Assets: how they work
There are two types of asset files.
Uploaded assets - also called attachments - are files like PDFs, CSVs and images which are uploaded via the publishing apps and attached to documents.
How users access assets
In production, the GOV.UK content delivery network is in front of our assets hostname.
In other environments there’s no CDN and the assets hostname points directly to our origin servers.
Assets at origin
Assets are served by the cache machines in all environments.
The cache machines proxy requests to the application hostnames based
on the first segment of the path. For example
is proxied to the hostname for
government-frontend in that environment.
All other assets that have a path that don’t match fall back to the static application.
There are currently two systems for uploading, managing and serving user-supplied assets on GOV.UK.
Asset Manager is an API that is called internally by Publisher, Specialist Publisher, Manuals Publisher and Travel Advice Publisher to manage their uploads. It serves the uploaded assets on assets.publishing.service.gov.uk/media
Whitehall is a standalone publishing app that manages its own uploaded assets. It also serves those assets, via both assets.publishing.service.gov.uk/government/uploads and www.gov.uk/government/uploads, even for content that has been migrated to the publishing platform.
How uploaded assets are stored and served
Both Whitehall and Asset Manager store the asset files on an NFS share. This is hosted on the backend and whitehall-backend machines, and asset requests therefore go via the cache, frontend-lb, (whitehall-)frontend, backend-lb and backend machines.
It should be noted that both applications do actually serve the asset requests, rather than letting nginx serve directly from the share. This is to enable the following features:
- Assets are not served until they have been virus scanned; a placeholder image or page is shown for assets that are not finished scanning.
- Assets can be access-limited so that only authorised users can see them. This is only done by Whitehall.
- Asset files can be replaced, and a request to the original path redirects to the replacement. Currently only Whitehall and Specialist Publisher support this.