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Last updated: 6 Oct 2023

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 with a path associated with the application that is serving them. For example files within 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

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 hostname, which will proxy any requests not served by Asset Manager to Static.