Use EDF schedule algorithm for WeightedRoundRobin

pkg/server/service/loadbalancer/wrr/wrr_test.go

@@ -13,11 +13,13 @@ func Int(v int) *int { return &v }
 
 type responseRecorder struct {
 	*httptest.ResponseRecorder
-	save map[string]int
+	save     map[string]int
+	sequence []string
 }
 
 func (r *responseRecorder) WriteHeader(statusCode int) {
 	r.save[r.Header().Get("server")]++
+	r.sequence = append(r.sequence, r.Header().Get("server"))
 	r.ResponseRecorder.WriteHeader(statusCode)
 }
 
@@ -112,3 +114,29 @@ func TestSticky(t *testing.T) {
 	assert.Equal(t, 0, recorder.save["first"])
 	assert.Equal(t, 3, recorder.save["second"])
 }
+
+// TestBalancerBias makes sure that the WRR algorithm spreads elements evenly right from the start,
+// and that it does not "over-favor" the high-weighted ones with a biased start-up regime.
+func TestBalancerBias(t *testing.T) {
+	balancer := New(nil)
+
+	balancer.AddService("first", http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		rw.Header().Set("server", "A")
+		rw.WriteHeader(http.StatusOK)
+	}), Int(11))
+
+	balancer.AddService("second", http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+		rw.Header().Set("server", "B")
+		rw.WriteHeader(http.StatusOK)
+	}), Int(3))
+
+	recorder := &responseRecorder{ResponseRecorder: httptest.NewRecorder(), save: map[string]int{}}
+
+	for i := 0; i < 14; i++ {
+		balancer.ServeHTTP(recorder, httptest.NewRequest(http.MethodGet, "/", nil))
+	}
+
+	wantSequence := []string{"A", "A", "A", "B", "A", "A", "A", "A", "B", "A", "A", "A", "B", "A"}
+
+	assert.Equal(t, wantSequence, recorder.sequence)
+}