200 OK (from memory cache) and 200 OK (from disk cache)

200 OK（from memory cache） Memory cache

No access to the server , Read cache directly from memory .

At this time, the obtained data is cached in memory , When kill After the process （ Browser closed ）, The data will not exist （ Resources have been released ）.

If you open the page again , There will be no 200 OK（from memory cache）.

characteristic

1. Memory cache capacity is small
2. Fast read ： The memory cache will compile the parsed file , Directly into the memory of the process , Occupy a certain memory resource of the process , In order to facilitate the next run-time use of fast reading
3. timeliness ： Very short cache timeliness , Will be released as the process is released

200 OK（from disk cache） Hard disk cache

No access to the server , Read cache directly from disk , When kill After the process （ Close the browser ）, The data still exist .

The next time you open the page, it's still from disk cache.

principle

The hard disk cache directly writes the cache to the hard disk file , To read the cache, the hard disk file stored in the cache should be I/O operation , Then re parse the cache content , Read complex , Slower than memory cache .

Capacity

Large hard disk cache capacity

304 Not Modified

Access server , Found that the data is not updated , The server returns this status code . then , Read data from cache .

Comparison of several States

state	type	explain
200	form memory cache	Do not request network resources , The resource is in memory , General script 、 typeface 、 The picture will be stored in memory
200	form disk ceche	Do not request network resources , In the disk , Generally, non scripts are stored on the disk , Such as css etc.
200	Resource size value	Download the latest resources from the server
304	Message size	The requesting server found that the resource was not updated , Using local resources

Style sheets are usually on disk , It won't be cached in memory , because CSS Style loading once can render the web page . however , The script may be executed at any time , If the script is on disk , To execute this script, you need to get it from disk to memory . such IO It costs a lot , May cause the browser to lose response . therefore , Scripts are usually in memory .

For large files , The high probability is not stored in memory , Whereas the priority . Current system memory usage is high , Files are stored in the hard disk first .
In the browser , The browser will be in js And pictures and other files are directly stored in memory cache after parsing , When refreshing the page, you only need to read it directly from the memory cache (from memory cache); and css The file will be stored in the hard disk file , So every time you render a page, you need to read the cache from the hard disk (from disk cache).

Three level cache principle

1. Look for memory first , If there is , Load directly from memory

2. If there is no memory , Then go to the hard disk for , If there is a direct load

3. If there's no hard disk , Then make a network request

4. The loaded resources are cached to the hard disk and memory

The browser runtime is coordinated by several processes , So the operating system to save memory , Will exchange a part of the memory resources back to the disk swap area , Of course, the exchange is strategic , For example, the most commonly used one is LRU. So when the cache resource does not expire , If the resource is in memory, then from memory, If only on disk from disk.

Example ：

Load a picture resource sequence ：

visit -> 200 -> Exit browser

Once again into the -> 200(from disk cache) -> Refresh -> 200(from memory cache)

prohibit 200 Caching method

prohibit 200 OK (from disk cache) The cache method is ,ajax The request is to take parameters cache: false.