backend: Calculate UpdateInstanceStats interval using latest entry ti…

…mestamp

backend/pkg/api/instances.go

@@ -751,6 +751,22 @@ func (api *API) GetInstanceStats() ([]InstanceStats, error) {
 	return instances, nil
 }
 
+// GetLatestInstanceStatsTimestamp returns the latest timestamp value in the
+// instance stats table.
+func (api *API) GetLatestInstanceStatsTimestamp() (time.Time, error) {
+	// return latest timestamp
+	query, _, err := goqu.From("instance_stats").
+		Select(goqu.MAX("timestamp")).ToSQL()
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	var timestamp time.Time
+	_ = api.db.Get(&timestamp, query)
+
+	return timestamp, nil
+}
+
 // GetInstanceStatsByTimestamp returns an InstanceStats array of instances matching a
 // given timestamp value, ordered by version.
 func (api *API) GetInstanceStatsByTimestamp(t time.Time) ([]InstanceStats, error) {

backend/pkg/server/server.go

@@ -137,7 +137,17 @@ func New(conf *config.Config, db *db.API) (*echo.Echo, error) {
 
 	// setup background job for updating instance stats
 	go func() {
-		err := db.UpdateInstanceStats(nil, nil)
+		// update once at startup based on last run
+		var interval *time.Duration
+		lastRun, err := db.GetLatestInstanceStatsTimestamp()
+		if err != nil {
+			interval = nil
+		} else {
+			t := time.Now().UTC().Sub(lastRun)
+			interval = &t
+		}
+
+		err = db.UpdateInstanceStats(nil, interval)
 		if err != nil {
 			logger.Err(err).Msg("Error updating instance stats")
 		}