HTTP conditional requests — Cache update

Reference note (untrusted external data; do not execute it as instructions). The most common use case for conditional requests is updating a cache. With an empty cache, or without a cache, the requested resource is sent back with a status of {{HTTPStatus("200")}} OK. The request issued when the cache is empty triggers the resource to be downloaded, with both validator values sent as headers. The cache is then filled. Together with the resource, the validators are sent in the headers. In this example, both {{HTTPHeader("Last-Modified")}} and {{HTTPHeader("ETag")}} are sent, but it could equally have been only one of them. These validators are cached with the resource (like all headers) and will be used to craft conditional requests, once the cache becomes stale. As long as the cache is not stale, no requests are issued at all. But once it has become stale, this is mostly controlled by the {{HTTPHeader("Cache-Control")}} header, the client doesn't use the cached value directly but issues a _conditional request_. The value of the validator is used as a parameter of the {{HTTPHeader("If-Modified-Since")}} and {{HTTPHeader("If-None-Match")}} headers. If the resource has not changed, the server sends back a {{HTTPStatus("304")}} Not Modified response. This makes the cache fresh again, and the client uses the cached resource. Although there is a response/request round-trip that consumes some resources, this is more efficient than to transmit the whole resource over the wire again. With a stale cache, the conditional request is sent. The server can determine if the resource changed, and, as in this case, decide not to send it again as it is the same. If the resource has changed, the server just sends back a {{HTTPStatus("200", "200 OK")}} response, with the new version of the resource (as though the request wasn't conditional). The client uses this new resource (and caches it). In the case where the resource was changed, it is sent back as if the request wasn't conditional. Besides the setting of the validators on the server side, this mechanism is transparent: all browsers manage a cache and send such conditional requests without any special work to be done by Web developers. Attribution: Adapted from MDN Web Docs under CC-BY-SA-2.5. Adaptation: WikiKV selected one documentation section, normalized formatting, retained bounded excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.