Only serve from replica when there is a request in scope

polling_stations/db_routers.py

@@ -2,21 +2,46 @@
 
 from django.conf import settings
 from django.db import DEFAULT_DB_ALIAS
+from django_middleware_global_request import get_request
+
+
+PRINCIPAL = settings.PRINCIPAL_DB_NAME
+REPLICA = DEFAULT_DB_ALIAS
 
 
 class ReplicationRouter(object):
     def db_for_read(self, model, **hints):
+        # In CI, there is only one DB connection
+        if os.environ.get("CIRCLECI"):
+            return DEFAULT_DB_ALIAS
+
+        # We don't need to scale requests touching these models
+        # but we do want to prevent race conditions
+        # when reading a record we just wrote
         if model._meta.label in (
             "file_uploads.Upload",
             "file_uploads.File",
-        ) and not os.environ.get("CIRCLECI"):
-            return settings.PRINCIPAL_DB_NAME
-        return DEFAULT_DB_ALIAS
+        ):
+            return PRINCIPAL
+
+        # We only care about trying to scale
+        # by serving traffic from a replica
+        # when we are serving HTTP traffic
+        request = get_request()
+        if request:
+            if request.path.startswith("/admin"):
+                return PRINCIPAL
+            return REPLICA
+
+        # in all other cases (e.g: management commands, shell)
+        # perform reads from the principal to prevent race conditions
+        return PRINCIPAL
 
     def db_for_write(self, model, **hints):
         if os.environ.get("CIRCLECI"):
             return DEFAULT_DB_ALIAS
-        return settings.PRINCIPAL_DB_NAME
+
+        return PRINCIPAL
 
     def allow_relation(self, obj1, obj2, **hints):
         return True
@@ -26,6 +51,6 @@ def allow_migrate(self, db, app_label, model_name=None, **hints):
 
 
 def get_principal_db_name():
-    if settings.PRINCIPAL_DB_NAME in settings.DATABASES:
-        return settings.PRINCIPAL_DB_NAME
-    return DEFAULT_DB_ALIAS
+    if PRINCIPAL in settings.DATABASES:
+        return PRINCIPAL
+    return REPLICA

polling_stations/settings/base.py

@@ -124,6 +124,7 @@ def get_ec2_ip():
 MIDDLEWARE = (
     "whitenoise.middleware.WhiteNoiseMiddleware",
     "corsheaders.middleware.CorsMiddleware",
+    "django_middleware_global_request.middleware.GlobalRequestMiddleware",
     "django.contrib.sessions.middleware.SessionMiddleware",
     "django.middleware.common.CommonMiddleware",
     "django.middleware.locale.LocaleMiddleware",
@@ -197,6 +198,7 @@ def get_ec2_ip():
     "dc_design_system",
     "dc_utils",
     "drf_spectacular",
+    "django_middleware_global_request",
 )
 
 PROJECT_APPS = (

requirements/base.in

@@ -11,6 +11,7 @@ django-dotenv
 django-extensions
 django-filter
 django-localflavor
+django-middleware-global-request
 django-sesame
 djangorestframework
 djangorestframework-csv

requirements/base.txt

@@ -198,6 +198,7 @@ django==4.2.13 \
     #   django-extensions
     #   django-filter
     #   django-localflavor
+    #   django-middleware-global-request
     #   django-sesame
     #   djangorestframework
     #   drf-spectacular
@@ -233,6 +234,12 @@ django-localflavor==4.0 \
     #   -c requirements/constraints.txt
     #   -r requirements/base.in
     #   dc-django-utils
+django-middleware-global-request==0.3.5 \
+    --hash=sha256:37c79c9cd80e73751ae2821696e20af13ba7628a0915ff1818d124e0e72980b1 \
+    --hash=sha256:7beb2c8ca9106aac641692cda2bac290bd7083a315446a12db06fa91b73903d6
+    # via
+    #   -c requirements/constraints.txt
+    #   -r requirements/base.in
 django-pipeline==3.0.0 \
     --hash=sha256:49a8bee298668100bb6e8a2144dff8c607baa5297820a2503793c38693f34103 \
     --hash=sha256:e9e08b084ef3ebf599795510519a8d44f2240b487782bebf4a8fcaf6302c31d1

requirements/constraints.txt

@@ -381,6 +381,7 @@ django==4.2.13 \
     #   django-extensions
     #   django-filter
     #   django-localflavor
+    #   django-middleware-global-request
     #   django-sesame
     #   djangorestframework
     #   drf-spectacular
@@ -415,6 +416,10 @@ django-localflavor==4.0 \
     # via
     #   -r requirements/base.in
     #   dc-django-utils
+django-middleware-global-request==0.3.5 \
+    --hash=sha256:37c79c9cd80e73751ae2821696e20af13ba7628a0915ff1818d124e0e72980b1 \
+    --hash=sha256:7beb2c8ca9106aac641692cda2bac290bd7083a315446a12db06fa91b73903d6
+    # via -r requirements/base.in
 django-pipeline==3.0.0 \
     --hash=sha256:49a8bee298668100bb6e8a2144dff8c607baa5297820a2503793c38693f34103 \
     --hash=sha256:e9e08b084ef3ebf599795510519a8d44f2240b487782bebf4a8fcaf6302c31d1