Browser cache execution process

HTTP Fields related to page caching in the protocol , Let's meet first ：

header	explain
Expires	Cache expiration date and time
Cache-Control	Set cache related configuration information
Last-Modified	Last modification time of the requested resource
ETag	The current value of the entity tag of the request variable , For example, the file MD5 value

（1） The user sends a request to the server through the browser for the first time to obtain data , The client has no corresponding cache , So you need to send request Request to get data ;

（2） After the server receives the request , After obtaining the data of the server and the permission of the server cache , return 200 And attach the corresponding resources and cache information on the response header ;

（3） When the user accesses the same resource again , The client will search the cache directory of the browser for the existence of the response cache file

（4） If the corresponding cache file is not found , Then go (2) Step

（5） If there is a cache file , Next, judge whether the cache file is expired , The criterion of expiration is (Expires),

（6） If it doesn't expire , Then directly return data from the local cache for display

（7） If Expires Be overdue , Next, you need to determine whether the cache file has changed

（8） There are two criteria for judging , One is ETag(Entity Tag), One is Last-Modified

（9） The judgment result is that there is no change , The server returns 304, Get data directly from the cache file

（10） If the judgment has changed , Get data from the server again , And negotiate according to the cache ( Whether the server needs to set the cache data ) To cache data .