Function whatsUpHandler

tutorials/whatsup/reference/main.go:169–219

whatsUpHandler returns all services that currently monitored by Prometheus. It uses prometheus client_golang client code to request PromQL query against given Prometheus server to return answer.

(
	apiClient v1.API,
	handled prometheus.Counter,
	lastNumElems prometheus.Gauge,
	handledDuration prometheus.Histogram,
)

// It uses prometheus client_golang client code to request PromQL query against given Prometheus server
// to return answer.
func whatsUpHandler(
	apiClient v1.API,
	handled prometheus.Counter,
	lastNumElems prometheus.Gauge,
	handledDuration prometheus.Histogram,
) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		start := time.Now()
		ctx := r.Context()
		w.Header().Set("Content-Type", "application/json")

		var upResponse model.Value
		if err := tracing.DoInSpan(ctx, "query Prometheus", func(ctx context.Context) error {
			res, warn, err := apiClient.Query(ctx, "up", time.Now())
			if err != nil {
				return err
			}

			if len(warn) > 0 {
				return errors.Newf("got warnings from Prometheus %v", warn)
			}
			upResponse = res
			return nil
		}); err != nil {
			// We return OK status, so browser can render nice result.
			w.WriteHeader(http.StatusOK)
			// NOTE: Pass-through error might be not always safe, sanitize on production.
			b, _ := json.Marshal(response{Error: err})
			_, _ = fmt.Fprintln(w, string(b))
			return
		}

		resp := response{}
		switch r := upResponse.(type) {
		case model.Vector:
			for _, s := range r {
				resp.Instances = append(resp.Instances, string(s.Metric["instance"]))
			}
		}

		w.WriteHeader(http.StatusOK)
		b, _ := json.Marshal(resp)
		_, _ = fmt.Fprintln(w, string(b))

		lastNumElems.Set(float64(len(resp.Instances)))
		handled.(prometheus.ExemplarAdder).
			AddWithExemplar(1, getExemplarFn(ctx))
		handledDuration.(prometheus.ExemplarObserver).
			ObserveWithExemplar(time.Since(start).Seconds(), getExemplarFn(ctx))
	}
}

func getExemplarFn(ctx context.Context) prometheus.Labels {
	if spanCtx := tracing.GetSpan(ctx); spanCtx.Context().IsSampled() {