HTTP Conditional Request

Conditional Request If-modified-since/if-unmodified-since/if-none-match/if-match/if-range for HTTP

• If-modified-since
  is a standard HTTP request header tag that, when sending an HTTP request, sends the last modification time (last-modified) of the browser-side cache page to the server, which compares the time to the last modification time of the actual file on the server.
  If the time is consistent, then return the HTTP status Code 304 (do not return the contents of the file), after the client receives, the local cache file is displayed directly to the browser.
  If the time is inconsistent, the HTTP status code 200 and the new file contents are returned, and when the client receives it, it discards the old file, caches the new file, and displays it in the browser.

• If-unmodified-since
  When sending an HTTP request, the last modification time (last-modified) of the browser-side cache page is sent to the server, and the server compares the time to the last modification time of the actual file on the server.
  If time is consistent, return to HTTP status Code 200 and start transferring files
  Returns 412 Precondition failed (preprocessing error) if time is consistent
• If-none-match
  When sending an HTTP request, the server previously sent the ETag, and the server responded to the etag to determine whether the change
  Returns 304 if the ETag is the same
  If the etag is different, response 200 returns the request file
• If-match
  When sending an HTTP request, the server previously sent the ETag, and the server responded to the etag to determine whether the change
  If the etag is the same, response 200 returns the request file
  If the etag is different, then you need to return 416 (range not satisfiable, the scope request cannot be satisfied)
• If-range
  If the response data is not changed, the server sends the missing portion of the client, otherwise the entire data is sent.

HTTP Conditional Request