The Vary mechanism allows multiple variants of the same URL to be cached side by side. Depending on header values provided by the client, the cache will select the correct variant to return to the client. This mechanism can become a problem when an attempt is made to vary on a header that is known to contain a wide range of possible values under normal use, for example the User-Agent header. Depending on the popularity of the particular web site thousands or millions of duplicate cache entries could be created for the same URL, crowding out other entries in the cache.