HTTP cache, forced cache, negotiated cache

What is caching ？

When the browser loads a web page , You need to load some resources ,html,css,js,img..., All requests are required for the first time , When the second time , The third time I visited this page , In fact, you can not send the request repeatedly , You can use http Cache policy , Make the web page load faster .

Mandatory cache

The browser sends a request to the server , The server should return to the client resource , But the client wants to cache these resources , You need to set the response header cache-control：max-age=31536000, In this way, the browser will cache resources , Check it the next time you ask max-age Has it expired , If it doesn't expire , Just take resources directly , If it's out of date , Just send a request , If you don't use caching, use no-cache

Negotiate the cache

Using negotiation cache , The browser makes a request for the first time , The server will return resources and resources identification , Then put the resources locally , At the next request , Send the request together with the resource ID to the server , The server determines the ID sent and the latest resource ID on the server , If it's the latest , The server returns 304 Status code , Get resources directly from the cache , If it's not the latest , Just go back to 200 Status code and the latest resource and resource identification

         Resource identification ：

         There are two kinds of , One is last-Modified： Last modification time of resource , It's only accurate to seconds , The key name when sending the request is ：if-Modified-Since

         One is ETag： The unique string corresponding to the resource , The key name when sending the request is ：if-None-Match, priority of use ETag