Websocket of Web real-time communication technology

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  “ The article is right Websocket Introduced and summarized ( contain python Programming code ).”

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Lead to read

• Why introduce Websocket?
• —— Because in HTTP In , The server cannot send a message to the client Take the initiative to push Updated resources
• PS：HTTP/2 Only static resources can be pushed ( Preset resources ), Unable to push real-time updated information
• HTTP—— Connection is established - Client request - Server response ( If there is no request, the server will not respond )
• Websocket—— Implement client - Server two-way peer-to-peer communication ( Both the server and the client can actively send messages )

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• Full duplex protocol ( full duplex —— Communication data can be transmitted in both directions at the same time , Half duplex —— Communication data can only be transmitted in one direction at the same time , Simplex —— Communication data can only be transmitted in one direction )
• Websocket—— Full duplex protocol
• Typical applications ： Barrage system

Websocket： The host port is occupied for a long time

HTTP request ： Listen to the port and transmit information through the port

• Tradition HTTP How to update information ：

• polling / A long connection

• Many websites are trying to implement push technology , The technology used is polling . Polling is done at specific intervals （ As per 1 second ）, Issued by the browser to the server HTTP request , The server then returns the latest data to the client's browser . This traditional model brings obvious disadvantages , That is, the browser needs to keep making requests to the server , However HTTP Requests may contain long headers , The amount of data that actually works is probably only a tiny fraction of that , Obviously this will waste a lot of bandwidth and other resources .

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Websocket Introduce

Basics

• Websocket And HTTP and HTTPS Use the same TCP port , You can bypass most firewalls . By default ,Websocket Agreement to use 80 port ; Running on the TLS When above , By default 443 port .

• establish WebSocket Connect

  • The client browser sends a message to the server HTTP request ( Request containing updated header information )

  • Server side parsing —— An answer message is generated to indicate that the upgrade is successful

  • Connection established successfully —— End to end communication —— Until a party initiates a shutdown request

• In the handshake ,Websocket Than HTTP One more handshake , Update the information ( Server response status code 101 Express ), As shown in the figure below

• Client message ：

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• Server message ：

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• Connection You have to set Upgrade, Indicates that the client wants to upgrade the connection .
• Upgrade Field must be set Websocket, I want to upgrade to Websocket agreement .

• Origin： For safe use , Prevent cross station attacks , Browsers generally use this to identify the original domain .

Python Websocket build

• Server setup code ：

import asyncioimport websocketsasync def echo(websocket, path):async for message in websocket:message = "I got your message: {}".format(message)await websocket.send(message)asyncio.get_event_loop().run_until_complete(websockets.serve(echo, '127.0.0.1', 8766))asyncio.get_event_loop().run_forever()

• Client build code ：

import asyncioimport websocketsasync def echo(uri):async with websockets.connect(uri) as websocket:while True:message = input("Write down your message:")await websocket.send(message)print("<", message)recv_text = await websocket.recv()print("> {}".format(recv_text))asyncio.get_event_loop().run_until_complete(echo('ws://127.0.0.1:8766'))

• The client connects to the server , After the server accepts the connection, both parties establish an end-to-end connection TCP Connect , Two way communication can be realized on this connection , After establishing this connection , There is no distinction between client and server between the two sides of communication , What is provided is End-to-end communication .

Websocket VS HTTP

• Websocket Cannot connect through proxy , Must be connected directly
• Websocket After establishment , Both sides of communication can actively communicate at any time
• Websocket After establishment, information is transmitted through data frames , Do not use request - Response structure
• Websocket Data frames are in order

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advantage

• Less control overhead . After the connection is established , When data is exchanged between server and client , The packet header used for protocol control is relatively small . Without the extension , For server to client content , This head is only the size of 2 to 10 byte （ Related to packet length ）; For client to server content , This head also needs to be added with extra 4 Byte relative to HTTP Request to carry the full head every time , This cost significantly reduces .

• Strong real time ： Because the protocol is full duplex , So the server can send data to the client at any time . be relative to HTTP The request needs to wait for the client to send the request to the server to respond , Delays are significantly less ; Even with Comet And so on Long polling Compare , It can also transfer data more than once in a short time .

• Better binary support .Websocket Defined binary frame , relative HTTP, Binary content can be handled more easily .

• Support extended protocols .Websocket Defined extensions , The user can extend the protocol 、 Implement some custom sub protocols . For example, some browsers support compression .
• Stateful protocol ( contrast HTTP No state ). And HTTP Different ,Websocket You need to establish a connection first , After that, some state information can be omitted during communication . and HTTP The request may need to carry status information in each request ( Such as cookies etc. ).
• WebSocket Not limited by browser homology policy
• WebSockets It is suitable for applications with high real-time requirements 、 The chat room 、 Multiplayer game 、 Real time document collaboration, etc

shortcoming

• The long-term maintenance of the server requires high costs
• Under the high concurrency scenario, the requirements for both sides of communication are high
• It occupies multiple process ports and is limited by general browsers ( for example ：firefox by 200)
• WebSocket The handshake phase is prone to security problems (Hijacking)

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Reference material

 https://zh.m.wikipedia.org/zh-hans/WebSocket

 http://www.52im.net/thread-1266-1-1.html

 Websocket Online testing sites ( Pay attention to the guarantee )

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