Limit the frequency of db sync

If exception occurs when processing a northbound event, one second delay
will be introduced by db sync, it leads to more exceptions because of
the delay. Avalanche will occur then. This patch limits the frequency of
db sync by one time every three minutes at maximum.

Closes-Bug: #1583992

Change-Id: I2f2635467b8fc5ab816ea4ac355e3e1cc488d5a2

dragonflow/db/api_nb.py

@@ -26,7 +26,8 @@ from oslo_serialization import jsonutils
 
 from dragonflow._i18n import _LI, _LW, _LE
 from dragonflow.common import utils as df_utils
-from dragonflow.db.db_common import DbUpdate, SEND_ALL_TOPIC
+from dragonflow.db.db_common import DbUpdate, SEND_ALL_TOPIC, \
+                                     DB_SYNC_MINIMUM_INTERVAL
 from dragonflow.db import pub_sub_api
 
 eventlet.monkey_patch()
@@ -157,6 +158,8 @@ class NbApi(object):
         eventlet.sleep(0)
 
     def _read_db_changes_from_queue(self):
+        sync_rate_limiter = df_utils.RateLimiter(
+            max_rate=1, time_unit=DB_SYNC_MINIMUM_INTERVAL)
         while True:
             self.next_update = self._queue.get(block=True)
             LOG.debug("Event update: %s", self.next_update)
@@ -176,7 +179,8 @@ class NbApi(object):
             except Exception as e:
                 if "ofport is 0" not in e.message:
                     LOG.exception(e)
-                self.apply_db_change(None, None, 'sync', None)
+                if not sync_rate_limiter():
+                    self.apply_db_change(None, None, 'sync', None)
 
     def apply_db_change(self, table, key, action, value):
         # determine if the action is allowed or not

dragonflow/db/db_common.py

@@ -15,6 +15,7 @@
 from oslo_utils import timeutils
 
 SEND_ALL_TOPIC = b'D'
+DB_SYNC_MINIMUM_INTERVAL = 180
 
 
 class DbUpdate(object):