How “304 Not Modified” works?

问题

• How \"304 Not Modified\" is generated?

• How does a browser determine whether the response to a http request is 304?

• Is it set by the browser or sent from the server?

• If sent by server, how does server know the data available in cache, also how does it set 304 to an image?

My guess, if generated by 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 data
  // what is that algorithm.?
      return result;
}

console.log(is_modified());

I am relying on 3rd party API provider to get data, parse & push it to DB. 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 & so on.. to determine the change in data, nor compare the result directly rather I md5(), sha1() & crc32() HASHed the result & works fine, but wondering the algorithm to determine 304.

I want to use same kind of algorithm to determine the change in data.

回答1:

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.

回答2:

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.

来源：https://stackoverflow.com/questions/20978189/how-304-not-modified-works