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Web Servers

It has been argued that the most instrumental factor in the growth of the Internet was the capability to graphically and universally serve up content—more specifically, the web server. This implies the software that provides the function to serve documents (standardized according to the Hypertext Markup Language, or HTML) via the Hypertext Transport Protocol (HTTP) to clients that are standardized to receive it (web browsers). As mentioned earlier, by far the most successful and widely used web server software is Apache, an open source implementation available for multiple platforms, including Linux and NetWare. Apache is part of the standard SUSE Linux distribution and is used extensively as part of most Linux implementations.

The entire category of web applications is huge and could be the subject of several books. This section outlines a simple overview with several possible considerations when moving to Linux and open source. Web server implementations generally fall into two categories: static and dynamic.

Every web server includes a root HTML directory, often the www/, html/, or htdocs/ directory by default. Files placed in this directory are served up when an HTTP request comes in from a web browser. If no specific file is requested, the index.html file is served. For static websites, creators will design the index.html file as the home page with links to other pages and graphics. Putting together a website can be as simple as creating a few HTML pages and copying them to the htdocs directory. If the server machine is on the network, using a browser to connect via the IP address renders the pages visible.

Apache can service websites from simple to very complex. Often, a web page (such as index.html) is a collection of scripts, processes, or objects that dynamically generate content and images. These page elements can be generated or processed at the browser, the server, or may be the result of multilayer processes and links distributed across hundreds of other web or application servers. Many of today's websites are complex aggregations of content, links, applications, and data streams from far-flung, multilayer sources. Apache can accommodate all of these (see Figure 3.11).

Figure 3.11Figure 3.11 A web server delivers pages that include multiple types of elements to a web browser.

Linux and Apache are the core elements of a basic web service. Combining these with other services, utilities, and tools is the difference between static sites and dynamic sites. Apache functionality can be expanded significantly through the use of additional programming, database, and customized modules. Some of these include the following:

  • Secure transmission—Using SSL, any communication or transmission between a client and the web server can be securely encrypted.

  • Authentication—With directory services such as Lightweight Directory Access Protocol (LDAP) and Novell eDirectory, access to specific web-based resources can be tightly controlled so that only authorized users have access.

  • Virtual hosting—A single instance of Apache on a single machine can host multiple websites with the single web server appearing as several independent web servers. Virtual hosts can be configured on the basis of different IP addresses or different hostnames.

  • Active contents—Active contents are HTML pages that are generated on the basis of variable input from the client. Apache offers three ways of generating active contents:

    • Server Side Includes (SSI)—These are directives that are embedded in an HTML page by means of special comments. Apache interprets the content of the comments and delivers the result as part of the HTML page.

    • Common Gateway Interface (CGI)—These are programs that are located in certain directories. Apache forwards the parameters transmitted by the client to these programs and returns the output of the programs. Programs are generally written in scripting languages, but can be in C, C++, or any other language.

    • Modules—Apache offers interfaces for executing any modules within the scope of request processing. Modules including scripting language modules for Perl, Python, PHP, and Ruby are available that work with Apache for the creation of any customized application or utility.

  • Content negotiation—Apache can deliver page content adaptable to the capabilities of a particular client. Browser-based clients can be phones, pagers, text-only devices, or full-blown PC-based browsers—Apache can determine client type and capabilities and customize content accordingly. Apache can also negotiate with the client to determine in what language the browser expects to receive content and deliver it accordingly.

In addition, Apache works in conjunction with the wide range of application servers that are enabling a new generation of web-accessible applications. These range from XML connectors that pull information from legacy applications and expose it for access, to new Java-based line-of-business applications, to web-based database front ends. Application server solutions are available from independent software vendors (ISVs), such as IBM (WebSphere), BEA (WebLogic), and others, as well as open source projects such as Jakarta Tomcat.

Web servers are "information doors" that can be elaborate, eye-catching, and complex or basic, text-only, and simple. They can also be found in many locations throughout an organization with open access to the outside or controlled and hidden access from the inside. Web servers are used to expose or publish content and provide access to a wide array of services. Think of what you access on a regular basis that is published via a web server. Content can include email, news, documentation, product information, catalogs, and shopping sites. Web servers facilitate full user portals with instant messaging, online calendaring, and scheduling, but can also be small, supporting a simple interface to a router. Increasingly, enterprise software applications, such as customer resource management (CRM), enterprise resource planning (ERP), human resources (HR), and financial systems are incorporating web-based user interfaces that expose rich content to a wide audience of employees, managers, partners, customers, and suppliers. Web servers are everywhere.

Implementing web servers based on Linux and Apache can be a gradual and incremental process that starts with peripheral services and applications and moves to core data center operations. Common first uses include providing static web content internally to an intranet. This might include company resource manuals or directories, project sites, or bulletin boards.

Next phases often include exposing database content, which can be anything from product listings or budget and finance information to internal groups. Organizations can easily create web-based database applications using open source databases, such as MySQL and PostgreSQL, and scripting tools, such as PHP and Perl. In addition, many existing open source business applications have been prebuilt based on Linux, Apache, MySQL, and PHP/Perl/Python. These are commonly referred to as LAMP solutions and are available as web-accessible business and productivity solutions.

As needs for content become more complex, application servers and custom-built applications are integrated. Depending on your organization's needs, time frame, and budget, you have the option of developing custom solutions in-house, working with consulting and development partners, or purchasing ready-made web-accessible enterprise solutions.

Web-based management tools are becoming a mandatory requirement for data centers and IT organizations. Novell's highly popular iManager aggregates all types of management utilities and functions to a common portal-like interface. Administrators are provided a dashboard view of entire operations with drill-down management capability in any one particular area.

In all cases, web servers are a key component of the solution, and Apache is the web server of choice. Apache enjoys the largest market share, and as a result provides the greatest degree of flexibility, compatibility, and security.

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