The process of browser accessing the website

The process of browser accessing the website ：
1、DNS analysis ：
（1） Why DNS analysis ？
Domain name resolution is to point the domain name to the website space IP, Let people through the registered domain name can easily access a website service . For websites , After the process of domain name resolution , Netizens can easily and conveniently visit the website through this domain name . If it is not parsed , The domain name cannot be used normally .

（2） System cache query
（3） Router cache 、ISP cache
（4）DNS recursive query

2、TCP Connect ：
（1） Three handshakes
（2） Four breakups

3、HTTP request ：

Once complete http Request processing ：
（1） Establishing a connection ： Receive or reject connection requests

（2） Receiving request ： The process of receiving a request for a resource in a client request message

（3） Processing requests ： The server parses the request message , And obtain the requested resource and request method and other related information , According to the method , resources , The header and optional body parts handle the request
Common requests Method: GET、POST、HEAD、PUT、DELETE、TRACE、OPTIONS

（4） Access resources ： The server obtains the resource requested in the request message web The server , That is, it's stored web Resource server , Responsible for providing static resources requested by the other party to the requester , Or resources generated after dynamic operation

（5） Build response message ： once Web The server identifies resources in addition to resources , To perform the actions described in the request method , And return the response message . The response message contains the response status code 、 Response head , If the response body is generated , It also includes the response body
1） Response subject ： If the transaction produces a response body , Put the content in the response message and send it back . The response message usually includes ： Describes the response body MIME Type of Content-Type The first one 、 Describes the length of the response body Content-Length、 The main content of the actual message
2）URL Redirect ：web The response of the service build is not a resource requested by the client , It's another access path to the resource
3）MIME type ： Web The server is responsible for determining the response subject MIME type . There are many ways to configure the server MIME Type and resource management

（6） Send response message ：Web When the server sends data through the connection, it also faces the same problems as receiving data . The server may have many connections to various clients , Some are idle , Some are sending data to the server , Some are sending back response data to clients . The server records the status of the connection , Special attention should also be paid to the handling of persistent connections . For non persistent connections , The server should send the whole message after , Close the connection at this end of yourself . For persistent connections , The connection may remain open , under these circumstances , The server should calculate correctly Content-Length The first one , Otherwise, the client will not know when the response ends

（7） Log ： Last , When the transaction ends ,Web The server will add an entry to the log file , To describe the executed transaction

HTTP The status code is used to represent the web server HTTP Responding to the state of 3 Digit code .

common HTTP Status code ：
200 （ success ） The server has successfully processed the request . Usually , This means that the server provides the requested web page .
301 （ A permanent move ） The requested page has been permanently moved to a new location . The server returned this response （ Yes GET or HEAD Response to the request ） when , The requester is automatically moved to a new location .
302 （ Temporary movement ） The server is currently responding to requests from pages in different locations , But the requester should continue to use the original location for future requests .
400 （ Wrong request ） The server does not understand the syntax of the request .
403 （ prohibit ） Server rejects request .
404 （ Not found ） The server could not find the requested page .
500 （ Server internal error ） Server encountered an error , Unable to complete request .
502 （ Bad Gateway ） Server as gateway or proxy , Invalid response received from upstream server .
503 （ Service not available ） The server is currently unavailable （ Maintenance due to overload or shutdown ）. Usually , It's just a temporary situation .
504 （ gateway timeout ） Server as gateway or proxy , But the request was not received from the upstream server in time .

4、 The rendering principle of browser ：
（1） structure DOM
（2） structure CSSOM Trees
（3） Build the render tree
（4） synthesis 、 draw

Browser workflow ：
（1） The browser first uses HTTP Agreement or HTTPS agreement , Request a page from the server
（2） Come back with the request HTML The code is parsed , To build a DOM Trees
（3） Calculation DOM On the tree CSS attribute
（4） according to CSS Attribute to render elements one by one , Get the bitmap in memory
（5） An optional step is to synthesize the bitmap , This will greatly increase the speed of subsequent painting
（6） After synthesis , Then draw it on the interface .

The responsibility of the browser process ： Mainly responsible for user interaction 、 Sub process management and file storage
Responsibilities of network process ： Provide network download function for rendering process and browser process
Responsibility of rendering process ： Download from the Internet HTML、JavaScript、CSS、 Pictures and other resources are parsed into pages that can be displayed and interacted