Add a target for waiting until PLC mirror catches up

cmd/plc-mirror/main.go

@@ -72,6 +72,7 @@ func runMain(ctx context.Context) error {
 		return fmt.Errorf("failed to create server: %w", err)
 	}
 	http.Handle("/", server)
+	http.HandleFunc("/ready", server.Ready)
 
 	log.Info().Msgf("Starting HTTP listener on %q...", config.MetricsPort)
 	http.Handle("/metrics", promhttp.Handler())

cmd/plc-mirror/mirror.go

@@ -89,6 +89,12 @@ func (m *Mirror) LastSuccess() time.Time {
 	return m.lastSuccessTimestamp
 }
 
+func (m *Mirror) LastRecordTimestamp(ctx context.Context) (string, error) {
+	ts := ""
+	err := m.db.WithContext(ctx).Model(&PLCLogEntry{}).Select("plc_timestamp").Order("plc_timestamp desc").Limit(1).Take(&ts).Error
+	return ts, err
+}
+
 func (m *Mirror) runOnce(ctx context.Context) error {
 	log := zerolog.Ctx(ctx)
 

cmd/plc-mirror/serve.go

@@ -45,6 +45,24 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	s.handler(w, req)
 }
 
+func (s *Server) Ready(w http.ResponseWriter, req *http.Request) {
+	convreq.Wrap(func(ctx context.Context) convreq.HttpResponse {
+		ts, err := s.mirror.LastRecordTimestamp(ctx)
+		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)
+		if delay > s.MaxDelay {
+			return respond.ServiceUnavailable(fmt.Sprintf("still %s behind", delay))
+		}
+		return respond.String("OK")
+	})(w, req)
+}
+
 func (s *Server) serve(ctx context.Context, req *http.Request) convreq.HttpResponse {
 	delay := time.Since(s.mirror.LastSuccess())
 	if delay > s.MaxDelay {