Update backend/pkg/api/instances.go

Co-authored-by: Ervin Racz <39372002+ervcz@users.noreply.github.com>

Author: t-lo

backend/pkg/api/instances.go

@@ -649,13 +649,14 @@ func (api *API) GetDefaultInterval() time.Duration {
 	return defaultStatsInterval
 }
 
-// instanceStatsQuery returns a SelectDataset prepared to return all active instances within a time period.
+// instanceStatsQuery returns a SelectDataset to estimate the active fleet size at a given point in time.
+// It answers "how many instances were part of the active fleet on day X",
+// not "how many instances specifically checked in on day X".
 //
-//	This is somewhat complicated by the fact that an instance created _before_ or _during_ our window
-//	can have checked in _after_ the window we're supposed to check. last_check_for_updates gets updated every
-//	time an instance checks for an update. Hence we filter for instances that:
-//	 1. have been created before timestamp 't'
-//	 2. have checked in _during_ or _after_ the time period
+// Since last_check_for_updates gets overwritten on every check-in, we cannot determine
+// whether an instance was active on a specific past day. Instead, we count instances that:
+//   1. existed at the time (created_ts <= timestamp)
+//   2. are still alive (last_check_for_updates > timestamp - duration)
 func (api *API) instanceStatsQuery(t *time.Time, duration *time.Duration) *goqu.SelectDataset {
 	if t == nil {
 		now := time.Now().UTC()