How does 304 Not Modified work exactly

Question

How are  304 Not Modified  responses generated  How does a browser determine whether the response to an HTTP request is 304  Is it set by the browser or sent from the server  If sent by the server  how does the server know the data available in cache  also how does it set 304 to an image    My guess  if it s generated by the browser   function is modified         return get data from cache       get data from url       function get data from cache         return some hash or xxx function cache data      function get data from url          return some hash or xxx function new data      function some hash or xxx function data            Do something with the data          What is that algorithm       return result     console log is modified       I am relying on a third party API provider to get data  parse  amp  push it to my database  The data may or may not change during every request  but the header always sends 200  I do not want to parse  check the last Unique ID in DB and so on    to determine the change in data  nor compare the result directly rather I md5    sha1   and crc32   hashed the result and works fine  but I m wondering about the algorithm to determine 304   I want to use the same kind of algorithm to determine the change in my data

User · Accepted Answer

When the browser puts something in its cache, it also stores the Last-Modified or ETag header from the server.

The browser then sends a request with the If-Modified-Since or If-None-Match header, telling the server to send a 304 if the content still has that date or ETag.

The server needs some way of calculating a date-modified or ETag for each version of each resource; this typically comes from the filesystem or a separate database column.

User · Answer

Last-Modified   The last modified date for the requested object If-Modified-Since   Allows a 304 Not Modified to be returned if last modified date is unchanged  ETag   An ETag is an opaque identifier assigned by a web server to a specific version of a resource found at a URL  If the resource representation at that URL ever changes  a new and different ETag is assigned  If-None-Match   Allows a 304 Not Modified to be returned if ETag is unchanged   the browser store cache with a date Last-Modified  or id ETag   when you need to request the URL again  the browser send request message with the header   the server will return 304 when the if statement is False  and browser will use cache

[http] How does "304 Not Modified" work exactly?

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