[DPE-3293] Changes external mongos (#350)

## Issue
Mongos charm does not support external connections

## Solution
Enable mongos to provide an external connection when it is requested by
the host charm

## Note
There are changes required in the shared MongoDB libs - but the bulk of
this work will be done in the Mongos Charm

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 = 6
+LIBPATCH = 7
 
 
 class ClusterProvider(Object):
@@ -275,7 +275,13 @@ def _on_relation_broken(self, event: RelationBrokenEvent) -> None:
 
     def is_mongos_running(self) -> bool:
         """Returns true if mongos service is running."""
-        with MongosConnection(None, f"mongodb://{MONGOS_SOCKET_URI_FMT}") as mongo:
+        connection_uri = f"mongodb://{self.charm.get_mongos_host()}"
+
+        # when running internally, connections through Unix Domain sockets do not need port.
+        if self.charm.is_external_client:
+            connection_uri = connection_uri + f":{Config.MONGOS_PORT}"
+
+        with MongosConnection(None, connection_uri) as mongo:
             return mongo.is_ready
 
     def update_config_server_db(self, config_server_db) -> bool:
@@ -285,7 +291,10 @@ def update_config_server_db(self, config_server_db) -> bool:
 
         mongos_config = self.charm.mongos_config
         mongos_start_args = get_mongos_args(
-            mongos_config, snap_install=True, config_server_db=config_server_db
+            mongos_config,
+            snap_install=True,
+            config_server_db=config_server_db,
+            external_connectivity=self.charm.is_external_client,
         )
         add_args_to_env("MONGOS_ARGS", mongos_start_args)
         return True

lib/charms/mongodb/v1/helpers.py

@@ -30,7 +30,7 @@
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 3
+LIBPATCH = 4
 
 # path to store mongodb ketFile
 KEY_FILE = "keyFile"
@@ -96,6 +96,7 @@ def get_mongos_args(
     config,
     snap_install: bool = False,
     config_server_db: str = None,
+    external_connectivity: bool = True,
 ) -> str:
     """Returns the arguments used for starting mongos on a config-server side application.
 
@@ -104,9 +105,9 @@ def get_mongos_args(
     """
     # suborinate charm which provides its own config_server_db, should only use unix domain socket
     binding_ips = (
-        f"--bind_ip {MONGODB_COMMON_DIR}/var/mongodb-27018.sock"
-        if config_server_db
-        else "--bind_ip_all"
+        "--bind_ip_all"
+        if external_connectivity
+        else f"--bind_ip {MONGODB_COMMON_DIR}/var/mongodb-27018.sock"
     )
 
     # mongos running on the config server communicates through localhost