Polling and connection

Polling and long polling

polling ： The client requests the server regularly ,   The client actively requests to update the data

• It consumes a lot of server memory and broadband resources , Because the server keeps requesting , A lot of times No new data updates , Therefore, most requests are invalid requests
• Data is not necessarily updated in real time , It depends on the set request interval , Basically there will be delays .

Long polling ： The client requests the server , But the server does not return , Instead, the content is returned to the client only when it is updated

• Solved two major problems of polling , Real time data update ;
• The only disadvantage is that the server consumes more memory when it is suspended .

A long connection 、 Short connection ？

stay HTTP/1.0 Short connection is used by default in . in other words , Client and server every time HTTP operation , Just one connection , Disconnect at end of task . When a client browser accesses HTML Or other types Web The page contains other Web resources （ Such as JavaScript file 、 image file 、CSS Documents, etc. ）, Every time I meet such a Web resources , The browser will recreate a HTTP conversation .

And from HTTP/1.1 rise , Use long connection by default , To maintain connection characteristics . Using long connected HTTP agreement , Will be added to the response header

Connection:keep-alive

With long connections , When a web page is opened , Transport between client and server HTTP Data TCP Connection will not close , When the client accesses this server again , Will continue to use this established connection .Keep-Alive Not permanently connected , It has a hold time , Different server software is available （ Such as Apache） Set this time in . To realize long connection, both client and server need to support long connection .

HTTP Long connection and short connection of protocol , Is essentially TCP Long connection and short connection of protocol .

In a long connected application scenario ,client Generally, the terminal will not actively close the connection , When client And server The connection between has not been closed , With more and more client connections ,server Will keep too many connections .