Include target in request logging

internal/server/logging_middleware.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"bufio"
+	"context"
 	"encoding/json"
 	"errors"
 	"io"
@@ -12,6 +13,10 @@ import (
 	"time"
 )
 
+type contextKey string
+
+var contextKeyTarget = contextKey("target")
+
 type LoggingMiddlewareLine struct {
 	Timestamp string `json:"@timestamp"`
 	Message   string `json:"message"`
@@ -25,6 +30,9 @@ type LoggingMiddlewareLine struct {
 	Event struct {
 		Duration int64 `json:"duration"`
 	} `json:"event"`
+	Destination struct {
+		Address string `json:"address"`
+	} `json:"destination"`
 	HTTP struct {
 		Request struct {
 			Method   string `json:"method"`
@@ -67,6 +75,10 @@ func NewLoggingMiddleware(w io.Writer, next http.Handler) *LoggingMiddleware {
 func (h *LoggingMiddleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	writer := newResponseWriter(w)
 
+	var target string
+	ctx := context.WithValue(r.Context(), contextKeyTarget, &target)
+	r = r.WithContext(ctx)
+
 	started := time.Now()
 	h.next.ServeHTTP(writer, r)
 	elapsed := time.Since(started)
@@ -90,6 +102,7 @@ func (h *LoggingMiddleware) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	line.Log.Level = "INFO"
 	line.Client.IP = clientIP
 	line.Client.Port = clientPort
+	line.Destination.Address = target
 	line.Event.Duration = elapsed.Nanoseconds()
 	line.HTTP.Request.Body.Bytes = r.ContentLength
 	line.HTTP.Request.Method = r.Method

internal/server/logging_middleware_test.go

@@ -16,6 +16,12 @@ import (
 func TestMiddleware_LoggingMiddleware(t *testing.T) {
 	out := &strings.Builder{}
 	middleware := NewLoggingMiddleware(out, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Record a value for the `target` context key.
+		target, ok := r.Context().Value(contextKeyTarget).(*string)
+		if ok {
+			*target = "upstream:3000"
+		}
+
 		w.Header().Set("Content-Type", "text/html")
 		w.WriteHeader(http.StatusCreated)
 		fmt.Fprintln(w, "goodbye")
@@ -35,6 +41,7 @@ func TestMiddleware_LoggingMiddleware(t *testing.T) {
 
 	assert.Equal(t, "Request", logline.Message)
 	assert.Equal(t, "INFO", logline.Log.Level)
+	assert.Equal(t, "upstream:3000", logline.Destination.Address)
 
 	assert.Equal(t, "http", logline.URL.Scheme)
 	assert.Equal(t, "example.com", logline.URL.Domain)

internal/server/router.go

@@ -66,6 +66,12 @@ func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	if target == nil {
 		w.WriteHeader(http.StatusServiceUnavailable)
 	} else {
+		// Record the target that served the request, if its context is available.
+		targetIdentifer, ok := req.Context().Value(contextKeyTarget).(*string)
+		if ok {
+			*targetIdentifer = target.Target()
+		}
+
 		target.ServeHTTP(w, req)
 	}
 }