Varying the Response for HTTP Cache

So far, it's been assumed that each URI has exactly one representation of the target resource. By default, HTTP caching is done by using the URI of the resource as the cache key. If two people request the same URI of a cacheable resource, the second person will receive the cached version.

Sometimes this isn't enough and different versions of the same URI need to be cached based on one or more request header values. For instance, if you compress pages when the client supports it, any given URI has two representations: one when the client supports compression, and one when it does not. This determination is done by the value of the Accept-Encoding request header.

In this case, you need the cache to store both a compressed and uncompressed version of the response for the particular URI and return them based on the request's Accept-Encoding value. This is done by using the Vary response header, which is a comma-separated list of different headers whose values trigger a different representation of the requested resource:

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Vary: Accept-Encoding, User-Agent

The Response object offers a clean interface for managing the Vary header:

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// sets one vary header
$response->setVary('Accept-Encoding');

// sets multiple vary headers
$response->setVary(['Accept-Encoding', 'User-Agent']);

The setVary() method takes a header name or an array of header names for which the response varies.