Query Prometheus API for stats.

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

gateway/metrics/add_metrics.go

@@ -0,0 +1,110 @@
+package metrics
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+
+	"github.com/alexellis/faas/gateway/requests"
+)
+
+func makeClient() http.Client {
+	// Fine-tune the client to fail fast.
+	return http.Client{}
+}
+
+// AddMetricsHandler wraps a http.HandlerFunc with Prometheus metrics
+func AddMetricsHandler(handler http.HandlerFunc, host string, port int) http.HandlerFunc {
+	client := makeClient()
+
+	prometheusQuery := NewPrometheusQuery(host, port, &client)
+	return func(w http.ResponseWriter, r *http.Request) {
+		// log.Printf("Calling upstream for function info\n")
+
+		recorder := httptest.NewRecorder()
+		handler.ServeHTTP(recorder, r)
+		upstreamCall := recorder.Result()
+
+		if upstreamCall.Body == nil {
+			return
+		}
+		defer upstreamCall.Body.Close()
+
+		if recorder.Code != http.StatusOK {
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte(fmt.Sprintf("Error pulling metrics from provider/backend. Status code: %d", recorder.Code)))
+			return
+		}
+
+		upstreamBody, _ := ioutil.ReadAll(upstreamCall.Body)
+		var functions []requests.Function
+
+		err := json.Unmarshal(upstreamBody, &functions)
+		if err != nil {
+			log.Println(err)
+
+			w.WriteHeader(http.StatusInternalServerError)
+			w.Write([]byte("Error parsing metrics from upstream provider/backend."))
+			return
+		}
+
+		// log.Printf("Querying Prometheus API\n")
+		// `sum(gateway_function_invocation_total{function_name=~".*", code=~".*"}) by (function_name, code)`)
+		expr := "sum(gateway_function_invocation_total%7Bfunction_name%3D~%22.*%22%2C+code%3D~%22.*%22%7D)+by+(function_name%2C+code)"
+		results, fetchErr := prometheusQuery.Fetch(expr)
+		if fetchErr != nil {
+			log.Printf("Error querying Prometheus API: %s\n", fetchErr.Error())
+
+			w.WriteHeader(http.StatusOK)
+			w.Write(upstreamBody)
+			return
+		}
+
+		mixIn(&functions, results)
+
+		bytesOut, marshalErr := json.Marshal(functions)
+		if marshalErr != nil {
+			log.Println(marshalErr)
+			return
+		}
+
+		// log.Printf("Writing bytesOut: %s\n", bytesOut)
+		w.WriteHeader(http.StatusOK)
+		w.Write(bytesOut)
+	}
+}
+
+func mixIn(functions *[]requests.Function, metrics *VectorQueryResponse) {
+	if functions == nil {
+		return
+	}
+
+	// Ensure values are empty first.
+	for i := range *functions {
+		(*functions)[i].InvocationCount = 0
+	}
+
+	for i, function := range *functions {
+		for _, v := range metrics.Data.Result {
+
+			if v.Metric.FunctionName == function.Name {
+				metricValue := v.Value[1]
+				switch metricValue.(type) {
+				case string:
+					// log.Println("String")
+					f, strconvErr := strconv.ParseFloat(metricValue.(string), 64)
+					if strconvErr != nil {
+						log.Printf("Unable to convert value for metric: %s\n", strconvErr)
+						continue
+					}
+					(*functions)[i].InvocationCount += f
+					break
+				}
+			}
+		}
+	}
+}