Assets: how they work
There are three types of asset files.
Static assets are stylesheets (CSS), JavaScript (JS) and image files which make GOV.UK look the way it does.
Uploaded assets - also called attachments - are files like PDFs, CSVs and images which are uploaded via the publishing apps and attached to documents.
Static templates - these are HTML snippets served by the Static application. These are used by applications to share common parts of the GOV.UK page layout (such as header and footer).
Static assets
These assets are served from https://www.gov.uk/assets with a path associated
with the application that is serving them. For example files within
https://www.gov.uk/assets/government-frontend are assets for the
Government Frontend application. These files
are cached by the GOV.UK content delivery network.
These assets are served by the cache machines, these will proxy requests to
the application based on the path. For example /assets/government-frontend/
will proxy asset requests to Government Frontend
Uploaded assets
Asset Manager is an API that is called internally
by GOV.UK publishing applications to manage
their uploads. It serves the uploaded assets on
assets.publishing.service.gov.uk.
How uploaded assets are stored and served
Asset files are stored in an S3 bucket (e.g. govuk-assets-production in
production) and Asset Manager instructs nginx to proxy requests to them.
Proxying was chosen, rather than linking directly to S3, to support 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.
- Asset files can be replaced, and a request to the original path redirects to the replacement. Currently only Whitehall and Specialist Publisher support this.
File size limit
There is a hard limit of 500 MB for assets.
Publishers may see timeouts if they attempt to upload very large attachments. In the past, we’ve worked around this by uploading a small file (in Whitehall) and then replacing the file in Asset Manager.
Static templates
The templates that Static hosts are used by GOV.UK frontend applications to render consistent aspects of GOV.UK pages. Production instances of GOV.UK applications access these by internal communication with Static.
These assets are also available publicly for development and preview versions
of GOV.UK applications. They allow apps to utilise these production resources
without needing to run an instance of Static manaul. Public access to Static
is provided by the assets.publishing.service.gov.uk hostname, which will
proxy any requests not served by Asset Manager to Static.