Support aliasing the redis relation (#75)

* Support aliases for the redis relation

lib/charms/redis_k8s/v0/redis.py

@@ -33,6 +33,7 @@
 """
 import logging
 import socket
+from typing import Dict, Optional
 
 from ops.charm import CharmEvents
 from ops.framework import EventBase, EventSource, Object
@@ -45,10 +46,11 @@
 
 # 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
 
 logger = logging.getLogger(__name__)
 
+DEFAULT_REALTION_NAME = "redis"
 
 class RedisRelationUpdatedEvent(EventBase):
     """An event for the redis relation having been updated."""
@@ -60,14 +62,16 @@ class RedisRelationCharmEvents(CharmEvents):
 
 
 class RedisRequires(Object):
-    def __init__(self, charm, _stored):
+
+    def __init__(self, charm, _stored, relation_name: str = DEFAULT_REALTION_NAME):
         """A class implementing the redis requires relation."""
-        super().__init__(charm, "redis")
-        self.framework.observe(charm.on.redis_relation_joined, self._on_relation_changed)
-        self.framework.observe(charm.on.redis_relation_changed, self._on_relation_changed)
-        self.framework.observe(charm.on.redis_relation_broken, self._on_relation_broken)
+        super().__init__(charm, relation_name)
+        self.framework.observe(charm.on[relation_name].relation_joined, self._on_relation_changed)
+        self.framework.observe(charm.on[relation_name].relation_changed, self._on_relation_changed)
+        self.framework.observe(charm.on[relation_name].relation_broken, self._on_relation_broken)
         self._stored = _stored
         self.charm = charm
+        self.relation_name = relation_name
 
     def _on_relation_changed(self, event):
         """Handle the relation changed event."""
@@ -89,19 +93,46 @@ def _on_relation_broken(self, event):
         # Trigger an event that our charm can react to.
         self.charm.on.redis_relation_updated.emit()
 
+    @property
+    def relation_data(self) -> Optional[Dict[str, str]]:
+        """Retrieve the relation data.
+
+        Returns:
+            Dict: dict containing the relation data.
+        """
+        relation = self.model.get_relation(self.relation_name)
+        if not relation or not relation.units:
+            return None
+        unit = next(iter(relation.units))
+        return relation.data[unit]
+
+    @property
+    def url(self) -> Optional[str]:
+        """Retrieve the Redis URL.
+
+        Returns:
+            str: the Redis URL.
+        """
+        relation_data = self.relation_data
+        if not relation_data:
+            return None
+        redis_host = relation_data.get("hostname")
+        redis_port = relation_data.get("port")
+        return f"redis://{redis_host}:{redis_port}"
+
 
 class RedisProvides(Object):
     def __init__(self, charm, port):
         """A class implementing the redis provides relation."""
-        super().__init__(charm, "redis")
+        super().__init__(charm, DEFAULT_REALTION_NAME)
         self.framework.observe(charm.on.redis_relation_changed, self._on_relation_changed)
         self._port = port
         self._charm = charm
 
     def _on_relation_changed(self, event):
         """Handle the relation changed event."""
-        event.relation.data[self.model.unit]['hostname'] = self._get_master_ip()
-        event.relation.data[self.model.unit]['port'] = str(self._port)
+        event.relation.data[self.model.unit]["hostname"] = self._get_master_ip()
+        event.relation.data[self.model.unit]["port"] = str(self._port)
         # The reactive Redis charm also exposes 'password'. When tackling
         # https://github.com/canonical/redis-k8s/issues/7 add 'password'
         # field so that it matches the exposed interface information from it.