Add documentation for deploying with cloud storage

docs/advanced_topics/deploying.rst

@@ -17,3 +17,20 @@ On other PAASs and IAASs
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 We know of Wagtail sites running on `Heroku <http://spapas.github.io/2014/02/13/wagtail-tutorial/>`_, Digital Ocean and elsewhere. If you have successfully installed Wagtail on your platform or infrastructure, please :doc:`contribute </contributing/index>` your notes to this documentation!
+
+Cloud storage
+~~~~~~~~~~~~~
+
+Wagtail follows `Django's conventions for managing uploaded files <https://docs.djangoproject.com/en/2.0/topics/files/>`_, and can be configured to store uploaded images and documents on a cloud storage service such as Amazon S3; this is done through the `DEFAULT_FILE_STORAGE <https://docs.djangoproject.com/en/2.0/ref/settings/#std:setting-DEFAULT_FILE_STORAGE>`_ setting in conjunction with an add-on package such as `django-storages <https://django-storages.readthedocs.io/>`_. Be aware that setting up remote storage will not entirely offload file handling tasks from the application server - some Wagtail functionality requires files to be read back by the application server. In particular, documents are served through a Django view in order to enforce permission checks, and original image files need to be read back whenever a new resized rendition is created.
+
+Note that the django-storages Amazon S3 backends (``storages.backends.s3boto.S3BotoStorage`` and ``storages.backends.s3boto3.S3Boto3Storage``) **do not correctly handle duplicate filenames** in their default configuration. When using these backends, ``AWS_S3_FILE_OVERWRITE`` must be set to ``True``.
+
+If you are also serving Wagtail's static files from remote storage (using Django's `STATICFILES_STORAGE <https://docs.djangoproject.com/en/2.0/ref/settings/#std:setting-STATICFILES_STORAGE>`_ setting), you'll need to ensure that it is configured to serve `CORS HTTP headers <https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS>`_, as current browsers will reject remotely-hosted font files that lack a valid header. For Amazon S3, refer to the documentation `Setting Bucket and Object Access Permissions <https://docs.aws.amazon.com/AmazonS3/latest/user-guide/set-permissions.html>`_, or (for the ``storages.backends.s3boto.S3BotoStorage`` backend only) add the following to your Django settings:
+
+.. code-block:: python
+
+    AWS_HEADERS = {
+        'Access-Control-Allow-Origin': '*'
+    }
+
+For other storage services, refer to your provider's documentation, or the documentation for the Django storage backend library you're using.