Web (1)

1. The Web has three main semantic components:
A. Uniform Resource Identifier (URI): A Universal Naming mechanism used to identify resources on the Web
URI: http://www.foo.com/coolpic.gif
In informal cases, a URI is usually composed of three parts: the server name used for communications with the server (for example, http?##( (( for example, www.foo.com#and the server Resource Name (for example, coolpic.gif)
The most popular URI format is uniform resource location address (URL). The URI and URL concepts are different.
B. Hypertext Markup Language (HTML): standard language for generating hypertext documents
C. Hypertext Transfer Protocol (HTTP): the communication language between the Web client and the server
Http is a stateless protocol, that is, the client and server process message exchange independently and do not require any status information to be maintained during requests and responses.

Each HTTP transmission consists of two messages: the request message sent by the client and the Response Message from the server.
The client is the sender of the Request Message and the receiver of the Response Message;
The server is the sender of the Request Message and the Response Message;
The sender of a message is the upstream of the receiver, and the receiver is the downstream of the sender.

2. WEB software components
The three main WEB software components are: client, proxy, and server.
1) User Agent: generate an HTTP request and process the response. The most visible user agent is a WEB browser, which indicates that the user generates the request and executes a series of tasks, such as displaying WEB pages and storing users' bookmarks.
2) WEB Client: a program that sends an HTTP request to the WEB server
3) original server: a program that provides or generates WEB resources. A server is essentially a program that receives HTTP requests and sends responses. In practical applications, the client may send an HTTP request to the intermediate media-another WEB component, such as a proxy or gateway, on the way to the original server,
4) intermediate media: WEB components on the path between the client and the original server
5) proxy: the proxy acts as a server in front of the client and acts as an intermediate program in front of the server. The proxy can execute a series of functions, such as filtering requests to unwanted WEB sites, making the client "invisible" to a certain extent, and caching popular resources.
6) cookie: status information transmitted between the client and the original server

3. Communication between the WEB client and the server
The WEB Client identifies the WEB server with a host name (such as www.att.com/users.berkely.edu) instead of an IP address. The two applications exchange HTTP messages instead of IP packets. The Domain Name System (DNS) and Transmission Control Protocol (TCP) implement these two functions. Before connecting to the WEB server, the WEB Client first sends a system call to the DNS server. The DNS server returns the IP address www.att.com. With this IP address, the WEB Client starts communication with the WEB server. Then, the client establishes a TCP connection with the server. It is a logical channel that provides two-way communication for two applications. It is implemented using the operating system of the two hosts, TCP hides the details of sending and receiving data over the internet. Once the connection is established successfully, the client can use a TCP connection to send an HTTP request message to the server. The server can also use an HTTP Response Message to answer the question.
A browser is a WEB client that allows you to create and send HTTP requests, and then receive, analyze, and display responses.

4. terms related to WEB traffic and Performance
1) latency: the time between an action and the first response
2) latency perceived by the user: the time between the time when the user action is sent to the initial display of the content
3) bandwidth: the amount of traffic that can be transmitted per unit of time
4) workload or load: the total input volume received by WEB components within a period of time

5. WEB Application Related Terms
1) high-speed cache: it is used to reduce users' perceived latency and store messages for servers and network loads, the cache can be located on the user's browser, on the original server, or on the machine on the path between the user and the original server [on the intermediate machine]
2) high-speed cache consistency: although the high-speed cache can improve the performance, the response content of the high-speed cache may be different from the version obtained on the original server, the cache consistency mechanism is used to ensure that the content in the cache is up-to-date with that on the original server, and to reduce the possibility of returning expired messages from the cache.