Working principle of HTTP protocol: a must-read for front-end

Why does the front-end need to understand the HTTP protocol? When you cooperate with the back-end, you need to access the back-end data interface. At this time, there are two key points that you must understand when connecting with the back-end: HTTP protocol and AJAX technology.

AJAX is a technology that accesses server data.

HTTP protocol: It is a regulation. When we use AJAX technology on the front end to access back-end data, we must use HTTP protocol to request information, so HTTP and AJAX have a very close relationship.

Let's take a look at the HTTP protocol.

1. Development History of Front-end Development

1.1 Server-Side Rendering

Server side rendering (SSR): Early web pages were completed through back-end rendering.

• The client sends a request;
• The server receives the request and returns the corresponding HTML document;
• The page refreshes and the client loads the new HTML document.

shortcoming:

• When a user clicks a button on a page to send a request to the server, essentially only some data on the page has changed, but at this time the server has to return the entire redrawn page to the browser for loading, which obviously goes against the programmer's "DRY (Don't repeat yourself)" principle;
• Moreover, forcing the server to return the entire HTML document when only some data has changed will itself bring unnecessary overhead to the network bandwidth.

2.2 Separation of front-end and back-end

The front end only needs to write the client code independently, and the back end only needs to write the server code independently to provide the data interface. The front end accesses the back end data interface through AJAX requests and displays the Model in the View.

2. Introduction to HTTP

HTTP protocol: Hypertext Transfer Protocol (HTTP).

• It is a simple request-response protocol;
• It usually runs on top of TCP;
• It specifies what messages the client may send to the server and what responses it may receive;
• The headers of request and response messages are given in ASCII form, while the message content has a MIME-like format.
• This simple model was instrumental in the early success of the Web because it made development and deployment extremely straightforward.

III. Development Stage

3.1 0.9

The 0.9 protocol is a simple and fast protocol suitable for various data information. It is an unordered protocol for exchanging information and is limited to text. Since the content cannot be negotiated, the handshake and protocol of both parties do not specify what the content of both parties is, that is, pictures cannot be displayed or processed. Only get requests are supported.

3.2 1.0

It supports request methods such as POST and HEAD, request headers, response headers, and more data types (no longer limited to text data). However, each request from the browser needs to establish a TCP connection with the server, and the TCP connection will be disconnected immediately after the request is processed. Each time a connection is established, performance loss is increased.

3.3 1.1 (currently the most widely used version)

In the 1.0 protocol, both parties stipulated the connection method and connection type, which has greatly expanded the scope of HTTP, added request methods such as PUT and DELETE, and adopted persistent connections (Connection: keep-alive), so that multiple requests can share the same TCP connection.

3.4 2.0

HTTP2.0 is preceded by HTTP1.0 and HTTP1.1. Although there were only two versions before, the protocol specifications contained in these two versions are so huge that any experienced engineer would be troubled.

New versions of network protocols do not immediately replace old versions. In fact, 1.0 and 1.1 coexisted for a long time afterwards, which was determined by the slow update of network infrastructure.

4. Working Principle

HTTP is based on the client/server model and is connection-oriented.

A typical HTTP transaction process is as follows:

• The client establishes a connection with the server;
• The client makes a request to the server;
• The server accepts the request and returns the corresponding file as a response;
• The client and server close the connection.

characteristic:

• The HTTP connection between the client and the server is a one-time connection. It limits each connection to only one request. When the server returns the response to this request, it immediately closes the connection and re-establishes the connection for the next request. This one-time connection is mainly considered that the WWW server faces thousands of users on the Internet and can only provide a limited number of connections. Therefore, the server will not keep a connection in a waiting state. Releasing the connection in time can greatly improve the execution efficiency of the server.
• HTTP is a stateless protocol, that is, the server does not retain any state when transacting with the client. This greatly reduces the memory burden of the server, thereby maintaining a faster response speed. HTTP is an object-oriented protocol. It allows the transmission of any type of data object. It identifies the content and size of the transmitted data by data type and length, and allows compressed transmission of data. When a user defines a hypertext link in an HTML document, the browser will establish a connection with the specified server through the TCP/IP protocol.
• HTTP supports persistent connections, in HTTP/0.9 and 1.0, the connection is closed after a single request/response pair. In HTTP/1.1, a keep-alive mechanism was introduced, where the connection can be reused for multiple requests. Such persistent connections can significantly reduce request latency, because the client does not need to renegotiate the TCP 3-Way-Handshake connection after sending the first request. Another positive side effect is that, in general, the connection becomes faster over time due to TCP's slow start mechanism.

5. Operation

HTTP is based on a request/response paradigm.

After a client establishes a connection with a server, it sends a request to the server in the format of a uniform resource identifier, a protocol version number, followed by MIME information including request modifiers, client information, and possible content. After receiving the request, the server gives the corresponding response information in the format of a status line including the protocol version number of the information, a success or error code, followed by MIME information including server information, entity information, and possible content.

In fact, to put it simply, any server, in addition to HTML files, also has an HTTP resident program to respond to user requests. Your browser is an HTTP client that sends requests to the server. When you enter a start file in the browser or click a hyperlink, the browser sends an HTTP request to the server. This request is sent to the URL specified by the IP address. The resident program receives the request and sends back the requested file after performing necessary operations.

In this process, the data sent and received on the network has been divided into one or more packets. Each packet includes:

• The data to be transmitted;
• Control information, which tells the network how to handle the data packet.

TCP/IP determines the format of each data packet. If you were not told in advance, you would not know that the information is divided into many small pieces for transmission and then reassembled.

Most HTTP communications are initiated by a user agent and involve a request for a resource on an origin server. In the simplest case, this is probably done over a single connection between a user agent (UA) and an origin server (O).

The situation becomes more complicated when one or more intermediaries appear in the request/response chain.

There are three types of intermediaries: Proxy, Gateway, and Tunnel.

• A proxy accepts requests based on the absolute format of the URI, rewrites all or part of the message, and sends the formatted request to the server using the URI identifier.
• A gateway is a receiving proxy that acts as a layer above some other server and can translate requests to the underlying server protocol if necessary.
• A channel acts as a relay point between two connections without changing the message. Channels are often used when the communication needs to go through an intermediary (such as a firewall) or when the intermediary cannot understand the content of the message.