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A conditional GET is an HTTP GET request that may return an HTTP 304 response (instead of HTTP 200). An HTTP 304 response indicates that the resource has not been modified since the previous GET, and so the resource is not returned to the client in such a response. See RFC 7232 for details.
There are at least two (not completely independent) approaches to conditional GET:
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The use of conditional GET has significant benefits, on both the client and the server (and the intervening network as well). On the InCommon metadata server, roughly 3/4 of all metadata requests result in HTTP 304. That translates into many thousands of metadata requests per day that conveniently avoid the unnecessary overhead of metadata refresh. For a file whose size is more than 11MB and growing, that represents a significant cost savings.
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