Add feature: scale from zero to 1 replicas

This change allows functions to be "idled" or scaled to zero
replicas and then be invoked later on. There is a penalty to
scaling up - the API gateway proxy will block until the function
is ready.

A cache is included to off-set the calls to upstream API to check
on readiness along with unit tests.

Testing via scaling to zero replicas and then invoking function.
On Swarm I observed 3 seconds on an Intel Nuc i5 for scaling back
from zero replicas.

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

gateway/handlers/scaling.go

@@ -0,0 +1,147 @@
+// Copyright (c) OpenFaaS Project. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package handlers
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"net/http/httptest"
+	"time"
+
+	"github.com/openfaas/faas/gateway/requests"
+)
+
+// ScalingConfig for scaling behaviours
+type ScalingConfig struct {
+	MaxPollCount         uint
+	FunctionPollInterval time.Duration
+	CacheExpiry          time.Duration
+}
+
+// MakeScalingHandler creates handler which can scale a function from
+// zero to 1 replica(s).
+func MakeScalingHandler(next http.HandlerFunc, upstream http.HandlerFunc, config ScalingConfig) http.HandlerFunc {
+	cache := FunctionCache{
+		Cache:  make(map[string]*FunctionMeta),
+		Expiry: config.CacheExpiry,
+	}
+
+	return func(w http.ResponseWriter, r *http.Request) {
+
+		functionName := getServiceName(r.URL.String())
+
+		if replicas, hit := cache.Get(functionName); hit && replicas > 0 {
+			next.ServeHTTP(w, r)
+			return
+		}
+
+		replicas, code, err := getReplicas(functionName, upstream)
+		cache.Set(functionName, replicas)
+
+		if err != nil {
+			var errStr string
+			if code == http.StatusNotFound {
+				errStr = fmt.Sprintf("unable to find function: %s", functionName)
+
+			} else {
+				errStr = fmt.Sprintf("error finding function %s: %s", functionName, err.Error())
+			}
+
+			log.Printf(errStr)
+			w.WriteHeader(code)
+			w.Write([]byte(errStr))
+			return
+		}
+
+		if replicas == 0 {
+			minReplicas := uint64(1)
+
+			err := scaleFunction(functionName, minReplicas, upstream)
+			if err != nil {
+				errStr := fmt.Errorf("unable to scale function [%s], err: %s", functionName, err)
+				log.Printf(errStr.Error())
+
+				w.WriteHeader(http.StatusInternalServerError)
+				w.Write([]byte(errStr.Error()))
+				return
+			}
+
+			for i := 0; i < int(config.MaxPollCount); i++ {
+				replicas, _, err := getReplicas(functionName, upstream)
+				cache.Set(functionName, replicas)
+
+				if err != nil {
+					errStr := fmt.Sprintf("error: %s", err.Error())
+					log.Printf(errStr)
+
+					w.WriteHeader(http.StatusInternalServerError)
+					w.Write([]byte(errStr))
+					return
+				}
+
+				if replicas > 0 {
+					break
+				}
+
+				time.Sleep(config.FunctionPollInterval)
+			}
+		}
+
+		next.ServeHTTP(w, r)
+	}
+}
+
+func getReplicas(functionName string, upstream http.HandlerFunc) (uint64, int, error) {
+
+	replicasQuery, _ := http.NewRequest(http.MethodGet, fmt.Sprintf("/system/function/%s", functionName), nil)
+	rr := httptest.NewRecorder()
+
+	upstream.ServeHTTP(rr, replicasQuery)
+	if rr.Code != 200 {
+		log.Printf("error, query replicas status: %d", rr.Code)
+
+		var errBody string
+		if rr.Body != nil {
+			errBody = string(rr.Body.String())
+		}
+
+		return 0, rr.Code, fmt.Errorf("unable to query function: %s", string(errBody))
+	}
+
+	replicaBytes, _ := ioutil.ReadAll(rr.Body)
+	replicaResult := requests.Function{}
+	json.Unmarshal(replicaBytes, &replicaResult)
+
+	return replicaResult.AvailableReplicas, rr.Code, nil
+}
+
+func scaleFunction(functionName string, minReplicas uint64, upstream http.HandlerFunc) error {
+	scaleReq := ScaleServiceRequest{
+		Replicas:    minReplicas,
+		ServiceName: functionName,
+	}
+
+	scaleBytesOut, _ := json.Marshal(scaleReq)
+	scaleBytesOutBody := bytes.NewBuffer(scaleBytesOut)
+	setReplicasReq, _ := http.NewRequest(http.MethodPost, fmt.Sprintf("/system/scale-function/%s", functionName), scaleBytesOutBody)
+
+	rr := httptest.NewRecorder()
+	upstream.ServeHTTP(rr, setReplicasReq)
+
+	if rr.Code != 200 {
+		return fmt.Errorf("scale to 1 replica status: %d", rr.Code)
+	}
+
+	return nil
+}
+
+// ScaleServiceRequest request to scale a function
+type ScaleServiceRequest struct {
+	ServiceName string `json:"serviceName"`
+	Replicas    uint64 `json:"replicas"`
+}