[DPE-3179] Use specified DB + Application roles from host charm

lib/charms/mongos/v0/mongos_client_interface.py

@@ -0,0 +1,133 @@
+# Copyright 2023 Canonical Ltd.
+# See LICENSE file for licensing details.
+
+"""In this class, we manage relations between config-servers and shards.
+
+This class handles the sharing of secrets between sharded components, adding shards, and removing
+shards.
+"""
+import logging
+from ops.framework import Object
+from ops.charm import CharmBase
+from charms.data_platform_libs.v0.data_interfaces import (
+    DatabaseProvides,
+    DatabaseRequires,
+)
+
+
+logger = logging.getLogger(__name__)
+DATABASE_KEY = "database"
+USER_ROLES_KEY = "extra-user-roles"
+MONGOS_RELATION_NAME = "mongos_proxy"
+
+# TODO - the below LIBID, LIBAPI, and LIBPATCH are not valid and were made manually. These will be
+# created automatically once the charm has been published. The charm has not yet been published
+# due to:
+# https://discourse.charmhub.io/t/request-ownership-of-reserved-mongos-charm/12735
+
+# The unique Charmhub library identifier, never change it
+LIBID = "58ad1ccca4974932ba22b97781b9b2a0"
+
+# Increment this major API version when introducing breaking changes
+LIBAPI = 0
+
+# Increment this PATCH version before using `charmcraft publish-lib` or reset
+# to 0 if you are raising the major API version
+LIBPATCH = 1
+
+"""Library to manage the relation for the application between mongos and the deployed application.
+In short, this relation ensure that:
+1. mongos receives the specified database and users roles needed by the host application
+2. the host application receives the generated username, password and uri for connecting to the 
+sharded cluster.
+
+This library contains the Requires and Provides classes for handling the relation between an
+application and mongos. The mongos application relies on the MongosProvider class and the deployed
+application uses the MongoDBRequires class.
+
+The following is an example of how to use the MongoDBRequires class to specify the roles and 
+database name:
+
+```python
+from charms.mongos.v0.mongos_client_interface import MongosRequirer
+
+
+class ApplicationCharm(CharmBase):
+
+    def __init__(self, *args):
+        super().__init__(*args)
+
+        # relation events for mongos client
+        self._mongos_client = MongosRequirer(
+            self,
+            database_name="my-test-db",
+            extra_user_roles="admin",
+        )
+```
+
+To receive the username, password, and uri:
+# TODO this is to be implemented in a future PR
+"""
+
+
+class MongosProvider(Object):
+    """Manage relations between the mongos router and the application on the mongos side."""
+
+    def __init__(self, charm: CharmBase, relation_name: str = MONGOS_RELATION_NAME) -> None:
+        """Constructor for MongosProvider object."""
+        self.relation_name = relation_name
+        self.charm = charm
+        self.database_provides = DatabaseProvides(self.charm, relation_name=self.relation_name)
+
+        super().__init__(charm, self.relation_name)
+        self.framework.observe(
+            charm.on[self.relation_name].relation_changed, self._on_relation_changed
+        )
+
+        # TODO Future PRs handle relation broken
+
+    def _on_relation_changed(self, event) -> None:
+        """Handles updating the database and extra user roles."""
+        if not self.charm.unit.is_leader():
+            return
+
+        relation_data = event.relation.data[event.app]
+        new_database_name = relation_data.get(DATABASE_KEY, self.charm.database)
+        new_extra_user_roles = relation_data.get(USER_ROLES_KEY, self.charm.extra_user_roles)
+
+        if new_database_name != self.charm.database:
+            self.charm.set_database(new_database_name)
+
+        if new_extra_user_roles != self.charm.extra_user_roles:
+            if isinstance(new_extra_user_roles, str):
+                new_extra_user_roles = [new_extra_user_roles]
+
+            self.charm.set_user_roles(new_extra_user_roles)
+
+
+class MongosRequirer(Object):
+    """Manage relations between the mongos router and the application on the application side."""
+
+    def __init__(
+        self,
+        charm: CharmBase,
+        database_name: str,
+        extra_user_roles: str,
+        relation_name: str = MONGOS_RELATION_NAME,
+    ) -> None:
+        """Constructor for MongosRequirer object."""
+        self.relation_name = relation_name
+        self.charm = charm
+
+        if not database_name:
+            database_name = f"{self.charm.app}"
+
+        self.database_requires = DatabaseRequires(
+            self.charm,
+            relation_name=self.relation_name,
+            database_name=database_name,
+            extra_user_roles=extra_user_roles,
+        )
+
+        super().__init__(charm, self.relation_name)
+        # TODO Future PRs handle relation broken

src/charm.py

@@ -9,6 +9,7 @@
 from pathlib import Path
 
 from charms.mongodb.v0.mongodb_secrets import SecretCache
+from charms.mongos.v0.mongos_client_interface import MongosProvider
 from typing import Set, List, Optional, Dict
 from charms.mongodb.v0.mongodb_secrets import generate_secret_label
 from charms.mongodb.v1.mongos import MongosConfiguration
@@ -53,6 +54,7 @@
 
         self.cluster = ClusterRequirer(self)
         self.secrets = SecretCache(self)
+        self.mongos_provider = MongosProvider(self)
         # 1. add users for related application (to be done on config-server charm side)
         # 2. update status indicates missing relations
 
@@ -237,9 +239,9 @@
         # TODO future PR - generate different URI for data-integrator as that charm will not
         # communicate to mongos via the Unix Domain Socket.
 
-    def set_user_role(self, roles: List[str]):
+    def set_user_roles(self, roles: List[str]) -> None:
         """Updates the roles for the mongos user."""
-        roles_str = roles.join(",")
+        roles_str = ",".join(roles)
         self.app_peer_data[USER_ROLES_TAG] = roles_str
 
         if len(self.model.relations[Config.Relations.CLUSTER_RELATIONS_NAME]) == 0:
@@ -253,7 +255,7 @@
             config_server_rel.id, {USER_ROLES_TAG: roles_str}
         )
 
-    def set_database(self, database: str):
+    def set_database(self, database: str) -> None:
         """Updates the database requested for the mongos user."""
         self.app_peer_data[DATABASE_TAG] = database
 
@@ -268,7 +270,6 @@
             config_server_rel.id, {DATABASE_TAG: database}
         )
 
-    # TODO future PR add function to set database field
     # END: helper functions
 
     # BEGIN: properties