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Middleware and XML?><html><head><title>Middleware and XML (CGI Programming with Perl)</title><link href="../style/style1.css" type="text/css" rel="stylesheet" /><meta name="DC.Creator" content="Scott Guelich, Gunther Birznieks and Shishir Gundavaram" /><meta scheme="MIME" content="text/xml" name="DC.Format" /><meta content="en-US" name="DC.Language" /><meta content="O'Reilly & Associates, Inc." name="DC.Publisher" /><meta scheme="ISBN" name="DC.Source" content="1565924193L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="CGI Programming with Perl" /><meta content="Text.Monograph" name="DC.Type" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" alt="Book Home" usemap="#banner-map" border="0" /><map name="banner-map"><area alt="CGI Programming with Perl" href="index.htm" coords="0,0,466,65" shape="rect" /><area alt="Search this book" href="jobjects/fsearch.htm" coords="467,0,514,18" shape="rect" /></map><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch13_05.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm">CGI Programming with Perl</a></td><td width="172" valign="top" align="right"><a href="ch14_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><hr align="left" width="515" /><h1 class="chapter">Chapter 14. Middleware and XML</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4><p><a href="ch14_01.htm">Communicating with Other Servers</a><br><a href="ch14_02.htm">An Introduction to XML</a><br><a href="ch14_03.htm">Document Type Definition</a><br><a href="ch14_04.htm">Writing an XML Parser</a><br><a href="ch14_05.htm">CGI Gateway to XML Middleware</a><br></p></div><p>CGI programming has been used to make individual web applicationsfrom simple guestbooks to complex programs such as a calendar capableof managing the schedules of large groups. Traditionally, theseprograms have been limited to displaying data and receiving inputdirectly from users.</p><p>However, as with all popular technologies, CGI is being pushed beyondthese traditional uses. Going beyond CGI applications that interactwith users, the focus of this chapter is on how CGI can be a powerfulmeans of communicating with other programs.</p><p>We have seen how CGI programs can act as a gateway to a variety ofresources such as databases, email, and a host of other protocols andprograms. However, a CGI program can also perform some sophisticatedprocessing on the data it gets so that it effectively becomes a dataresource itself. This is the definition of <a name="INDEX-2717" /><a name="INDEX-2718" />CGI<em class="emphasis">middleware</em>. In this context, the CGI applicationsits between the program it is serving data to and the resources thatit is interacting with.</p><p>The variety of <a name="INDEX-2719" />search engines that exist provides a goodexample of why CGI middleware can be useful. In the early history ofthe Web, there were only a few search engines to choose from. Now,there are many. The results these engines produce are usually notidentical. Finding out about a rare topic is not an easy task if youhave to jump from engine to engine to retry the search.</p><p>Instead of trying multiple queries, you would probably rather issueone query and get back results from many search engines in aconsolidated form with duplicate responses already filtered out. Tomake this a reality, the search engines themselves must become CGImiddleware engines, talking to one CGI script that consolidates theresults.</p><p>Furthermore, a <a name="INDEX-2720" /><a name="INDEX-2721" />CGI middleware layer can beused to consolidate databases other than ones on the Internet. Forexample, a company-wide directory service could be programmed tosearch several internal phone directory databases such as customerdata and human resources data as well as using an Internet phoneresource such as <a href="http://www.four11.com/">http://www.four11.com/</a> if the information islacking internally, as shown in <a href="ch14_01.htm#ch14-33108">Figure 14-1</a>.</p><a name="ch14-33108" /><div class="figure"><img width="334" src="figs/cgi2.1401.gif" height="241" alt="Figure 14-1" /></div><h4 class="objtitle">Figure 14-1. Consolidated phone directory interface using CGI middleware</h4><p>Two technologies to illustrate the use of CGI middleware will bedemonstrated later in this chapter. First, we will look at how toperform network connections from your CGI scripts in order to talk toother servers. Then, we introduce eXtensible Markup Language (XML), aplatform-independent way of transferring data between programs.We'll show an example using Perl's XML parser.</p><div class="sect1"><a name="ch14-59873" /><h2 class="sect1">14.1. Communicating with Other Servers</h2><p>Let's look <a name="INDEX-2723" /> <a name="INDEX-2,724" /> <a name="INDEX-2,725" /> <a name="INDEX-2,726" />at the typical communicationscheme between a <a name="INDEX-2727" />client and a server. Consider anelectronic mail application, for example. Most<a name="INDEX-2728" />email applications save the user'smessages in a particular file, typically in the<em class="emphasis">/var/spool/mail</em> directory. When you send mail tosomeone on a different host, the mail application must find therecipient's mail file on that server and append your message toit. How does the mail program achieve this task, since it cannotmanipulate files on a remote host directly?</p><p>The answer to this question is <em class="emphasis">interprocesscommunication</em><a name="INDEX-2729" /> (IPC). Typically, thereexists a process on the remote host, which acts as a messenger fordealing with email services. When you send a message, the localprocess on your host communicates with this remote agent across anetwork to deliver mail. As a result, the <a name="INDEX-2730" /><a name="INDEX-2731" /><a name="INDEX-2732" />remote process is called a server(because it services an issued request), and the<a name="INDEX-2733" /><a name="INDEX-2734" />local process is referred to as aclient. The Web works along the same philosophy: the<a name="INDEX-2735" /><a name="INDEX-2736" />browser represents the client that issues arequest to an HTTP server that interprets and executes the request.</p><p>The most important thing to remember here is that the client and theserver must speak the same language. In other words, a particularclient is designed to work with a specific server. So, for example,an email client, such as Eudora, cannot communicate with a webserver. But if you know the stream of data expected by a server, andthe output it produces, you can write an application thatcommunicates with the server, as you will see later in this chapter.</p><a name="ch14-1-fm2xml" /><div class="sect2"><h3 class="sect2">14.1.1. Sockets</h3><p>Most <a name="INDEX-2737" /><a name="INDEX-2738" /><a name="INDEX-2739" /><a name="INDEX-2740" />companies have atelephone switchboard that acts as a gateway for calls coming in andgoing out. A socket can be likened to a telephone switchboard. If youwant to connect to a remote host, you need to first create a socketthrough which the communications would occur. This is similar todialing "9" to go through the company switchboard to theoutside world.</p><p>Similarly, if you want to create a server that accepts<a name="INDEX-2741" />connections from remote (or local) hosts,you need to set up a socket that listens for connections. The socketis identified on the Internet by the <a name="INDEX-2742" /><a name="INDEX-2743" /><a name="INDEX-2744" />host's IP address and the portthat it listens on. Once a connection is established, a new socket iscreated to handle this connection, so that the original socket can goback and listen for more connections. The telephone switchboard worksin the same manner: as it handles outside phone calls, it routes themto the appropriate extension and goes back to accept more calls.</p><p>For the sake of discussion, think of a socket simply as a pipebetween two locations. You can send and receive information throughthat pipe. This concept will make it easier for you to understandsocket I/O.</p></div><a name="ch14-2-fm2xml" /><div class="sect2"><h3 class="sect2">14.1.2. IO::Socket</h3><p>The <a name="INDEX-2745" /><a name="INDEX-2746" /><a name="INDEX-2747" />IO::Socket module, which is includedwith the standard Perl distribution, makes socket programming simple.<a href="ch14_01.htm#ch14-91444">Example 14-1</a> provides a short program that takes aURL from the user, requests the resource via a GET method, thenprints the headers and content.</p><a name="ch14-91444" /><div class="example"><h4 class="objtitle">Example 14-1. socket_get.pl </h4><blockquote><pre class="code">#!/usr/bin/perl -wTuse strict;use IO::Socket;use URI;my $location = shift || die "Usage: $0 URL\n";my $url = new URI( $location );my $host = $url->host;my $port = $url->port || 80;my $path = $url->path || "/";my $socket = new IO::Socket::INET (PeerAddr => $host, PeerPort => $port, Proto => 'tcp') or die "Cannot connect to the server.\n";$socket->autoflush (1);print $socket "GET $path HTTP/1.1\n", "Host: $host\n\n";print while (<$socket>);$socket->close;</pre></blockquote></div><p>We use the <a name="INDEX-2748" /><a name="INDEX-2749" />URI module discussed in <a href="ch02_01.htm">Chapter 2, "The Hypertext Transport Protocol "</a>, to break the URL supplied by the user intocomponents. Then we create a new instance of the IO::Socket::INETobject and pass it the host, port number, and the communicationsprotocol. And the module takes care of the rest of the details.</p><p>We make the socket unbuffered by using the<tt class="function">autoflush</tt><a name="INDEX-2750" /> method. Notice in thenext set of code that we can use the<a name="INDEX-2751" /><a name="INDEX-2752" />instance variable<tt class="literal">$socket</tt> as a file handle as well. This means thatwe can read from and write to the socket through this variable.</p><p>This is a relatively simple program, but there is an even easier wayto retrieve web resources from Perl: LWP.</p></div><a name="ch14-3-fm2xml" /><div class="sect2"><h3 class="sect2">14.1.3. LWP</h3><p><em class="emphasis">LWP</em><a name="INDEX-2753" /><a name="INDEX-2754" />,which stands for <em class="firstterm">libwww-perl</em>, is animplementation of the W3C's <em class="firstterm">libwww</em>package for Perl by Gisle Aas and Martijn Koster, with contributionsfrom a host of others. LWP allows you to create a fully configurableweb <a name="INDEX-2755" />client in Perl. You cansee an example of some of what LWP can do in <a href="ch08_02.htm#ch08-74342">Section 8.2.5, "Trusting the Browser"</a>.</p><p>With LWP, we can write our <a name="INDEX-2756" /> <a name="INDEX-2,757" />web agent as shown in <a href="ch14_01.htm#ch14-59139">Example 14-2</a>.</p><a name="ch14-59139" /><div class="example"><h4 class="objtitle">Example 14-2. lwp_full_get.pl </h4><blockquote><pre class="code">#!/usr/bin/perl -wTuse strict;use LWP::UserAgent;use HTTP::Request;my $location = shift || die "Usage: $0 URL\n";my $agent = new LWP::UserAgent;my $req = new HTTP::Request GET => $location; $req->header('Accept' => 'text/html');my $result = $agent->request( $req );print $result->headers_as_string, $result->content;</pre></blockquote></div><p>Here we create a user agent object as well as an HTTP request object.We ask the user agent to fetch the result of the HTTP request andthen print out the headers and content of this response.</p><p>Finally, let's look at <a name="INDEX-2758" />LWP::Simple. LWP::Simple does not offerthe same flexibility as the full LWP module, but it is much easier touse. In fact, we can rewrite our previous example to be even shorter;see <a href="ch14_01.htm#ch14-31064">Example 14-3</a>.</p><a name="ch14-31064" /><div class="example"><h4 class="objtitle">Example 14-3. lwp_simple_get.pl </h4><blockquote><pre class="code">#!/usr/bin/perl -wTuse strict;use LWP::Simple;my $location = shift || die "Usage: $0 URL\n";getprint( $location );</pre></blockquote></div><p>There is a slight difference between this and the previous example.It does not print the HTTP headers, just the content. If we want toaccess the headers, we would need to use the <a name="INDEX-2759" /> <a name="INDEX-2,760" /> <a name="INDEX-2,761" /> <a name="INDEX-2,762" />full LWP moduleinstead.</p></div></div><hr align="left" width="515" /><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch13_05.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td width="172" valign="top" align="right"><a href="ch14_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td width="172" valign="top" align="left">13.5. PerlMagick</td><td width="171" valign="top" align="center"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td width="172" valign="top" align="right">14.2. An Introduction to XML</td></tr></table></div><hr align="left" width="515" /><img src="../gifs/navbar.gif" alt="Library Navigation Links" usemap="#library-map" border="0" /><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. All rights reserved.</font></p><map name="library-map"><area href="../index.htm" coords="1,1,83,102" shape="rect" /><area href="../lnut/index.htm" coords="81,0,152,95" shape="rect" /><area href="../run/index.htm" coords="172,2,252,105" shape="rect" /><area href="../apache/index.htm" coords="238,2,334,95" shape="rect" /><area href="../sql/index.htm" coords="336,0,412,104" shape="rect" /><area href="../dbi/index.htm" coords="415,0,507,101" shape="rect" /><area href="../cgi/index.htm" coords="511,0,601,99" shape="rect" /></map></body></html>⌨️ 快捷键说明
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