If-None-Match - HTTP
the if-none-match http request header makes the request conditional.
... when used in combination with if-modified-since, if-none-match has precedence (if the server supports it).
... for other methods, and in particular for put, if-none-match used with the * value can be used to save a file not known to exist, guaranteeing that another upload didn't happen before, losing the data of the previous put; this problem is a variation of the lost update problem.
...And 3 more matches

HTTP conditional requests - HTTP
if-none-match succeeds if the etag of the distant resource is different to each listed in this header.
...to prevent this, conditional requests can be used: by adding if-none-match with the special value of '*', representing any etag.
... the request will succeed, only if the resource didn't exist before: if-none-match will only work with http/1.1 (and later) compliant servers.

HTTP Index - HTTP
158 if-none-match conditional requests, http, http header, reference, request header the if-none-match http request header makes the request conditional.
...this happens when the request method is safe, like a get or a head request, or when the request is conditional and uses a if-none-match or a if-modified-since header.
...this happens with conditional requests on methods other than get or head when the condition defined by the if-unmodified-since or if-none-match headers is not fulfilled.

HTTP caching - HTTP
note that a stale resource is not evicted or ignored; when the cache receives a request for a stale resource, it forwards this request with a if-none-match to check if it is in fact still fresh.
...if the etag header was part of the response for a resource, the client can issue an if-none-match in the header of future requests – in order to validate the cached resource.

Cache-Control - HTTP
if-none-match or if-modified-since) to check for updates, even when the user explicitly refreshes the page.
...conditional headers such as if-none-match should not be set.

HTTP headers - HTTP
conditional requests using if-match and if-none-match use this value to change the behavior of the request.
... if-none-match makes the request conditional, and applies the method only if the stored resource doesn't match any of the given etags.

Request header - MDN Web Docs Glossary: Definitions of Web-related terms
.mozilla.org user-agent: mozilla/5.0 (macintosh; intel mac os x 10.9; rv:50.0) gecko/20100101 firefox/50.0 accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 accept-language: en-us,en;q=0.5 accept-encoding: gzip, deflate, br referer: https://developer.mozilla.org/testpage.html connection: keep-alive upgrade-insecure-requests: 1 if-modified-since: mon, 18 jul 2016 02:36:04 gmt if-none-match: "c561c68d0ba92bbeb8b0fff2a9199f722e3a621a" cache-control: max-age=0 strictly speaking, the content-length header in this example is not a request header like the others, but an entity header: post /myform.html http/1.1 host: developer.mozilla.org user-agent: mozilla/5.0 (macintosh; intel mac os x 10.9; rv:50.0) gecko/20100101 firefox/50.0 content-length: 128 learn more technical knowl...

Request.cache - Web APIs
fetch("some.json", {cache: "reload"}) .then(function(response) { /* consume the response */ }); // download a resource with cache busting when dealing with a // properly configured server that will send the correct etag // and date headers and properly handle if-modified-since and // if-none-match request headers, therefore we can rely on the // validation to guarantee a fresh response.

ETag - HTTP
if a user visits a given url again (that has an etag set), and it is stale (too old to be considered usable), the client will send the value of its etag along in an if-none-match header field: if-none-match: "33a64df551425fcc55e4d42a148795d9f25f89d4" the server compares the client's etag (sent with if-none-match) with the etag for its current version of the resource, and if both values match (that is, the resource has not changed), the server sends back a 304 not modified status, without a body, which tells the client that the cached version of the response is sti...

If-Modified-Since - HTTP
when used in combination with if-none-match, it is ignored, unless the server doesn't support if-none-match.

Index - HTTP
79 if-none-match conditional requests, http, http header, reference, request header the if-none-match http request header makes the request conditional.

Link prefetching FAQ - HTTP
this header enables caching, but requires an if-modified-since or if-none-match validation request before the serving the document out of the browser's cache.

HTTP Messages - HTTP
request headers, like user-agent, accept-type, modify the request by specifying it further (like accept-language), by giving context (like referer), or by conditionally restricting it (like if-none).

304 Not Modified - HTTP
this happens when the request method is safe, like a get or a head request, or when the request is conditional and uses a if-none-match or a if-modified-since header.

412 Precondition Failed - HTTP
this happens with conditional requests on methods other than get or head when the condition defined by the if-unmodified-since or if-none-match headers is not fulfilled.