fix: keep the timestamp of last record in memory instead of querying the DB every time

cmd/plc-mirror/mirror.go

@@ -38,6 +38,7 @@ type Mirror struct {
 
 	mu                   sync.RWMutex
 	lastSuccessTimestamp time.Time
+	lastRecordTimestamp  time.Time
 }
 
 func NewMirror(ctx context.Context, upstream string, db *gorm.DB) (*Mirror, error) {
@@ -91,10 +92,33 @@ func (m *Mirror) LastSuccess() time.Time {
 	return m.lastSuccessTimestamp
 }
 
-func (m *Mirror) LastRecordTimestamp(ctx context.Context) (string, error) {
+func (m *Mirror) LastRecordTimestamp(ctx context.Context) (time.Time, error) {
+	m.mu.RLock()
+	t := m.lastRecordTimestamp
+	m.mu.RUnlock()
+	if !t.IsZero() {
+		return t, nil
+	}
+
 	ts := ""
 	err := m.db.WithContext(ctx).Model(&PLCLogEntry{}).Select("plc_timestamp").Order("plc_timestamp desc").Limit(1).Take(&ts).Error
-	return ts, err
+	if err != nil {
+		return time.Time{}, err
+	}
+	dbTimestamp, err := time.Parse(time.RFC3339, ts)
+	if err != nil {
+		return time.Time{}, fmt.Errorf("parsing timestamp %q: %w", ts, err)
+	}
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if t.IsZero() {
+		t = dbTimestamp
+	}
+	if t.After(dbTimestamp) {
+		return t, nil
+	}
+	return dbTimestamp, nil
 }
 
 func (m *Mirror) runOnce(ctx context.Context) error {
@@ -138,6 +162,8 @@ func (m *Mirror) runOnce(ctx context.Context) error {
 		decoder := json.NewDecoder(resp.Body)
 		oldCursor := cursor
 
+		var lastTimestamp time.Time
+
 		for {
 			var entry plc.OperationLogEntry
 			err := decoder.Decode(&entry)
@@ -155,6 +181,7 @@ func (m *Mirror) runOnce(ctx context.Context) error {
 			t, err := time.Parse(time.RFC3339, row.PLCTimestamp)
 			if err == nil {
 				lastEventTimestamp.Set(float64(t.Unix()))
+				lastTimestamp = t
 			} else {
 				log.Warn().Msgf("Failed to parse %q: %s", row.PLCTimestamp, err)
 			}
@@ -174,6 +201,12 @@ func (m *Mirror) runOnce(ctx context.Context) error {
 			return fmt.Errorf("inserting log entry into database: %w", err)
 		}
 
+		if !lastTimestamp.IsZero() {
+			m.mu.Lock()
+			m.lastRecordTimestamp = lastTimestamp
+			m.mu.Unlock()
+		}
+
 		log.Info().Msgf("Got %d log entries. New cursor: %q", len(newEntries), cursor)
 	}
 	return nil

cmd/plc-mirror/serve.go

@@ -51,11 +51,7 @@ func (s *Server) Ready(w http.ResponseWriter, req *http.Request) {
 		if err != nil {
 			return respond.InternalServerError(err.Error())
 		}
-		t, err := time.Parse(time.RFC3339, ts)
-		if err != nil {
-			return respond.InternalServerError(err.Error())
-		}
-		delay := time.Since(t)
+		delay := time.Since(ts)
 		if delay > s.MaxDelay {
 			return respond.ServiceUnavailable(fmt.Sprintf("still %s behind", delay))
 		}