If we are below a failure threshold and a probe fails, use the destin…

…ation's previous active health state as the next health state.

This has the effect of a destination with a health state of Unknown in the unknown state until the threshold is reached, at which time the destination transitions into Unhealthy.

src/ReverseProxy/Health/ConsecutiveFailuresHealthPolicy.cs

@@ -40,9 +40,10 @@ public void ProbingCompleted(ClusterState cluster, IReadOnlyList<DestinationProb
         for (var i = 0; i < probingResults.Count; i++)
         {
             var destination = probingResults[i].Destination;
+            var previousState = destination.Health.Active;
 
             var count = _failureCounters.GetOrCreateValue(destination);
-            var newHealth = EvaluateHealthState(threshold, probingResults[i].Response, count);
+            var newHealth = EvaluateHealthState(threshold, probingResults[i].Response, count, previousState);
             newHealthStates[i] = new NewActiveDestinationHealth(destination, newHealth);
         }
 
@@ -55,7 +56,7 @@ private double GetFailureThreshold(ClusterState cluster)
         return thresholdEntry.GetParsedOrDefault(_options.DefaultThreshold);
     }
 
-    private static DestinationHealth EvaluateHealthState(double threshold, HttpResponseMessage? response, AtomicCounter count)
+    private static DestinationHealth EvaluateHealthState(double threshold, HttpResponseMessage? response, AtomicCounter count, DestinationHealth previousState)
     {
         DestinationHealth newHealth;
         if (response is not null && response.IsSuccessStatusCode)
@@ -68,7 +69,7 @@ private static DestinationHealth EvaluateHealthState(double threshold, HttpRespo
         {
             // Failure
             var currentFailureCount = count.Increment();
-            newHealth = currentFailureCount < threshold ? DestinationHealth.Healthy : DestinationHealth.Unhealthy;
+            newHealth = currentFailureCount < threshold ? previousState : DestinationHealth.Unhealthy;
         }
 
         return newHealth;

test/ReverseProxy.Tests/Health/ConsecutiveFailuresHealthPolicyTests.cs

@@ -21,7 +21,7 @@ public void ProbingCompleted_FailureThresholdExceeded_MarkDestinationUnhealthy()
         var options = Options.Create(new ConsecutiveFailuresHealthPolicyOptions { DefaultThreshold = 2 });
         var policy = new ConsecutiveFailuresHealthPolicy(options, new DestinationHealthUpdaterStub());
         var cluster0 = GetClusterInfo("cluster0", destinationCount: 2);
-        var cluster1 = GetClusterInfo("cluster0", destinationCount: 2, failureThreshold: 3);
+        var cluster1 = GetClusterInfo("cluster1", destinationCount: 2, failureThreshold: 3);
 
         var probingResults0 = new[] {
             new DestinationProbingResult(cluster0.Destinations.Values.First(), new HttpResponseMessage(HttpStatusCode.InternalServerError), null),
@@ -40,16 +40,19 @@ public void ProbingCompleted_FailureThresholdExceeded_MarkDestinationUnhealthy()
 
         // First probing attempt
         policy.ProbingCompleted(cluster0, probingResults0);
-        Assert.All(cluster0.Destinations.Values, d => Assert.Equal(DestinationHealth.Healthy, d.Health.Active));
+        Assert.Equal(DestinationHealth.Unknown, cluster0.Destinations.Values.First().Health.Active);
+        Assert.Equal(DestinationHealth.Healthy, cluster0.Destinations.Values.Skip(1).First().Health.Active);
         policy.ProbingCompleted(cluster1, probingResults1);
-        Assert.All(cluster1.Destinations.Values, d => Assert.Equal(DestinationHealth.Healthy, d.Health.Active));
+        Assert.Equal(DestinationHealth.Healthy, cluster1.Destinations.Values.First().Health.Active);
+        Assert.Equal(DestinationHealth.Unknown, cluster1.Destinations.Values.Skip(1).First().Health.Active);
 
         // Second probing attempt
         policy.ProbingCompleted(cluster0, probingResults0);
         Assert.Equal(DestinationHealth.Unhealthy, cluster0.Destinations.Values.First().Health.Active);
         Assert.Equal(DestinationHealth.Healthy, cluster0.Destinations.Values.Skip(1).First().Health.Active);
         policy.ProbingCompleted(cluster1, probingResults1);
-        Assert.All(cluster1.Destinations.Values, d => Assert.Equal(DestinationHealth.Healthy, d.Health.Active));
+        Assert.Equal(DestinationHealth.Healthy, cluster1.Destinations.Values.First().Health.Active);
+        Assert.Equal(DestinationHealth.Unknown, cluster1.Destinations.Values.Skip(1).First().Health.Active);
 
         // Third probing attempt
         policy.ProbingCompleted(cluster0, probingResults0);