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<?label 10.2. DBM Files?><html><head><title>DBM Files (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="ch10_01.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="ch10_03.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><hr align="left" width="515" /><h2 class="sect1">10.2. DBM Files</h2><p><a name="INDEX-2046" />DBM files provide many advantages<a name="INDEX-2047" />over text files for database purposes, andbecause Perl provides such a simple, transparent interface to workingwith DBM files, they are a popular choice for programming tasks thatdon't require a full RDBMS. DBM files are simply on-disk<a name="INDEX-2048" />hash tables. You can quickly look upvalues by key and efficiently update and delete values in place.</p><p>To use a DBM file, you must tie a Perl hash to the file using one ofthe DBM modules. <a href="ch10_02.htm#ch10-86514">Example 10-3</a> shows some code thatuses the <a name="INDEX-2049" /><a name="INDEX-2050" />DB_File module to tie a hash to the file<em class="filename">user_email.db</em>.</p><a name="ch10-86514" /><div class="example"><h4 class="objtitle">Example 10-3. email_lookup.cgi </h4><blockquote><pre class="code">#!/usr/bin/perl -wTuse strict;use DB_File;use Fcntl;use CGI;my $q = new CGI;my $username = $q->param( "user" );my $dbm_file = "/usr/local/apache/data/user_email.db";my %dbm_hash;my $email;tie %dbm_hash, "DB_File", $dbm_file, O_RDONLY or die "Unable to open dbm file $dbm_file: $!";if ( exists $dbm_hash{$username} ) { $email = $q->a( { href => "mailto:$dbm_hash{$username}" }, $dbm_hash{$user_name} );else { $email = "Username not found";}untie %dbm_hash;print $q->header( "text/html" ), $q->start_html( "Email Lookup Results" ), $q->h2( "Email Lookup Results" ), $q->hr, $q->p( "Here is the email address for the username you requested: " ), $q->p( "Username: $username", $q->br, "Email: $email" ), $q->end_html;</pre></blockquote></div><p>There are many different formats of DBM files, and likewise there aremany different DBM modules available. <a name="INDEX-2051" /> <a name="INDEX-2,052" />Berkeley DB and GDBM are the mostpowerful. However, for web development Berkeley DB, and thecorresponding DB_File module, is the most popular choice. UnlikeGDBM, it provides a simple way for you to lock the database so thatconcurrent writes do not truncate and corrupt your file.</p><a name="ch10-8-fm2xml" /><div class="sect2"><h3 class="sect2">10.2.1. DB_File</h3><p><a name="INDEX-2053" /><a name="INDEX-2054" /><a name="INDEX-2055" />DB_Filesupports Version 1.xx functionality for Berkeley DB; Berkeley DBVersions 2.xx and 3.xx add numerous enhancements. DB_File iscompatible with these later versions, but it supports only the 1.xxAPI. Perl support for version 2.xx and later is provided by theBerkeleyDB module. DB_File is much simpler and easier to use,however, and continues to be the more popular option. If Berkeley DBis not installed on your system, you can get it from <a href="http://www.sleepycat.com/">http://www.sleepycat.com/</a>. The DB_File andBerkeleyDB modules are on CPAN. DB_File is also included in thestandard Perl distribution (although it is installed only if BerkeleyDB is present).</p><p>Using DB_File is quite simple, as we saw earlier. You simply need totie a hash to the DBM file you want to use and then you can treat itlike a regular hash. The<tt class="function">tie</tt><a name="INDEX-2056" /><a name="INDEX-2057" /> function takes at least twoarguments: the hash you want to tie and the name of the DBM moduleyou are using. Typically, you also provide the name of the DBM fileyou want to use and access flags from<a name="INDEX-2058" /> <a name="INDEX-2,059" /><a name="INDEX-2060" />Fcntl.You can also specify the file permission for the new file if you arecreating a file.</p><p>Often, you access<a name="INDEX-2061" /> <a name="INDEX-2,062" /><a name="INDEX-2063" />hash files on a read/write basis.This complicates the code somewhat because of<a name="INDEX-2064" /> <a name="INDEX-2,065" />filelocking:</p><blockquote><pre class="code">use Fcntl;use DB_File;my %hash;local *DBM;my $db = tie %hash, "DB_File", $dbm_file, O_CREAT | O_RDWR, 0644 or die "Could not tie to $dbm_file: $!";my $fd = $db->fd; # Get file descriptoropen DBM, "+<&=$fd" or die "Could not dup DBM for lock: $!"; # Get dup filehandleflock DBM, LOCK_EX; # Lock exclusivelyundef $db; # Avoid untie probs..# All your code goes here; treat %hash like a normal, basic hash..untie %hash; # Clears buffers, then saves, closes, and unlocks file</pre></blockquote><p>We use the <tt class="literal">O_CREAT</tt> and <tt class="literal">O_RDWR</tt>flags imported by Fcntl to indicate that we want to open the DBM filefor read/write access and create the file if it does not exist. If anew file is created, on Unix systems it is assigned 0644 as its filepermissions (although <tt class="command">umask</tt> may restrict thisfurther). If <tt class="function">tie</tt> succeeds, we store theresulting DB_File object in<tt class="literal">$db</tt><a name="INDEX-2066" />.</p><p>The only reason we need <tt class="literal">$db</tt> is to get the filedescriptor of DB_File's underlying DBM file. By using this, wecan open a read/write filehandle to this file descriptor. Finally,this gives us a filehandle we can lock with<tt class="function">flock</tt>. We then undefine <tt class="literal">$db</tt>.</p><p>The reason we clear <tt class="literal">$db</tt> is not just to conserveRAM. Typically, when you are done working with a tied hash, you<tt class="function">untie</tt><a name="INDEX-2067" /> <a name="INDEX-2,068" /><a name="INDEX-2069" /> it, just as you would<tt class="function">close</tt> a file, and if you do not explicitly<tt class="function">untie</tt> it, then Perl automatically does this foryou as soon as all references to the DB_File go out of scope. Thecatch is that <tt class="function">untie</tt> clears only the variablethat it is untying; the DBM file isn't actually written andfreed until DB_File's<tt class="function">DESTROY</tt><a name="INDEX-2070" />method is called -- when all references to the object have goneout of scope. In our code earlier, we have two references to thisobject: <tt class="literal">%hash</tt> and <tt class="literal">$db</tt>, so inorder for the DBM file to be written and saved, both these referencesneed to be cleared.</p><p>If this is confusing, then don't worry about the specifics.Just remember that whenever you get a DB_File object (such as<tt class="literal">$db</tt> above) in order to do file locking, undefineit as soon as you have locked the filehandle. Then<tt class="function">untie</tt> will act like <tt class="function">close</tt>and always be the command that frees your DBM file.</p><p>DB_File provides a very simple, efficient solution when you need tostore name-value pairs. Unfortunately, if you need to store morecomplex data structures, you must still encode and decode them sothat they can be stored as scalars. Fortunately, another moduleaddresses this issue.</p></div><a name="ch10-9-fm2xml" /><div class="sect2"><h3 class="sect2">10.2.2. MLDBM</h3><p>If you look at the bottom of the Perl manpage, you will see that thethree great virtues of a programmer are<em class="emphasis">laziness</em>, <em class="emphasis">impatience</em>, and<em class="emphasis">hubris</em>. <a name="INDEX-2071" /><a name="INDEX-2072" /> <a name="INDEX-2,073" />MLDBM is all about laziness, but in avirtuous way. With MLDBM, you don't have to worry aboutencoding and decoding your Perl data in order to fit the confines ofyour storage medium. You can just save and retrieve it as Perl.</p><p>MLDBM turns another DBM like DB_File into a<a name="INDEX-2074" />multilevel DBM that is notrestricted to simple <a name="INDEX-2075" />key-value pairs. It uses a serializer toconvert complex <a name="INDEX-2076" />Perl structures into a representation thatcan be stored and deserialized back into Perl again. Thus, you can dothings like this:</p><blockquote><pre class="code"># File locking omitted for brevitytie %hash, "MLDBM", $dbm_file, O_CREAT | O_RDWR, 0644;$hash{mary} = { name => "Mary Smith", position => "Vice President", phone => [ "650-555-1234", "800-555-4321" ], email => 'msmith@widgets.com',};</pre></blockquote><p>Later, you can retrieve this information directly:</p><blockquote><pre class="code">my $mary = $hash{mary};my $position = $mary->{position};</pre></blockquote><p>Note that because MLDBM is so transparent it will allow you to ignorethe fact that data is stored in <a name="INDEX-2077" />name-value pairs:</p><blockquote><pre class="code">my $work_phone = $hash{mary}{phone}[1];</pre></blockquote><p>However, be careful because this only works when you are<a name="INDEX-2078" />reading, not when you are writing. You muststill write the data as a key-value pair. This will silently fail:</p><blockquote><pre class="code">$hash{mary}{email} = 'mary_smith@widgets.com';</pre></blockquote><p>You should do this instead:</p><blockquote><pre class="code">my $mary = $hash{mary}; # Get a copy of Mary's record$mary{email} = 'mary_smith@widgets.com'; # Modify the copy$hash{mary} = $mary; # Write the copy to the hash</pre></blockquote><p>MLDBM keeps track of blessed objects, so it works exceptionally wellfor storing objects in Perl:</p><blockquote><pre class="code">use Employee;my $mary = new Employee( "Mary Smith" );$mary->position( "Vice President" );$mary->phone( "650-555-1234", "800-555-4321" );$mary->email( 'msmith@widgets.com' );$hash{mary} = $mary;</pre></blockquote><p>and for retrieving them:</p><blockquote><pre class="code">use Employee;my $mary = $hash{mary};print $mary->email;</pre></blockquote><p>When retrieving objects, be sure you use the corresponding module (inthis case, a fictional module called Employee) before you try toaccess the data.</p><p>MLDBM does have limitations. It cannot store and retrieve filehandlesor code references (at least not across multiple CGI requests).</p><p>When you use MLDBM, you must tell it which DBM module to use as wellas which module to use for serializing and deserializing the data.The options include <a name="INDEX-2079" /> <a name="INDEX-2,080" /> <a name="INDEX-2,081" />Storable, Data::Dumper, and FreezeThaw.Storable is the fastest, but Data::Dumper is included with Perl.</p><p>When you use <a name="INDEX-2082" /> <a name="INDEX-2,083" />MLDBM with DB_File, you can lock theunderlying <a name="INDEX-2084" />DBM file just likeyou would with DB_File:</p><blockquote><pre class="code">use Fcntl;use MLDBM qw( DB_File Storable );my %hash;local *DBM;my $db = tie %hash, "MLDBM", $dbm_file, O_CREAT | O_RDWR, 0644 or die "Could not tie to $dbm_file: $!";my $fd = $db->fd; # Get file descriptoropen DBM, "+<&=$fd" or die "Could not dup DBM for lock: $!"; # Get dup filehandleflock DBM, LOCK_EX; # Lock exclusivelyundef $db; # Avoid untie probs..# All your code goes here; treat %hash like a normal, complex hash..untie %hash; # Clears buffers then saves, closes, <a name="INDEX-2085" />and unlocks file</pre></blockquote></div><hr align="left" width="515" /><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch10_01.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="ch10_03.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td width="172" valign="top" align="left">10. 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