7 @@ Flags:
       --healthcheck_timeout duration                                     the health check timeout period (default 1m0s)
       --heartbeat_enable                                                 If true, vttablet records (if master) or checks (if replica) the current time of a replication heartbeat in the sidecar database's heartbeat table. The result is used to inform the serving state of the vttablet via healthchecks.
       --heartbeat_interval duration                                      How frequently to read and write replication heartbeat. (default 1s)
-      --heartbeat_on_demand_duration duration                            If non-zero, heartbeats are only written upon consumer request, and only run for up to given duration following the request. Frequent requests can keep the heartbeat running consistently; when requests are infrequent heartbeat may completely stop between requests
+      --heartbeat_on_demand_duration duration                            If non-zero, heartbeats are only written upon consumer request, and only run for up to given duration following the request. Frequent requests can keep the heartbeat running consistently. Automatically set to 5s when --heartbeat_enable is unset.
   -h, --help                                                             help for vtcombo
       --hot_row_protection_concurrent_transactions int                   Number of concurrent transactions let through to the txpool/MySQL for the same hot row. Should be > 1 to have enough 'ready' transactions in MySQL and benefit from a pipelining effect. (default 5)
       --hot_row_protection_max_global_queue_size int                     Global queue limit across all row (ranges). Useful to prevent that the queue can grow unbounded. (default 1000)

go/flags/endtoend/vttablet.txt

@@ -196,7 +196,7 @@ Flags:
       --health_check_interval duration                                   Interval between health checks (default 20s)
       --heartbeat_enable                                                 If true, vttablet records (if master) or checks (if replica) the current time of a replication heartbeat in the sidecar database's heartbeat table. The result is used to inform the serving state of the vttablet via healthchecks.
       --heartbeat_interval duration                                      How frequently to read and write replication heartbeat. (default 1s)
-      --heartbeat_on_demand_duration duration                            If non-zero, heartbeats are only written upon consumer request, and only run for up to given duration following the request. Frequent requests can keep the heartbeat running consistently; when requests are infrequent heartbeat may completely stop between requests
+      --heartbeat_on_demand_duration duration                            If non-zero, heartbeats are only written upon consumer request, and only run for up to given duration following the request. Frequent requests can keep the heartbeat running consistently. Automatically set to 5s when --heartbeat_enable is unset.
   -h, --help                                                             help for vttablet
       --hot_row_protection_concurrent_transactions int                   Number of concurrent transactions let through to the txpool/MySQL for the same hot row. Should be > 1 to have enough 'ready' transactions in MySQL and benefit from a pipelining effect. (default 5)
       --hot_row_protection_max_global_queue_size int                     Global queue limit across all row (ranges). Useful to prevent that the queue can grow unbounded. (default 1000)

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

@@ -183,7 +183,7 @@ func registerTabletEnvFlags(fs *pflag.FlagSet) {
 
 	fs.BoolVar(&enableHeartbeat, "heartbeat_enable", false, "If true, vttablet records (if master) or checks (if replica) the current time of a replication heartbeat in the sidecar database's heartbeat table. The result is used to inform the serving state of the vttablet via healthchecks.")
 	fs.DurationVar(&heartbeatInterval, "heartbeat_interval", 1*time.Second, "How frequently to read and write replication heartbeat.")
-	fs.DurationVar(&heartbeatOnDemandDuration, "heartbeat_on_demand_duration", 0, "If non-zero, heartbeats are only written upon consumer request, and only run for up to given duration following the request. Frequent requests can keep the heartbeat running consistently; when requests are infrequent heartbeat may completely stop between requests")
+	fs.DurationVar(&heartbeatOnDemandDuration, "heartbeat_on_demand_duration", 0, "If non-zero, heartbeats are only written upon consumer request, and only run for up to given duration following the request. Frequent requests can keep the heartbeat running consistently. Automatically set to 5s when --heartbeat_enable is unset.")
 
 	fs.BoolVar(&currentConfig.EnforceStrictTransTables, "enforce_strict_trans_tables", defaultConfig.EnforceStrictTransTables, "If true, vttablet requires MySQL to run with STRICT_TRANS_TABLES or STRICT_ALL_TABLES on. It is recommended to not turn this flag off. Otherwise MySQL may alter your supplied values before saving them to the database.")
 	flagutil.DualFormatBoolVar(fs, &enableConsolidator, "enable_consolidator", true, "This option enables the query consolidator.")
@@ -251,6 +251,17 @@ func Init() {
 	if heartbeatOnDemandDuration < 0 {
 		heartbeatOnDemandDuration = 0
 	}
+	if heartbeatOnDemandDuration == 0 && !enableHeartbeat {
+		// Neither heartbeat-related flag was set. We want to always have some heartbeat capability,
+		// because the tablet throttler depends on the availability of a heartbeat.
+		// To that effect, we choose to set a minimal and relaxed heartbeat setup: a 5s on-demand lease.
+		// Ad on-demand heartbeats go, this has no impact when the throttler is disabled, and no impact
+		// when the throttler is enabled but otherwise not engaged with some consumer. Heartbeats are
+		// only leased and generated when the throttler is requested by some consumer (e.g. vreplication)
+		// to provide throttling advice. We keep the lease to a very low 5s, so that heartbeats are only
+		// generated while an active consumer is requesting them, and only up to 5s afterwards.
+		heartbeatOnDemandDuration = 5 * time.Second
+	}
 	currentConfig.ReplicationTracker.HeartbeatInterval = heartbeatInterval
 	currentConfig.ReplicationTracker.HeartbeatOnDemand = heartbeatOnDemandDuration
 

go/vt/vttablet/tabletserver/tabletenv/config_test.go

@@ -214,6 +214,7 @@ func TestFlags(t *testing.T) {
 	want.Healthcheck.UnhealthyThreshold = 2 * time.Hour
 	want.ReplicationTracker.HeartbeatInterval = time.Second
 	want.ReplicationTracker.Mode = Disable
+	want.ReplicationTracker.HeartbeatOnDemand = 5 * time.Second
 	assert.Equal(t, want.DB, currentConfig.DB)
 	assert.Equal(t, want, currentConfig)
 
@@ -266,21 +267,36 @@ func TestFlags(t *testing.T) {
 	assert.Equal(t, want, currentConfig)
 
 	enableHeartbeat = true
+	heartbeatOnDemandDuration = 0
 	heartbeatInterval = 1 * time.Second
 	currentConfig.ReplicationTracker.Mode = ""
 	currentConfig.ReplicationTracker.HeartbeatInterval = 0
 	Init()
 	want.ReplicationTracker.Mode = Heartbeat
 	want.ReplicationTracker.HeartbeatInterval = time.Second
+	want.ReplicationTracker.HeartbeatOnDemand = 0
 	assert.Equal(t, want, currentConfig)
 
 	enableHeartbeat = false
+	heartbeatOnDemandDuration = 0
 	heartbeatInterval = 1 * time.Second
 	currentConfig.ReplicationTracker.Mode = ""
 	currentConfig.ReplicationTracker.HeartbeatInterval = 0
 	Init()
 	want.ReplicationTracker.Mode = Disable
 	want.ReplicationTracker.HeartbeatInterval = time.Second
+	want.ReplicationTracker.HeartbeatOnDemand = 5 * time.Second
+	assert.Equal(t, want, currentConfig)
+
+	enableHeartbeat = false
+	heartbeatOnDemandDuration = 7 * time.Second
+	heartbeatInterval = 1 * time.Second
+	currentConfig.ReplicationTracker.Mode = ""
+	currentConfig.ReplicationTracker.HeartbeatInterval = 0
+	Init()
+	want.ReplicationTracker.Mode = Disable
+	want.ReplicationTracker.HeartbeatInterval = time.Second
+	want.ReplicationTracker.HeartbeatOnDemand = 7 * time.Second
 	assert.Equal(t, want, currentConfig)
 
 	enableReplicationReporter = true