Add new config option use_aggregated_uve_db

Provide an option to enable/disable serve UVE queries from the aggregated
UVE db. In a scale setup, HA event (alarm-gen down), alarm-gen may take a
long time to aggregate the UVEs depending on the number of UVEs, number
of partitions it owns, etc., So, enabling this option would reduce the
down time in case of HA events (collector down - time taken for generators
 to connect to the new collector and resync the UVEs, alarm-gen down - no
impact on UVE queries, only the alarms would be reevaluated)

Change-Id: I600e2c33671a1c16545732cd64d1493678095198
Closes-Bug: #1719236

src/opserver/opserver.py

@@ -488,6 +488,7 @@ def _reject_auth_headers(self):
     def __init__(self, args_str=' '.join(sys.argv[1:])):
         self.gevs = []
         self._args = None
+        self._usecache = True
         self._parse_args(args_str)
         print args_str
 
@@ -907,6 +908,7 @@ def _parse_args(self, args_str=' '.join(sys.argv[1:])):
             'admin_port'        : OpServerAdminPort,
             'cloud_admin_role'  : CLOUD_ADMIN_ROLE,
             'api_server_use_ssl': False,
+            'use_aggregated_uve_db' : 'true',
         }
         defaults.update(SandeshConfig.get_default_options(['DEFAULTS']))
         redis_opts = {
@@ -1055,6 +1057,8 @@ def _parse_args(self, args_str=' '.join(sys.argv[1:])):
             help="Port with local auth for admin access")
         parser.add_argument("--api_server_use_ssl",
             help="Use SSL to connect with API server")
+        parser.add_argument("--use_aggregated_uve_db",
+            help="enable/disbale read uve from aggregated uve db")
         SandeshConfig.add_parser_arguments(parser)
         self._args = parser.parse_args(remaining_argv)
         if type(self._args.collectors) is str:
@@ -1085,6 +1089,9 @@ def _parse_args(self, args_str=' '.join(sys.argv[1:])):
         auth_conf_info['admin_port'] = self._args.admin_port
         auth_conf_info['api_servers'] = self._args.api_server
         self._args.auth_conf_info = auth_conf_info
+        if self._args.use_aggregated_uve_db.lower() == 'false' or\
+           self._args.use_aggregated_uve_db == '0':
+            self._usecache = False
         self._args.conf_file = args.conf_file
         self._args.sandesh_config = \
             SandeshConfig.from_parser_arguments(self._args)
@@ -2566,10 +2573,11 @@ def get_agp(self):
         return self.agp
 
     def run(self):
-        self._uvedbstream.start()
+        if self._usecache:
+            self._uvedbstream.start()
+            self.gevs.append(self._uvedbstream)
 
         self.gevs += [
-            self._uvedbstream,
             gevent.spawn(self.start_webserver),
             gevent.spawn(self.start_uve_server),
             ]