Web应用架构：原理、协议与实践全览

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《Web应用架构：原则、协议与实践》是一本经典的IT教材，全面而系统地探讨了Web技术的基础和进阶知识。作者站在高屋建瓴的角度，帮助读者理解Web技术的发展历程，从早期的网页到网站，再到现代的Web应用程序，涵盖了Web2.0时代的在线社区和协作，以及AJAX技术的兴起。本书的核心焦点在于Web的核心协议，如TCP/IP架构和HTTP协议，以及HTML、XML、XHTML、Web服务等相关语言和应用。第1章“介绍”简述了Web的历史，从最初的Web页面到网站，再到Web应用程序的兴起，强调了Web2.0时代的新特征和AJAX技术带来的变革。这一章还明确了本书的范围，明确指出读者可以在这里学到的内容，并附有参考文献。第2章“核心互联网协议”深入探讨了TCP/IP协议的背景和历史，阐述了其在互联网中的关键作用。接着讲解了Web如何利用这些协议提供服务，包括HTTP协议的起源和发展。这部分内容对于理解Web服务器如何运作至关重要。第3章“Web诞生：HTTP”详述了HTTP协议的历史演变，包括URL的概念和HTTP的基本原理，以及如何通过头部信息优化信息传输。这一章节提供了丰富的历史视角和协议细节。第4章“HTML及其根源”介绍了HTML的基本概念，它是标准通用标记语言，是构建网页的基础。章节内容涵盖了HTML的渲染过程，使读者明白如何创建可读性强的网页内容。第5章“XML语言和应用”深入剖析了XML的核心、XHTML与Web服务的关系，以及XSL在数据转换中的作用。这章展示了XML在Web开发中的广泛运用，以及如何与其他技术结合。第6章“Web服务器”详细解释了服务器的基本操作，包括动态内容的交付机制，这对于理解Web应用的后端架构至关重要。这一章有助于读者掌握服务器端如何响应客户端请求。《Web应用架构：原则、协议与实践》不仅涵盖了Web技术的基础知识，还深入探讨了它们之间的相互作用和实践应用，是一本适合系统学习和深入研究Web开发的必备参考资料。每个章节的内容都紧密围绕着Web架构的关键组成部分，为读者提供了扎实的技术基础和实际应用指导。

1.1 History and Pre-History of the Web

Back in 1989, at CERN (the scientific research laboratory near Geneva, Switzerland),

Tim Berners-Lee presented a proposal for an information management system that

would enable the sharing of knowledge and resources over a computer network. We

now know this system as the worldwide web (the web). Building on the foundation of

existing Internet protocols and services, it lives up to its name as a ubiquitous network

providing information and communication services to hundreds of millions of people

around the world.

From the very beginnings of Internet technology, many people shared the dream of

using the Internet as a universal medium for exchanging information over computer

networks. Internet file-sharing services (such as FTP and Gopher) and message forum

services (such as Netnews) provided increasingly powerful mechanisms for

information exchange and brought us closer to fulfilling those goals.

Ted Nelson’s Xanadu project aspired to make that dream a reality, but the goals

were lofty and never fully realized. Nelson coined the term “hypertext” as a way of

describing “non-sequential writing - text that branches and allows choice to the

reader.” Unlike the static text of print media, hypertext was intended for use with an

interactive computer screen. It is open, fluid and mutable, and can be connected to

other pieces of hypertext by “links”.

It took Tim Berners-Lee to “marry together” (in his words) the notion of hypertext

with the power of the Internet, bringing those initial dreams to fruition in a way that the

earliest developers of both hypertext and Internet technology might never have

imagined. His vision was to connect literally everything, in a uniform and universal

way. Berners-Lee originally promoted the web as a virtual library, a document control

system for sharing information resources among researchers. On-line documents could

be accessed via a unique document address, a universal resource locator (URL).

These documents could be cross-referenced via hypertext links.

From its humble beginnings, the web has expanded exponentially to serve a wide

variety of purposes for a wide variety of people:

• Given its origins, it seems natural that educational institutions and research

laboratories were among the very first users of the web, employing it to share

documents and other resources across the Internet.

• Popular adoption of the web by individuals followed gradually, originally

through on-line services, such as America On-line (now AOL) that slowly but

surely integrated with the web and the Internet. Initially, personal usage of the

web was limited to e-mail and web surfing. Eventually, people began building

their own web sites where they posted everything from on-line photo albums to

personal journals that would become known as “blogs.”

• Over time, businesses saw the potential of the web and began to employ it for e-

commerce, providing an on-line medium for buying and selling goods

interactively. As more and more people were connected to the web, the draw

of a new source of revenue and the fear that competitors would get there first

and undercut traditional revenue streams made e-commerce increasingly

important to business. E-commerce applications are now being used for

everything - from displaying catalogs of available merchandise to providing the

means for customers to purchase goods and services securely on-line.

• As the impact of e-commerce grew, the back-end applications supporting e-

commerce became increasingly important to the companies using it. The front-

end, customer-facing web sites needed to be up to date, synchronized with

inventory systems so that customers would know what items were or weren’t

immediately available. Automated fulfillment systems connected to the on-line

ordering mechanisms became commonplace. With that, secure login and

registration, including collection of credit card information and payment

processing, became an absolute requirement for viable e-commerce sites.

• With the maturing of the web, a tug of war arose between sites offering free and

paid content. Google was successful in tilting the balance in favor of free

content supported by ads. Even the Wall Street Journal , which pioneered the

notion of paid subscriptions for on-line content, has been opening up more and

more of its content to non-subscribers. Critical technological challenges for

free sites include tracking visitor behavior, providing adaptive personalization,

and offering advanced community-building tools.

The web did not come into existence in a vacuum. It was built on top of core Internet

protocols that had been in existence for many years prior to the inception of the web.

Understanding the relationship between web technology and the underlying Internet

protocols is fundamental to the design and implementation of true web applications. In

fact, it is the exploitation of that relationship that distinguishes a web page or web site

from a web application.

1.2 From Web Pages to Web Sites

The explosive growth of the web at least partially can be attributed to its grass-roots

proliferation as a tool for personal publishing. The fundamental technology behind the

web is relatively simple. A computer connected to the Internet running a web server

was all that was necessary to serve documents. Both CERN and the National Center

for Supercomputer Applications (NCSA) at the University of Illinois had developed

freely available web server software. A small amount of HTML knowledge (and the

proper computing resources) got you something that could be called a web site. Early

web sites were just loosely connected sets of pages, branching hierarchically from a

home page; now, a web site is much more than just a conglomeration of web pages.

When the web was in its infancy, academic institutions and technology companies

owned the only computers that were connected to the Internet and could run server

software. In those days, personal computers sitting on people’s desks were still a

rarity. If you wanted access to any sort of computing power, you used a terminal that

let you “log in” to a large server or mainframe over a direct connection or dial-up

phone line.

Still, creating and posting web pages quickly became popular among people that had

access to scarce computing power. The original HTML language was simple enough

that, even without the more sophisticated tools we have at our disposal today, it was an

easy task for someone to create a web page. (Some would say too easy.) In the end, all

that was needed to create first generation web pages was a simple text editor.

There is a big difference between a bunch of web pages and a web site. A web site

is more than just a group of web pages that happen to be connected to each other

through hypertext links:

• First, there are content-related concerns. Maintaining thematic consistency of

content is important in giving a site some degree of identity.

• There are also aesthetic concerns. In addition to having thematically related

content, a web site should also have a common look and feel across all of its

pages, so that site visitors know what they are looking at. This means utilizing a

common style: page layout, graphic design, and typographical elements.

• Then there are architectural concerns. As a site grows in size and becomes

more complex, it is critically important to organize its content. This includes

not just the layout of content on individual pages, but also the interconnections

between the pages (site navigation). If the site becomes so complex that

visitors cannot navigate their way through, even with the help of site maps and

navigation bars, then it needs to be reorganized and restructured.

1.3 From Web Sites to Web Applications

Initially, what people shared over the Internet consisted mostly of static information

found in files. They might edit those files, but there were few truly dynamic

information services.

Granted, there were a few exceptions: search applications for finding files in on-

line archives and services that provided information about the current weather, or the

availability of cans from a soda-dispensing machine. (One of the first web applications

that Tim Berners-Lee demonstrated at CERN was designed to look up numbers in an

on-line phone book using a web browser.) However, for the most part, the information

resources on the web were static documents.

The advent of the dynamic web, which resulted from the proliferation of dynamic

information services, changed all that. The new services ranged from CGI scripts to

search engines to packages that connected web applications to relational databases. No

longer was it sufficient to build a web site (as opposed to a motley collection of web

pages). It became necessary to design a web application.

What is a “web application?” It is a client-server application that uses a web

browser as its client program. It delivers interactive services through web servers

distributed over the Internet (or an intranet). A web site simply delivers content from

static files. A web application can present dynamically tailored content based on

request parameters, tracked user behaviors, and security considerations.

Web applications power information portals, retail sites, and corporate intranets.

They not only provide information and interact with site visitors, but also collect and

update information, maintain access controls, and support on-line transactions.

A prime example of a dynamic web application is an on-line shopping site. Site

visitors browse on-line catalogs, search for items they want to purchase, add those

items to an on-line shopping cart, and place their orders through a secure interface that

encrypts personal information. To enable this functionality, the web application

communicates with warehouses that maintain catalog and inventory information,

storing this information in a database. Users can search the on-line database to obtain

information about products. The application provides a way for users to select items to

be added to their shopping carts, maintains shopping cart contents for the duration of

the browser session, and records order information in the database. It communicates

with outside financial institutions to verify credit card and address information, fulfills

the order by directing warehouses to ship the purchased items, and sends confirmation

e-mails to purchasers allowing them to track shipment of their orders.

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