Merge pull request #96 from s-kostyuk/graceful_degradation

Graceful degradation for local_announce module

This pull request changes the logic of initialization for local_announce module of the system and local announcement API itself.

From now the local_announce module will not be imported until it'll become explicitly needed to satisfy user requirements (i.e. until it will be explicitly enabled in everpl configuration file). If the module will be enabled by user but will not be able to be imported, the user will receive an error message and the whole platform will crash with exception.

Closes #94

dpl/core/controller.py

@@ -40,7 +40,6 @@
 from dpl.api.rest_api.messages_subapp import build_messages_subapp
 from dpl.api.rest_api.rest_api_provider import RestApiProvider
 
-from dpl.api.local_announce import LocalAnnounce
 
 module_logger = logging.getLogger(__name__)
 dpl_root_logger = logging.getLogger(name='dpl')
@@ -181,7 +180,31 @@ def __init__(self):
             auth_service=self._auth_service
         )
 
-        self._local_announce = LocalAnnounce()
+        # None will indicate that this module was disabled
+        self._local_announce = None
+
+        # if module ws enabled...
+        if 'local_announce' in self._apis_config['enabled_apis']:
+            self._initialize_local_announcement()
+
+    def _initialize_local_announcement(self):
+        """
+        Performs an import of local_announce module an initializes the
+        self._local_announce field with LocalAnnounce constructor
+
+        :return: None
+        """
+        try:  # ...try to import a module and initialize its instance
+            from dpl.api.local_announce import LocalAnnounce
+            self._local_announce = LocalAnnounce()
+        except ImportError as e:  # if something gone wrong...
+            # ... print an error ...
+            logging.error(
+                "Failed to enable local_announce module: %s. Install all "
+                "missing dependencies or disable local_announce module in "
+                "everpl config file" % e
+            )
+            raise  # ...and terminate
 
     def parse_arguments(self):
         """
@@ -350,6 +373,8 @@ async def _bootstrap_integrations(self):
         self._db_session_manager.remove_session()
 
     async def shutdown(self):
+        if self._local_announce is not None:
+            self._local_announce.shutdown_server()
+
         await self._rest_api.shutdown_server()
-        self._local_announce.shutdown_server()
         self._thing_service_raw.disable_all()