aldryn-apphook-reload

Reload urls of django CMS Apphooks without a restart

Introduction

Django CMS allows extending cms pages with Apphooks. Apphooks are saved in the Database, which means urls depend on the database contents. For changes to Apphooks to be reflected in reverse() and {% url ... %} calls, a webserver restart is usually necessary.

aldryn-apphook-reload will automatically reload urls from Django CMS apphooks, without the need of a webserver restart. It listens to cms.signals.urls_need_reloading and causes a reload.

The signal is only available in the process where the change to the database was made. In order for other processes to know when to reload (be it a gunicorn worker or a process on a other server) a token is saved in the database. This implies a performance hit: 1 database query per request.

Installation

• add aldryn_apphook_reload to INSTALLED_APPS.
• add aldryn_apphook_reload.middleware.ApphookReloadMiddleware to MIDDLEWARE (place it as close to the top as possible)
• run migrations: python manage.py migrate aldryn_apphook_reload

Advanced

If the process that triggers cms.signals.urls_need_reloading is a simple runserver under load ( ~2 requests per second), the reload sometimes fails on the other processes. This might be due to an unknown race condition, where the token in the database is refreshed already, but the new apphooks are not in the database yet. The other processes would try to reload right away and would reload the old apphooks. Tests with gunicorn in the default mode and in the gevent mode worked fine though.

Why not save the token in the cache backend for better performance? - Because altering the cache would happen right away, before the database transaction is committed at the end of the request. Thus other process would reload their urls prematurely.