responsive-and-fast-implementing-high-performance-responsive-design

The Vary header instructs the cache to add a given request header to
its cache key. For instance, a server may choose to only gzip a response
for clients that support it, as indicated by their Accept-Encoding
header. Since it doesn’t want downstream proxies to serve a compressed
response to a client that doesn’t support it, it returns Vary:
Accept-Encoding with its response.
For RESS, the most relevant uses are to Vary by User-Agent, which is
the formal way an incoming device declares itself; and Cookie, often
used for custom identifiers. Unfortunately, User-Agent and Cookie
are probably the longest request headers, and each can hold an extremely
large number of values—many more than the granularity we
actually want to split our content by.
This means the use of Vary for either header will likely create excessive
cache fragmentation (when every cache entry is applicable to a small
number of requests), and effectively disable third-party caches. In fact,
many caching proxies explicitly won’t bother trying, and will simply
not cache responses that use Vary on Cookie or User-Agent. As a
result, the only practical value in using Vary: User-Agent is to indicate
to search engines that your page may serve different content to
different clients.
The Cache-Control header guides downstream proxies on how to
cache a response, and supports separate instructions for shared proxies
and clients. For instance, specifying Cache-Control: private tells
downstream cache proxies to avoid caching a RESS response, while
still allowing the client (for which we tuned the response) to cache it.
Given the Vary header limitations, the caching effect of Cache-
Control: private and Vary: User-Agent is practically the same.
In summary, with today’s technology, using RESS implies that the best
we can do is disable downstream caching proxies. However, a new
proposed standard, the Key header, hopes to change this in the future.
The Key header proposes a way to explicitly state the variables to include
in a cache key, and do so in a standardized way. Key is not yet
supported by any notable caching tool or service, but backing for it
has been growing, which will hopefully lead to actual adoption.
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