Add API endpoints to ConfigDBConnector to support pre-loading data without blackout

common/configdb.h

@@ -71,10 +71,16 @@ class ConfigDBConnector_Native : public SonicV2Connector_Native
             return self.getDbName()
 
         ## Note: callback is difficult to implement by SWIG C++, so keep in python
-        def listen(self):
-            ## Start listen Redis keyspace events and will trigger corresponding handlers when content of a table changes.
+        def listen(self, start=True):
+            ## Start listen Redis keyspace event. Use start=False if you need to load in initial data to prevent blackout.
+            ## Then call process(cache) where cache is a dict of {table_name: data} of the initialized data to prevent
+            ## duplicate calls to the callback if the data has not changed. 
             self.pubsub = self.get_redis_client(self.db_name).pubsub()
             self.pubsub.psubscribe("__keyspace@{}__:*".format(self.get_dbid(self.db_name)))
+
+            if start: self.process()
+
+        def process(self, cache={}):
             while True:
                 item = self.pubsub.listen_message()
                 if item['type'] == 'pmessage':
@@ -84,6 +90,11 @@ class ConfigDBConnector_Native : public SonicV2Connector_Native
                         if table in self.handlers:
                             client = self.get_redis_client(self.db_name)
                             data = self.raw_to_typed(client.hgetall(key))
+                            if table in cache and row in cache[table]:
+                                if cache[table][row] == data:
+                                    continue
+                                else:
+                                    del cache[table][row]
                             self.__fire(table, row, data)
                     except ValueError:
                         pass    #Ignore non table-formated redis entries