Collect basic request metrics and expose using GoLang's expvar

README.md

@@ -24,6 +24,7 @@ Tiny Go webserver that prints os information and HTTP request to output
 - `key`: give me a key.
 - `port`: give me a port number. (default: 80)
 - `name`: give me a name. (it can be also defined with `WHOAMI_NAME` environment variable)
+- `metrics`: enable collecting metrics (default: disabled). When enabled, request metrics are available using the `/debug/vars` request path.  
 
 ## Examples
 

app.go

@@ -3,6 +3,7 @@ package main
 import (
 	"bytes"
 	"encoding/json"
+	"expvar"
 	"flag"
 	"fmt"
 	"io"
@@ -29,17 +30,23 @@ const (
 )
 
 var (
-	cert string
-	key  string
-	port string
-	name string
+	cert           string
+	key            string
+	port           string
+	name           string
+	metricsEnabled bool
+
+	totalRequestCount         expvarInt
+	concurrentRequestCount    expvarInt
+	maxConcurrentRequestCount expvarMaxInt
 )
 
 func init() {
 	flag.StringVar(&cert, "cert", "", "give me a certificate")
 	flag.StringVar(&key, "key", "", "give me a key")
 	flag.StringVar(&port, "port", "80", "give me a port number")
 	flag.StringVar(&name, "name", os.Getenv("WHOAMI_NAME"), "give me a name")
+	flag.BoolVar(&metricsEnabled, "metrics", false, "enable collecting metrics")
 }
 
 var upgrader = websocket.Upgrader{
@@ -49,13 +56,14 @@ var upgrader = websocket.Upgrader{
 
 func main() {
 	flag.Parse()
+	publishExpvarMetrics()
 
-	http.HandleFunc("/data", dataHandler)
-	http.HandleFunc("/echo", echoHandler)
-	http.HandleFunc("/bench", benchHandler)
-	http.HandleFunc("/", whoamiHandler)
-	http.HandleFunc("/api", apiHandler)
-	http.HandleFunc("/health", healthHandler)
+	http.HandleFunc("/data", wrapHandlerIfNeeded(dataHandler))
+	http.HandleFunc("/echo", wrapHandlerIfNeeded(echoHandler))
+	http.HandleFunc("/bench", wrapHandlerIfNeeded(benchHandler))
+	http.HandleFunc("/", wrapHandlerIfNeeded(whoamiHandler))
+	http.HandleFunc("/api", wrapHandlerIfNeeded(apiHandler))
+	http.HandleFunc("/health", wrapHandlerIfNeeded(healthHandler))
 
 	fmt.Println("Starting up on port " + port)
 
@@ -274,3 +282,34 @@ func fillContent(length int64) io.ReadSeeker {
 
 	return bytes.NewReader(b)
 }
+
+func publishExpvarMetrics() {
+	if metricsEnabled {
+		metrics := expvar.Map{}
+		metrics.Set("totalRequestCount", &totalRequestCount)
+		metrics.Set("concurrentRequestCount", &concurrentRequestCount)
+		metrics.Set("maxConcurrentRequestCount", &maxConcurrentRequestCount)
+		expvar.Publish("metrics", &metrics)
+	}
+}
+
+func wrapHandlerIfNeeded(handler http.HandlerFunc) http.HandlerFunc {
+	if !metricsEnabled {
+		return handler
+	}
+	return func(writer http.ResponseWriter, request *http.Request) {
+		onHandleRequestStart()
+		handler(writer, request)
+		onHandleRequestFinish()
+	}
+}
+
+func onHandleRequestStart() {
+	count := concurrentRequestCount.Add(1)
+	totalRequestCount.Add(1)
+	maxConcurrentRequestCount.Update(count)
+}
+
+func onHandleRequestFinish() {
+	concurrentRequestCount.Add(-1)
+}

expvar.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"strconv"
+	"sync/atomic"
+)
+
+// ----- Int -----
+
+type expvarInt struct {
+	i int64
+}
+
+func (v *expvarInt) Value() int64 {
+	return atomic.LoadInt64(&v.i)
+}
+
+func (v *expvarInt) String() string {
+	return strconv.FormatInt(atomic.LoadInt64(&v.i), 10)
+}
+
+func (v *expvarInt) Add(i int64) int64 {
+	return atomic.AddInt64(&v.i, i)
+}
+
+// ----- MaxInt -----
+
+type expvarMaxInt struct {
+	i int64
+}
+
+func (v *expvarMaxInt) Value() int64 {
+	return atomic.LoadInt64(&v.i)
+}
+
+func (v *expvarMaxInt) String() string {
+	return strconv.FormatInt(atomic.LoadInt64(&v.i), 10)
+}
+
+func (v *expvarMaxInt) Update(newMax int64) {
+	max := v.Value()
+	if max < newMax {
+		if !atomic.CompareAndSwapInt64(&v.i, max, newMax) {
+			v.Update(newMax)
+		}
+	}
+}