fix: space pkm response to max once per minute

flow/connectors/postgres/cdc.go

@@ -26,6 +26,7 @@ import (
 	"github.com/PeerDB-io/peerdb/flow/model"
 	"github.com/PeerDB-io/peerdb/flow/model/qvalue"
 	"github.com/PeerDB-io/peerdb/flow/otel_metrics"
+	"github.com/PeerDB-io/peerdb/flow/peerdbenv"
 	"github.com/PeerDB-io/peerdb/flow/shared"
 )
 
@@ -383,7 +384,12 @@ func PullCdcRecords[Items model.Items](
 
 	pkmRequiresResponse := false
 	waitingForCommit := false
-
+	pkmEmptyBatchThrottleEnabled, err := peerdbenv.PeerDBPKMEmptyBatchThrottleEnabled(ctx, req.Env)
+	if err != nil {
+		logger.Error("failed to get PeerDBPKMEmptyBatchThrottleEnabled", slog.Any("error", err))
+		// No need to fail here, just continue without throttling
+	}
+	lastEmptyBatchPkmSentTime := time.Now()
 	for {
 		if pkmRequiresResponse {
 			if cdcRecordsStorage.IsEmpty() && int64(clientXLogPos) > req.ConsumedOffset.Load() {
@@ -392,6 +398,7 @@ func PullCdcRecords[Items model.Items](
 					return err
 				}
 				req.ConsumedOffset.Store(int64(clientXLogPos))
+				lastEmptyBatchPkmSentTime = time.Now()
 			}
 
 			if err := sendStandbyAfterReplLock("pkm-response"); err != nil {
@@ -501,7 +508,13 @@ func PullCdcRecords[Items model.Items](
 			if pkm.ServerWALEnd > clientXLogPos {
 				clientXLogPos = pkm.ServerWALEnd
 			}
-			pkmRequiresResponse = true
+			if pkmEmptyBatchThrottleEnabled {
+				if pkm.ReplyRequested || time.Since(lastEmptyBatchPkmSentTime) >= 1*time.Minute {
+					pkmRequiresResponse = true
+				}
+			} else {
+				pkmRequiresResponse = true
+			}
 
 		case pglogrepl.XLogDataByteID:
 			xld, err := pglogrepl.ParseXLogData(msg.Data[1:])

flow/peerdbenv/dynamicconf.go

@@ -237,6 +237,14 @@ var DynamicSettings = [...]*protos.DynamicSetting{
 		ApplyMode:        protos.DynconfApplyMode_APPLY_MODE_IMMEDIATE,
 		TargetForSetting: protos.DynconfTarget_ALL,
 	},
+	{
+		Name:             "PEERDB_PKM_EMPTY_BATCH_THROTTLE_ENABLED",
+		Description:      "Throttle sending KeepAlive response to 1 minute (or if reply is requested) when no records are processed",
+		DefaultValue:     "false",
+		ValueType:        protos.DynconfValueType_BOOL,
+		ApplyMode:        protos.DynconfApplyMode_APPLY_MODE_AFTER_RESUME,
+		TargetForSetting: protos.DynconfTarget_ALL,
+	},
 }
 
 var DynamicIndex = func() map[string]int {
@@ -497,3 +505,7 @@ func PeerDBMaintenanceModeEnabled(ctx context.Context, env map[string]string) (b
 func UpdatePeerDBMaintenanceModeEnabled(ctx context.Context, pool *pgxpool.Pool, enabled bool) error {
 	return UpdateDynamicSetting(ctx, pool, "PEERDB_MAINTENANCE_MODE_ENABLED", ptr.String(strconv.FormatBool(enabled)))
 }
+
+func PeerDBPKMEmptyBatchThrottleEnabled(ctx context.Context, env map[string]string) (bool, error) {
+	return dynamicConfBool(ctx, env, "PEERDB_PKM_EMPTY_BATCH_THROTTLE_ENABLED")
+}