Make min-drop-probability of PHC optional

The default value (0.0) makes some sense and is equivalent to how
PHC was working before.

Signed-off-by: Roman Zavodskikh <roman.zavodskikh@zalando.de>

docs/operation/operation.md

@@ -907,13 +907,17 @@ in previous period are, the stronger reduction is) amount of requests compared t
 
 Having this, we expect less requests to fail because a lot of them would be sent to endpoints that seem to be healthy instead.
 
-To enable this feature, you need to provide `-passive-health-check` option having all forementioned parameters
-(`period`, `min-requests`, `min-drop-probability`, `max-drop-probability`) defined,
-for instance: `-passive-health-check=period=1s,min-requests=10,min-drop-probability=0.05,max-drop-probability=0.9`.
+To enable this feature, you need to provide `-passive-health-check` option having forementioned parameters
+(`period`, `min-requests`, `min-drop-probability`, `max-drop-probability`) defined.
+`period`, `min-requests`, `max-drop-probability` are required parameters, it is not possible for PHC to be enabled without
+them explicitly defined by user. `min-drop-probability` is implicitly defined as `0.0` if not explicitly set by user.
+Valid examples of `-passive-health-check` are:
 
-You need to define all parameters on your side, there are no defaults, and skipper will not run if PHC params are passed only partially.
++ `-passive-health-check=period=1s,min-requests=10,min-drop-probability=0.05,max-drop-probability=0.9`
++ `-passive-health-check=period=1s,min-requests=10,max-drop-probability=0.9`
 
-However, Skipper will run without this feature, if no `-passive-health-check` is provided at all.
+If `-passive-health-check` option is provided, but some required parameters are not defined, Skipper will not start.
+Skipper will run without this feature, if no `-passive-health-check` is provided at all.
 
 The parameters of `-passive-health-check` option are:
 

proxy/proxy.go

@@ -169,10 +169,16 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 	}
 
 	result := &PassiveHealthCheck{}
-	keysInitialized := make(map[string]struct{})
+	requiredParams := map[string]struct{}{
+		"period":               {},
+		"max-drop-probability": {},
+		"min-requests":         {},
+	}
 
 	for key, value := range o {
+		delete(requiredParams, key)
 		switch key {
+		/* required parameters */
 		case "period":
 			period, err := time.ParseDuration(value)
 			if err != nil {
@@ -191,15 +197,6 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 				return false, nil, fmt.Errorf("passive health check: invalid minRequests value: %s", value)
 			}
 			result.MinRequests = int64(minRequests)
-		case "min-drop-probability":
-			minDropProbability, err := strconv.ParseFloat(value, 64)
-			if err != nil {
-				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
-			}
-			if minDropProbability < 0 || minDropProbability > 1 {
-				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
-			}
-			result.MinDropProbability = minDropProbability
 		case "max-drop-probability":
 			maxDropProbability, err := strconv.ParseFloat(value, 64)
 			if err != nil {
@@ -209,14 +206,23 @@ func InitPassiveHealthChecker(o map[string]string) (bool, *PassiveHealthCheck, e
 				return false, nil, fmt.Errorf("passive health check: invalid maxDropProbability value: %s", value)
 			}
 			result.MaxDropProbability = maxDropProbability
+
+		/* optional parameters */
+		case "min-drop-probability":
+			minDropProbability, err := strconv.ParseFloat(value, 64)
+			if err != nil {
+				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
+			}
+			if minDropProbability < 0 || minDropProbability > 1 {
+				return false, nil, fmt.Errorf("passive health check: invalid minDropProbability value: %s", value)
+			}
+			result.MinDropProbability = minDropProbability
 		default:
 			return false, nil, fmt.Errorf("passive health check: invalid parameter: key=%s,value=%s", key, value)
 		}
-
-		keysInitialized[key] = struct{}{}
 	}
 
-	if len(keysInitialized) != 4 {
+	if len(requiredParams) != 0 {
 		return false, nil, fmt.Errorf("passive health check: missing required parameters")
 	}
 	if result.MinDropProbability >= result.MaxDropProbability {

proxy/proxy_test.go

@@ -2454,6 +2454,22 @@ func TestInitPassiveHealthChecker(t *testing.T) {
 			expectedParams:  nil,
 			expectedError:   fmt.Errorf("passive health check: missing required parameters"),
 		},
+		{
+			inputArg: map[string]string{
+				"period":               "1m",
+				"min-requests":         "10",
+				"max-drop-probability": "0.9",
+				/* using default max-drop-probability */
+			},
+			expectedEnabled: true,
+			expectedParams: &PassiveHealthCheck{
+				Period:             1 * time.Minute,
+				MinRequests:        10,
+				MaxDropProbability: 0.9,
+				MinDropProbability: 0.0,
+			},
+			expectedError: nil,
+		},
 	} {
 		t.Run(fmt.Sprintf("case_%d", i), func(t *testing.T) {
 			enabled, params, err := InitPassiveHealthChecker(ti.inputArg)