Perform check before updating host in DistributedHostStore

By checking to see if the host needs to be updated, we can
avoid invalidating the local cache in many cases.

This also helps to alleviate slow pingall times with fwd
enabled.

Change-Id: I7d58a488cd91878e6028ddfaae407e849fc10211

@@ -113,6 +113,42 @@ public class DistributedHostStore
         log.info("Stopped");
     }
 
+    private boolean shouldUpdate(DefaultHost existingHost,
+                                 ProviderId providerId,
+                                 HostId hostId,
+                                 HostDescription hostDescription,
+                                 boolean replaceIPs) {
+        if (existingHost == null) {
+            return true;
+        }
+
+        if (!Objects.equals(existingHost.providerId(), providerId) ||
+                !Objects.equals(existingHost.mac(), hostDescription.hwAddress()) ||
+                !Objects.equals(existingHost.vlan(), hostDescription.vlan()) ||
+                !Objects.equals(existingHost.location(), hostDescription.location())) {
+            return true;
+        }
+
+        if (replaceIPs) {
+            if (!Objects.equals(hostDescription.ipAddress(),
+                                existingHost.ipAddresses())) {
+                return true;
+            }
+        } else {
+            if (!existingHost.ipAddresses().containsAll(hostDescription.ipAddress())) {
+                return true;
+            }
+        }
+
+        // check to see if any of the annotations provided by hostDescription
+        // differ from those in the existing host
+        return hostDescription.annotations().keys().stream()
+                    .anyMatch(k -> !Objects.equals(hostDescription.annotations().value(k),
+                                                   existingHost.annotations().value(k)));
+
+
+    }
+
     // TODO No longer need to return HostEvent
     @Override
     public HostEvent createOrUpdateHost(ProviderId providerId,
@@ -120,9 +156,10 @@ public class DistributedHostStore
                                         HostDescription hostDescription,
                                         boolean replaceIPs) {
         // TODO: We need a way to detect conflicting changes and abort update.
-        //       (BOC) Compute might do this for us.
-
-        hosts.compute(hostId, (id, existingHost) -> {
+        host.computeIf(hostId,
+                       existingHost -> shouldUpdate(existingHost, providerId, hostId,
+                                                    hostDescription, replaceIPs),
+                       (id, existingHost) -> {
             HostLocation location = hostDescription.location();
 
             final Set<IpAddress> addresses;