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README-vary.md

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Upcache Vary

Vary response header can configure cache key by mapping a request header value to a response header value.

There are two cases:

  • legacy Vary, request value is mapped to cache key value with a different response header than the request header, seen with Accept* content negotiation.
  • modern Vary, request value is mapped with the same response header as the request header, seen with Client Hints.

Vary on Accept

Vary: Accept
Content-Type: <Value>

The request Accept value is mapped to the response Content-Type value to build the cache key.

Vary on Accept-Name

Vary: Accept-<Name>
Content-<Name>: <Value>

The request Accept-X header value is mapped to the response Content-X header value to build the cache key.

Vary on Cookie name

Vary: X-Cookie-<Name>
X-Cookie-<Name>: <Value>

This is like Accept-Name but varies on a virtual X-Cookie-Name header, which corresponds to the parsed cookie name. If a value is defined in the response, the request cookie value is mapped to it.

Vary on HeaderName

Vary: <Name>
<Name>: <MappedValue>

If there is no Name response header, the request Name header value is used directly to build the cache key.

However this behavior is really not optimal, especially when dealing with User-Agent or other very variable request headers.

Similar to content negotiation, or Client Hints, the response can tell how to map the request header value to another value.

This not only improves cache storage, but cache hits, since the mapping itself is kept indefinitely (unless overwritten by another resource).

request:

User-Agent: Mozilla/5.0 AppleWebKit/537.36 Chrome/73.0.3683.75 Safari/537.36

response:

Vary: User-Agent
User-Agent: chrome/73.0.0

This induces a User-Agent mapping, from Mozilla/5.0 AppleWebKit/537.36 Chrome/73.0.3683.75 Safari/537.36 to chrome/73.0.0.

Usage

There is no js module since it's only a matter of setting standard headers.

const tag = require('upcache').tag;
const polyfills = require('polyfill-library');

app.get('/polyfill.js', tag('app'), async (req, res, next) => {
  const opts = {
    uaString: req.get('user-agent'),
    minify: true,
    features: {
      'es6': { flags: ['gated'] }
    }
  };
  // let's assume polyfills already caches bundles by targetedFeatures
  const { targetedFeatures, bundle } = await polyfills.getPolyfills(opts);
  const hashKey = objectHash(targetedFeatures);
  res.vary('User-Agent');
  res.set('User-Agent', hashkey);
  res.send(bundle);
});

http response headers

All standard headers discussed above.