Ensure zone stats and metrics do not contain stale data for zones that no longer have servers.

ribbon-core/src/main/java/com/netflix/loadbalancer/ZoneAwareLoadBalancer.java

@@ -17,6 +17,7 @@
 */
 package com.netflix.loadbalancer;
 
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -30,6 +31,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import com.google.common.annotations.VisibleForTesting;
+import com.google.common.collect.Sets;
 import com.netflix.client.ClientFactory;
 import com.netflix.config.DynamicBooleanProperty;
 import com.netflix.config.DynamicDoubleProperty;
@@ -84,6 +86,14 @@ public class ZoneAwareLoadBalancer<T extends Server> extends DynamicServerListLo
         	String zone = entry.getKey().toLowerCase();
             getLoadBalancer(zone).setServersList(entry.getValue());
         }
+        // check if there is any zone that no longer has a server
+        // and set the list to empty so that the zone related metrics does not
+        // contain stale data
+        for (Map.Entry<String, BaseLoadBalancer> existingLBEntry: balancers.entrySet()) {
+            if (!zoneServersMap.keySet().contains(existingLBEntry.getKey())) {
+                existingLBEntry.getValue().setServersList(Collections.emptyList());
+            }
+        }
     }    
     
     private Map<String, ZoneSnapshot> createSnapshot() {

ribbon-core/src/test/java/com/netflix/loadbalancer/ZoneAwareLoadBalancerTest.java

@@ -275,6 +275,15 @@ public class ZoneAwareLoadBalancerTest {
             result.add(server);
         }
         assertEquals(expected, result);
+        servers = new ArrayList<Server>();        
+        servers.add(createServer(1, "a"));
+        servers.add(createServer(2, "a"));
+        servers.add(createServer(1, "b"));
+        servers.add(createServer(2, "b"));
+        balancer.setServersList(servers);
+        assertTrue(balancer.getLoadBalancer("us-east-1c").getServerList(false).isEmpty());
+        AvailabilityFilteringRule rule = (AvailabilityFilteringRule) balancer.getLoadBalancer("us-east-1c").getRule();
+        assertEquals(0, rule.getAvailableServersCount());
     }
     
     @Test