HTTP cache ＜ strong cache and negotiation cache ＞

HTTP cache

HTTP The basic operation of caching

1. The browser checks whether it has a cache , If not, ask the server
2. If the browser has , You need to check whether the cache has expired
3. Use if it doesn't expire , Verify with the server when it expires
4. The server tells you that you can continue to use it without expiration , The server tells you that it has expired to get a new one

in general ： Browser first check , Let's see if the browser itself is expired , Then check with the server to see if it is expired

Strong cache

The server sends... Through the response header Catch-Control, Set the expiration time of the resource

1. public            These resources can be cached by any object （ Such as ： client , proxy server , wait ）
2. private           These resources can only be cached on the client 
3. max-age=xx        Set the maximum time for resource caching , Time out indicates that the cache has expired , The request is sent to the server （ Company ： second ）
4. no-cache          Before loading cache resources , Force a request to be sent to the server “ Negotiate the cache ”
5. no-store          Is not cached at all 

Negotiate the cache

The server returns the resource and resource ID , When the client sends a subsequent request , Send the request and resource ID to the server , If not the latest resources , Server return 200、 Latest resource and latest resource identification ; If it is the latest resource , Then the server returns 304, Get resources directly from the cache .

Resource identification ：

• Last-Modified： The time when the resource was last modified （ understand ）
• ETag： The resource corresponds to a unique string

ETag： Requests initiated on the client , The request header will carry if-none-match, The value is... Of the last response ETag, The server will ETag Contrast , Same back 304, Just get it from the local cache .

difference

The main difference between the two is ： Strong caching does not make requests to the server , The negotiation cache will