server: build new zone and replace live copy in sync

Instead of clearing the live zone and iterating through and adding
records from OpenNebula we build an entirely new zone, sync it with
OpenNebula, and instantaneously swap it with the currently live zone.
This minimizes the amount of time that the DNS server is missing entries
while its syncing with OpenNebula.

onedns/resolver.py

@@ -25,6 +25,7 @@ class DynamicResolver(server.BaseResolver):
         Initialise resolver from zone list
         Stores RRs as a list of (label, type, rr) tuples
         """
+        self.domain = domain
         self.zone = zone.Zone(domain)
         self._tcp_server = None
         self._udp_server = None

onedns/server.py

@@ -1,3 +1,4 @@
+from onedns import zone
 from onedns import resolver
 from onedns import exception
 from onedns.clients import one
@@ -19,6 +20,7 @@ class OneDNS(resolver.DynamicResolver):
             raise exception.NoNetworksError(vm)
 
     def _get_vm_dns_entries(self, vm):
+        self._check_for_networks(vm)
         entries = {}
         hostname = vm.name
         primary_ip = vm.template.nics[0].ip
@@ -29,19 +31,17 @@ class OneDNS(resolver.DynamicResolver):
             entries[nicname] = nic.ip
         return entries
 
-    def add_vm(self, vm):
-        self._check_for_networks(vm)
+    def add_vm(self, vm, zone=None):
         dns_entries = self._get_vm_dns_entries(vm)
         log.info("Adding VM {id}: {vm}".format(id=vm.id, vm=vm.name))
         for name, ip in dns_entries.items():
-            self.add_host(name, ip)
+            self.add_host(name, ip, zone=zone)
 
-    def remove_vm(self, vm):
-        self._check_for_networks(vm)
+    def remove_vm(self, vm, zone=None):
         dns_entries = self._get_vm_dns_entries(vm)
         log.info("Removing VM {id}: {vm}".format(id=vm.id, vm=vm.name))
         for name, ip in dns_entries.items():
-            self.remove_host(name, ip)
+            self.remove_host(name, ip, zone=zone)
 
     def add_vm_by_id(self, vm_id):
         vm = self._one.get_vm_by_id(vm_id)
@@ -52,10 +52,11 @@ class OneDNS(resolver.DynamicResolver):
         return self.remove_vm(vm)
 
     def sync(self, vms=None):
+        z = zone.Zone(self.domain)
         vms = vms or self._one.vms()
-        self.clear()
         for vm in vms:
             try:
-                self.add_vm(vm)
+                self.add_vm(vm, zone=z)
             except exception.NoNetworksError as e:
                 e.log(warn=True)
+        self.load(z)