up.protocol Vary

HTTP header

Request headers that influenced a response should be listed in a Vary response header.

This tells Unpoly to partition its cache for that URL so that each request header value gets a separate cache entries.

You can set a Vary header manually from your server-side code. You may also be using a library like unpoly-rails that sets the Vary header automatically.

Tip

Server-side Unpoly apps may inspect request headers to customize or shorten responses, e.g. by omitting content that isn't targeted. When a response is optimized in that fasion, a Vary header should be sent.

Example

The user makes a request to /sitemap in order to updates a fragment .menu. Unpoly makes a request like this:

GET /sitemap HTTP/1.1
X-Up-Target: .menu

The server may choose to optimize its response by only render only the HTML for the .menu fragment. It responds with the HTTP seen below. Note that it includes a Vary header indicating that the X-Up-Target header has influenced the response body:

Vary: X-Up-Target

<div class="menu">...</div>

After observing the Vary: X-Up-Target header, Unpoly will partition cache entries to /sitemap by X-Up-Target value. That means a request targeting .menu is no longer a cache hit for a request targeting a different selector.

See How cache entries are matched for more examples.

Resources

• RFC 9110: Vary
• MDN: Vary
• Key CDN: Vary