Cache the automatic version pin to avoid repeated lookups

The compute rpcapi module was looking up the desired automatic
version pin each time it was initialized. Turns out, we create and
destroy this object too often, which means we do a lot of those
lookups unnecessarily. That's a larger problem that will be harder
to solve, but we can be smarter about the DB impact of that.

This adds a cache of the last version we discovered and avoids doing
the lookup when possible. It makes the reset() method invalidate the
cache prior to reinitializing the object so that we will do the lookup
on SIGHUP as desired.

Closes-Bug: #1524444
Change-Id: Ia1c58efa7105d32973f6921c2144b17c30b764c2

nova/compute/manager.py

@@ -720,6 +720,7 @@ class ComputeManager(manager.Manager):
 
     def reset(self):
         LOG.info(_LI('Reloading compute RPC API'))
+        compute_rpcapi.LAST_VERSION = None
         self.compute_rpcapi = compute_rpcapi.ComputeAPI()
 
     def _get_resource_tracker(self, nodename):

nova/compute/rpcapi.py

@@ -47,6 +47,7 @@ rpcapi_cap_opt = cfg.StrOpt('compute',
 CONF.register_opt(rpcapi_cap_opt, 'upgrade_levels')
 
 LOG = logging.getLogger(__name__)
+LAST_VERSION = None
 
 
 def _compute_host(host, instance):
@@ -333,6 +334,9 @@ class ComputeAPI(object):
         self.client = self.get_client(target, version_cap, serializer)
 
     def _determine_version_cap(self, target):
+        global LAST_VERSION
+        if LAST_VERSION:
+            return LAST_VERSION
         service_version = objects.Service.get_minimum_version(
             context.get_admin_context(), 'nova-compute')
         history = service_obj.SERVICE_VERSION_HISTORY
@@ -350,6 +354,7 @@ class ComputeAPI(object):
                           'service history for version %(version)i'),
                       {'version': service_version})
             return target.version
+        LAST_VERSION = version_cap
         LOG.info(_LI('Automatically selected compute RPC version %(rpc)s '
                      'from minimum service version %(service)i'),
                  {'rpc': version_cap,

nova/tests/unit/compute/test_rpcapi.py

@@ -55,6 +55,7 @@ class ComputeRpcAPITestCase(test.NoDBTestCase):
     def test_auto_pin(self, mock_get_min):
         mock_get_min.return_value = 1
         self.flags(compute='auto', group='upgrade_levels')
+        compute_rpcapi.LAST_VERSION = None
         rpcapi = compute_rpcapi.ComputeAPI()
         self.assertEqual('4.4', rpcapi.client.version_cap)
         mock_get_min.assert_called_once_with(mock.ANY, 'nova-compute')
@@ -63,6 +64,7 @@ class ComputeRpcAPITestCase(test.NoDBTestCase):
     def test_auto_pin_fails_if_too_old(self, mock_get_min):
         mock_get_min.return_value = 1955
         self.flags(compute='auto', group='upgrade_levels')
+        compute_rpcapi.LAST_VERSION = None
         self.assertRaises(exception.ServiceTooOld,
                           compute_rpcapi.ComputeAPI)
 
@@ -70,10 +72,21 @@ class ComputeRpcAPITestCase(test.NoDBTestCase):
     def test_auto_pin_kilo(self, mock_get_min):
         mock_get_min.return_value = 0
         self.flags(compute='auto', group='upgrade_levels')
+        compute_rpcapi.LAST_VERSION = None
         rpcapi = compute_rpcapi.ComputeAPI()
         self.assertEqual('4.0', rpcapi.client.version_cap)
         mock_get_min.assert_called_once_with(mock.ANY, 'nova-compute')
 
+    @mock.patch('nova.objects.Service.get_minimum_version')
+    def test_auto_pin_caches(self, mock_get_min):
+        mock_get_min.return_value = 1
+        self.flags(compute='auto', group='upgrade_levels')
+        compute_rpcapi.LAST_VERSION = None
+        compute_rpcapi.ComputeAPI()
+        compute_rpcapi.ComputeAPI()
+        mock_get_min.assert_called_once_with(mock.ANY, 'nova-compute')
+        self.assertEqual('4.4', compute_rpcapi.LAST_VERSION)
+
     def _test_compute_api(self, method, rpc_method,
                           expected_args=None, **kwargs):
         ctxt = context.RequestContext('fake_user', 'fake_project')