(In lecture 20, I introduced HTTP cache control, and in lecture 21, I introduced HTTP proxy service. Then, the combination of the two is the "cache agent" in this lesson, which is a proxy service that supports cache control.When I talked about caching before, I mainly talked about the cache control on the client (browser), which can reduce the response time, save bandwidth and improve the user experience of the client.However, on the HTTP transmission chain, not only the client has cache, but also the cache on the server is very valuable, so that the request does not need to go through the whole subsequent processing process to get the "latest" response result.Especially those data that "read more and write less", such as popular models and popular product details pages, may have thousands of requests per second. Even if only caching for a few seconds, it can block huge access traffic and makeRPS (requests per second) is reduced by several orders of magnitude, which reduces the concurrency pressure of the application server and significantly improves the performance.The caching function of HTTP server is mainly realized through proxy server (i.e. caching proxy). Although various caches (such as Memcache, redis, varnish, etc.) often appear in the source server system, they have nothing to do with HTTP and are not mentioned here)