🔧(backend) configure RedisCache in production settings

In development, sessions are saved in local memory. It's working well,
however it doesn't adapt to a kubernetized setup. Several pods need
to access the current sessions, which need to be stored in a single
source of truth.

With a local memory cache, pods cannot read session saved in other pods.
We end up returning 401 errors, because we cannot authenticate the user.

I preferred setting up a proper cache than storing sessions in database,
because in the long run it would be a performance bottleneck. Cache will
decrease data access latency when reading current sessions.

I added a Redis cache backend to the production settings. Sessions would
be persisted to Redis. In K8s, a Redis operator will make sure the cached
data are not lost.

Two new dependencies were added, redis and django-redis.

I followed the installation guide of django-redis dependency. These
setting were tested deploying the app to a local K8s cluster.

src/backend/impress/settings.py

@@ -277,6 +277,7 @@ class Base(Configuration):
 
     # Session
     SESSION_ENGINE = "django.contrib.sessions.backends.cache"
+    SESSION_CACHE_ALIAS = "default"
     SESSION_COOKIE_AGE = 60 * 60 * 12
 
     # OIDC - Authorization Code Flow
@@ -531,6 +532,20 @@ class Production(Base):
     AWS_STORAGE_BUCKET_NAME = values.Value("tf-default-impress-media-storage")
     AWS_S3_REGION_NAME = values.Value()
 
+    CACHES = {
+        "default": {
+            "BACKEND": "django_redis.cache.RedisCache",
+            "LOCATION": values.Value(
+                "redis://redis:6379/1",
+                environ_name="REDIS_URL",
+                environ_prefix=None,
+            ),
+            "OPTIONS": {
+                "CLIENT_CLASS": "django_redis.client.DefaultClient",
+            },
+        },
+    }
+
 
 class Feature(Production):
     """

src/backend/pyproject.toml

@@ -32,6 +32,8 @@ dependencies = [
     "django-cors-headers==4.3.1",
     "django-countries==7.5.1",
     "django-parler==2.3",
+    "redis==5.0.3",
+    "django-redis==5.4.0",
     "django-storages==1.14.2",
     "django-timezone-field>=5.1",
     "django==5.0.3",