Understanding of strong caching and negotiation caching

cache It refers to the resource copy saved in the proxy server or client disk . Using the cache can reduce access to the server , Therefore, the communication flow and communication time are saved .
Browser caching is to save resources , And it can speed up the speed of browsing the website , When a user visits a web page for the first time , The browser saves the requested document on the user's disk , When the second visit , It will be displayed directly from the local , Thus, the access speed of the page is improved .
Advantages of browser caching ：

1. Reduced redundant data transfer , Save network fee ;
2. Reduced server burden , Greatly improve the performance of the website ;
3. Speed up client loading web pages ;

Browser cache is divided into Strong cache and Negotiate the cache .

• When the browser makes a resource request , It will determine whether the request header hits the strong cache , If hit , Read cache directly from local , No request is sent to the server .
• When the strong cache fails to hit , Will send a request to the server , Determine whether the negotiation cache hit , If hit , The server will return the request , No resources will be returned , Tell the browser to read the cache locally ; If you don't hit , The server directly returns the resource .

difference ：
Strong cache hit , The server will not be requested , Direct request cache ; Negotiate cache hits , Will request the server , No content will be returned , Then read the cache .

from memory cache and from disk cache The difference between ：

from memory cache： Literally from memory , This resource is taken directly from memory , The server will not be requested , Generally, the resource has been loaded and cached in memory , When you close the page , The resource is freed by memory , Reopening the same page again is , There will be no from memory cache The situation of .
from disk cache： This resource is taken from the disk , This resource was also loaded at a certain time before , The server will not be requested , However, this resource will not be released when the page is closed , Because this resource is stored in the hard disk , Next time you open it, it will still from disk cache.

One 、 Strong cache

Strong cache , utilize HTTP Return to... In the head expires and cache-control Two fields to control , The cache time used to represent the resource （ Validity of cache ）.
This method will have a fixed time , But if the server data is updated , If the validity period of the strong cache has not expired , The data cannot be updated .

Expires

The field is http1.0 The norm of time , Its value is GMT Absolute time of format , When the browser makes the first request , The server will add... To the return header Expires, Next time request , If you hit the cache before this time .

Cache-Control

This value is based on max-age Determine the cache life cycle , Is in seconds , If in the life cycle time , Then hit cache .

Two 、 Negotiate the cache

Negotiate the cache , It is up to the server to determine whether cache resources are available , Therefore, the client and the server should communicate through some kind of identity , So that the server can judge whether the request resource can be accessed by cache .
utilize Last-Modified,If-Modified-Since and ETag,If-None-Match To achieve .

Last-Modified,If-Modified-Since

Last-Modified： Represented as the head part of the entity ,response return , Indicates the last update time of the resource ;
If-Modified-Since： By comparing the two times, we can determine whether the resource has been modified during the request , If not modified , Cache negotiation hits , The browser reads resources from the cache , If you don't hit , Resource has been modified , Returns a new Last-Modified Time and server resources .

ETag,If-None-Match

In some cases, it is not enough to judge the last modification date to verify whether the resource has been changed ：

1. There is a periodic rewrite of some resources , However, the actual content of resources has not changed ;
2. The modified information is not important , Such as notes, etc ;
3. Last-Modified Cannot be accurate to milliseconds , However, the update frequency of some resources is sometimes less than one second .

ETag： In response to the header field , A unique identifier that represents the content of a resource , With the server response return ;
If-None-Match： The server compares... In the request header If-None-Match And... In the current resource etag Is it consistent , To determine whether the resource has been modified , If not modified , Then hit cache , The browser reads resources from the cache , If it has been modified , The server will return a new etag, And return the resource .

The first time the browser requests ：

When the browser makes a subsequent request ：

The difference between strong cache and negotiation cache ：

Cache type	Access to resources in the form of	Status code	Send request to server
Strong cache	Fetch... From cache	200（from cache）	no , Get directly from the cache
Negotiate the cache	Fetch... From cache	304（Not Modified）	yes , Let the server know if the cache is available