Add documentation about image formats for multi server setup (#847)

Co-authored-by: Prokyonn <daniel.mathis@sector8.eu>

Author: alexander-schranz

cookbook/multi-server-setup.rst

@@ -26,6 +26,30 @@ for your media files.
                 path_prefix: 'optional path prefix'
                 region: 'eu-west-1'
 
+Image Formats
+-------------
+
+In the Media Storage, only original files are stored. The image formats are generated by Sulu the first time they are
+requested. That mechanism (reverse proxy cache) is not supported by storages like S3, Blob Storage, etc. So for
+every server, the image will be regenerated once. As long as no media are removed, that might not be a problem.
+You can configure ``bin/console sulu:media:format:cache:cleanup`` as a cron job per server to ensure that an image
+format is also removed on all servers.
+
+Still, in Kubernetes/container setups where you might lose the ``public/uploads/media`` directory every time a container restarts,
+you need to find different ways to cache images.
+You can make the ``public/uploads/media`` directory a mounted volume shared between all containers (which might not be good for performance).
+It is better to use some kind of CDN (Cloudflare, Fastly, etc.) or a reverse proxy cache via Nginx to cache generated image formats.
+
+If you configure a CDN/reverse proxy correctly, you do not require the cron job and can disable the image format filesystem cache completely via:
+
+.. code-block:: yaml
+
+    # config/packages/sulu_media.yaml
+    sulu_media:
+        format_cache:
+            save_image: false
+
+Never disable the image format cache if you do not use a CDN or reverse proxy cache that caches files for a very long time.
 
 HTTP Cache
 ----------
@@ -101,4 +125,3 @@ Alternatively, you can set a centralized ``session.save_handler`` directly in yo
 .. _Store Sessions in a Database: https://symfony.com/doc/current/session/database.html
 .. _configure a centralized caching adapter: https://symfony.com/doc/current/cache.html
 .. _configure Elasticsearch as a centralized search adapter: https://massivesearchbundle.readthedocs.io/en/latest/search_adapters.html#elasticsearch
-