Add call-id via middleware

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

gateway/README.md

@@ -5,7 +5,23 @@ The gateway will scale functions according to demand by altering the service rep
 
 Swagger docs: https://github.com/openfaas/faas/tree/master/api-docs
 
-**Environmental overrides:**
+## Logs
+
+Logs are available at the function level and can be accessed through Swarm or Kubernetes using native tooling. You can also install a Docker logging driver to aggregate your logs. By default functions will not write the request and response bodies to stdout. You can toggle this behaviour by setting `read_debug` for the request and `write_debug` for the response.
+
+## Tracing
+
+An "X-Call-Id" header is applied to every incoming call through the gateway and is usable for tracing and monitoring calls. We use a UUID for this string.
+
+Header:
+
+```
+X-Call-Id
+```
+
+Within a function this is available as `Http_X_Call_Id`.
+
+## Environmental overrides
 The gateway can be configured through the following environment variables: 
 
 | Option                 | Usage             |

gateway/handlers/callid_middleware.go

@@ -0,0 +1,24 @@
+package handlers
+
+import (
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/docker/distribution/uuid"
+)
+
+// MakeCallIDMiddleware middleware tags a request with a uid
+func MakeCallIDMiddleware(next http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		start := time.Now()
+		callID := uuid.Generate().String()
+		r.Header.Add("X-Call-Id", callID)
+		w.Header().Add("X-Call-Id", callID)
+
+		r.Header.Add("X-Start-Time", fmt.Sprintf("%d", start.UTC().UnixNano()))
+		w.Header().Add("X-Start-Time", fmt.Sprintf("%d", start.UTC().UnixNano()))
+
+		next(w, r)
+	}
+}

gateway/server.go

@@ -108,7 +108,7 @@ func main() {
 
 	prometheusQuery := metrics.NewPrometheusQuery(config.PrometheusHost, config.PrometheusPort, &http.Client{})
 	listFunctions := metrics.AddMetricsHandler(faasHandlers.ListFunctions, prometheusQuery)
-
+	faasHandlers.Proxy = internalHandlers.MakeCallIDMiddleware(faasHandlers.Proxy)
 	r := mux.NewRouter()
 
 	// r.StrictSlash(false)	// This didn't work, so register routes twice.