[DPE-3085] - Provide mongos with update config db (#315)

## Issue
When replicas are added to the config DB the related mongos charm should
get an updated connection string

## Future PRs
Adopt this change on `mongos` and add an integration tests

lib/charms/mongodb/v0/config_server_interface.py

@@ -35,7 +35,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 5
+LIBPATCH = 6
 
 
 class ClusterProvider(Object):
@@ -86,7 +86,6 @@ def _on_relation_changed(self, event) -> None:
         # create user and set secrets for mongos relation
         self.charm.client_relations.oversee_users(None, None)
 
-        # TODO Future PR, use secrets
         if self.charm.unit.is_leader():
             self.database_provides.update_relation_data(
                 event.relation.id,
@@ -98,6 +97,25 @@ def _on_relation_changed(self, event) -> None:
                 },
             )
 
+    def update_config_server_db(self, event):
+        """Provides related mongos applications with new config server db."""
+        if not self.pass_hook_checks(event):
+            logger.info("Skipping update_config_server_db: hook checks did not pass")
+            return
+
+        config_server_db = self.generate_config_server_db()
+
+        if not self.charm.unit.is_leader():
+            return
+
+        for relation in self.charm.model.relations[self.relation_name]:
+            self.database_provides.update_relation_data(
+                relation.id,
+                {
+                    CONFIG_SERVER_DB_KEY: config_server_db,
+                },
+            )
+
     def generate_config_server_db(self) -> str:
         """Generates the config server database for mongos to connect to."""
         replica_set_name = self.charm.app.name

src/charm.py

@@ -414,14 +414,7 @@ def _on_relation_joined(self, event: RelationJoinedEvent) -> None:
 
         self._on_relation_handler(event)
 
-        # app relations should be made aware of the new set of hosts
-        try:
-            self.client_relations.update_app_relation_data()
-            self.config_server.update_mongos_hosts()
-        except PyMongoError as e:
-            logger.error("Deferring on updating app relation data since: error: %r", e)
-            event.defer()
-            return
+        self._update_related_hosts(event)
 
     def _on_relation_handler(self, event: RelationEvent) -> None:
         """Adds the unit as a replica to the MongoDB replica set.
@@ -477,15 +470,6 @@ def _on_leader_elected(self, event: LeaderElectedEvent) -> None:
 
         self._update_hosts(event)
 
-        # app relations should be made aware of the new set of hosts
-        try:
-            self.client_relations.update_app_relation_data()
-            self.config_server.update_mongos_hosts()
-        except PyMongoError as e:
-            logger.error("Deferring on updating app relation data since: error: %r", e)
-            event.defer()
-            return
-
     def _on_relation_departed(self, event: RelationDepartedEvent) -> None:
         """Remove peer from replica set if it wasn't able to remove itself.
 
@@ -498,15 +482,6 @@ def _on_relation_departed(self, event: RelationDepartedEvent) -> None:
 
         self._update_hosts(event)
 
-        # app relations should be made aware of the new set of hosts
-        try:
-            self.client_relations.update_app_relation_data()
-            self.config_server.update_mongos_hosts()
-        except PyMongoError as e:
-            logger.error("Deferring on updating app relation data since: error: %r", e)
-            event.defer()
-            return
-
     def _on_storage_detaching(self, event: StorageDetachingEvent) -> None:
         """Before storage detaches, allow removing unit to remove itself from the set.
 
@@ -869,6 +844,19 @@ def _update_hosts(self, event: LeaderElectedEvent) -> None:
         self.process_unremoved_units(event)
         self.app_peer_data["replica_set_hosts"] = json.dumps(self._unit_ips)
 
+        self._update_related_hosts(event)
+
+    def _update_related_hosts(self, event) -> None:
+        # app relations should be made aware of the new set of hosts
+        try:
+            self.client_relations.update_app_relation_data()
+            self.config_server.update_mongos_hosts()
+            self.cluster.update_config_server_db(event)
+        except PyMongoError as e:
+            logger.error("Deferring on updating app relation data since: error: %r", e)
+            event.defer()
+            return
+
     def process_unremoved_units(self, event: LeaderElectedEvent) -> None:
         """Removes replica set members that are no longer running as a juju hosts."""
         with MongoDBConnection(self.mongodb_config) as mongo:
@@ -897,20 +885,12 @@ def _handle_reconfigure(self, event: UpdateStatusEvent):
 
         # remove any IPs that are no longer juju hosts & update app data.
         self._update_hosts(event)
+
         # Add in any new IPs to the replica set. Relation handlers require a reference to
         # a unit.
         event.unit = self.unit
         self._on_relation_handler(event)
 
-        # app relations should be made aware of the new set of hosts
-        try:
-            self.client_relations.update_app_relation_data()
-            self.config_server.update_mongos_hosts()
-        except PyMongoError as e:
-            logger.error("Deferring on updating app relation data since: error: %r", e)
-            event.defer()
-            return
-
     def _open_ports_tcp(self, ports: int) -> None:
         """Open the given port.
 

tests/unit/test_config_server_lib.py

@@ -53,3 +53,34 @@ def is_config_mock_call(*args):
         self.harness.set_leader(False)
         self.harness.add_relation_unit(relation_id, "mongos/2")
         self.harness.charm.cluster.database_provides.update_relation_data.assert_not_called()
+
+    def test_update_rel_data_failed_hook_checks(self):
+        """Tests that no relation data is set when the cluster is not ready."""
+
+        def is_not_config_mock_call(*args):
+            assert args == ("config-server",)
+            return False
+
+        self.harness.charm.app_peer_data["db_initialised"] = "True"
+
+        # fails due to being run on non-config-server
+        self.harness.charm.is_role = is_not_config_mock_call
+        self.harness.charm.cluster.update_config_server_db(mock.Mock())
+        self.harness.charm.cluster.database_provides.update_relation_data = mock.Mock()
+        self.harness.charm.cluster.database_provides.update_relation_data.assert_not_called()
+
+        # fails because db has not been initialized
+        del self.harness.charm.app_peer_data["db_initialised"]
+
+        def is_config_mock_call(*args):
+            assert args == ("config-server",)
+            return True
+
+        self.harness.charm.is_role = is_config_mock_call
+        self.harness.charm.cluster.update_config_server_db(mock.Mock())
+        self.harness.charm.cluster.database_provides.update_relation_data.assert_not_called()
+
+        # fails because not leader
+        self.harness.set_leader(False)
+        self.harness.charm.cluster.update_config_server_db(mock.Mock())
+        self.harness.charm.cluster.database_provides.update_relation_data.assert_not_called()