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<h1 class="chapter">Chapter 16. Simple Databases</h1>
<div class="htmltoc"><h4 class="tochead">Contents:</h4>
<p> <a href="#lperl3-CHP-16-SECT-1">DBM Files and DBM Hashes</a><br />
<a href="ch16_02.htm">Manipulating Data with pack and unpack</a><br />
<a href="ch16_03.htm">Fixed-length Random-access Databases</a><br />
<a href="ch16_04.htm">Variable-length (Text) Databases</a><br />
<a href="ch16_05.htm">Exercises</a><br /></p></div>
<p><a name="INDEX-1035" /></a>Databases
permit us to allow data to persist beyond the end of our program. The
kinds of databases we're talking about in this chapter are
merely simple ones; how to use full-featured database implementations
(Oracle, Sybase, Informix, mySQL, and others) is a topic that could
fill an entire book, and usually does. The databases in this chapter
are those that are simple enough to implement that you don't
need to know about modules to use them.<a href="#FOOTNOTE-346">[346]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-346" /></a><p>[346]To be sure,
on some of these, the core of Perl will load a module for you. But
you don't need to know anything about modules to use these
databases.</p> </blockquote>
<div class="sect1"><a name="lperl3-CHP-16-SECT-1" /></a>
<h2 class="sect1">16.1. DBM Files and DBM Hashes</h2>
<p><a name="INDEX-1036" /></a> <a name="INDEX-1037" /></a>Every
system thas has Perl also has a simple database already available in
the form of DBM files. This lets your program store data for quick
lookup in a file or in a pair of files. When two files are used, one
holds the data and the other holds a table of contents, but you
don't need to know that in order to use DBM files. We're
intentionally being a little vague about the exact implementation,
because that will vary depending upon your machine and configuration;
see the <tt class="literal">AnyDBM_file</tt><a name="INDEX-1038" /></a> manpage for more information. Also,
among the downloadable files from the O'Reilly website is a
utility called
<em class="filename">which_dbm,</em><a name="INDEX-1039" /></a>
<a name="INDEX-1040" /></a>
which tries to tell you which implementation you're using, how
many files there are, and what extensions they use, if any.
</p>
<p>Some DBM file implementations (we'll call it "a
file," even though it may be two actual files) have a limit of
around 1000 bytes for each key and value in the file. Your actual
limit may be larger or smaller than this number, but as long as you
aren't trying to store gigantic text strings in the file, it
shouldn't be a problem. There's no limit to the number of
individual data items in the file, as long as you have enough disk
space.
</p>
<p>In Perl, we can access the DBM file as a special kind of hash called
a DBM hash. This is a powerful concept, as we'll see.
</p>
<a name="lperl3-CHP-16-SECT-1.1" /></a><div class="sect2">
<h3 class="sect2">16.1.1. Opening and Closing DBM Hashes</h3>
<p>To associate a DBM database with a DBM hash (that is, to open it),
use the <tt class="literal">dbmopen</tt><a name="INDEX-1041" /></a> function,<a href="#FOOTNOTE-347">[347]</a> which looks similar to
<tt class="literal">open</tt>, in a way:
</p><blockquote class="footnote"> <a name="FOOTNOTE-347" /></a><p>[347]Here we
depart from other beginner documentation, which claims that
<tt class="literal">dbmopen </tt>is deprecated and suggests that you use
the more complicated <tt class="literal">tie </tt><a name="INDEX-1042" /></a>interface instead. We disagree, since
<tt class="literal">dbmopen </tt>works just fine, and it keeps you from
having to think harder about what you're doing. Keep the common
tasks simple!</p> </blockquote>
<blockquote><pre class="code">dbmopen(%DATA, "my_database", 0644)
or die "Cannot create my_database: $!";</pre></blockquote>
<p>The first parameter is the name of a Perl hash. (If this hash already
has values, the values are inaccessible while the DBM file is open.)
This hash becomes connected to the DBM database whose name was given
as the second parameter, often stored on disk as a pair of files with
the extensions <tt class="literal">.</tt><em class="emphasis">dir</em> and
<tt class="literal">.</tt><em class="emphasis">pag</em>. (The filename as given
in the second parameter shouldn't include either extension,
though; the extensions will be automatically added as needed.) In
this case, the files might be called
<em class="filename">my_database.dir</em> and
<em class="filename">my_database.pag</em>.
</p>
<p>Any legal hash name may be used as the name of the DBM hash, although
uppercase-only hash names are traditional because their resemblance
to filehandles reminds us that the hash is connected to a file. The
hash name isn't stored anywhere in the file, so you can call it
whatever you'd like.
</p>
<p>If the file doesn't exist, it will be created and given a
permission mode based upon the value in the third
parameter.<a href="#FOOTNOTE-348">[348]</a> The number is typically specified in
octal; the frequently used value of <tt class="literal">0644</tt> gives
read-only permission to everyone but the owner, who gets read/write
permission. If you're trying to open an existing file,
you'd probably rather have the <tt class="literal">dbmopen</tt>
fail if the file isn't found, so just use
<tt class="literal">undef</tt> as the third parameter.
</p><blockquote class="footnote"> <a name="FOOTNOTE-348" /></a><p>[348]The actual mode will be modified by the
<tt class="literal">umask</tt>; see the
<em class="emphasis">perlfunc</em>manpage for more
information.</p> </blockquote>
<p>The return value from the <tt class="literal">dbmopen</tt> is true if the
database could be opened or created, and false otherwise, just like
<tt class="literal">open</tt>. You should generally use <tt class="literal">or
die</tt> in the same spirit as <tt class="literal">open</tt>.
</p>
<p>The DBM hash typically stays open throughout the program. When the
program terminates, the association is terminated. You can also break
the association in a manner similar to closing a filehandle, by using
<tt class="literal">dbmclose</tt><a name="INDEX-1043" /></a>:
</p>
<blockquote><pre class="code">dbmclose(%DATA);</pre></blockquote>
</div>
<a name="lperl3-CHP-16-SECT-1.2" /></a><div class="sect2">
<h3 class="sect2">16.1.2. Using a DBM Hash</h3>
<p>Here's the beauty of the DBM hash: it works just like the
hashes you already understand! To read from the file, look at an
element of the hash. To write to the file, store something into the
hash. In short, it's like any other hash, but instead of being
stored in memory, it's stored on disk. And thus, when your
program opens it up again, the hash is already stuffed full of the
data from the previous invocation.
</p>
<p>All of the normal hash operations are available:</p>
<blockquote><pre class="code">$DATA{"fred"} = "bedrock"; # create (or update) an element
delete $DATA{"barney"}; # remove an element of the database
foreach my $key (keys %DATA) { # step through all values
print "$key has value of $DATA{$key}\n";
}</pre></blockquote>
<p>That last loop could have a problem, since <tt class="literal">keys</tt>
has to traverse the entire hash, possibly producing a very large list
of keys. If you are scanning through a DBM hash, it's generally
more memory-efficient to use the
<tt class="literal">each</tt><a name="INDEX-1044" /></a>
function:
</p>
<blockquote><pre class="code">while (my($key, $value) = each(%DATA)) {
print "$key has value of $value\n";
}</pre></blockquote>
<p>If you are accessing DBM files that are maintained by
<a name="INDEX-1045" /></a>C programs, you should be aware that
C programs generally tack on a trailing <a name="INDEX-1046" /></a>NUL (<tt class="literal">"\0"</tt>) character to
the end of their strings, for reasons known only to Kernighan and
Ritchie.<a href="#FOOTNOTE-349">[349]</a> The DBM library routines do not need this
NUL (they handle binary data using a byte count, not a NUL-terminated
string), and so the NUL is stored as part of the data.
</p><blockquote class="footnote"> <a name="FOOTNOTE-349" /></a><p>[349]Well, they're not the only ones:
it's because C uses the NUL byte as the end-of-string
marker.</p> </blockquote>
<p>To cooperate with these programs, you must therefore append a NUL
character to the end of your keys and values, and discard the NUL
from the end of the returned values to have the data make sense. For
example, to look up <tt class="literal">merlyn</tt> in the sendmail aliases
database on a Unix system, you might do something like this:
</p>
<blockquote><pre class="code">dbmopen(my %ALI, "/etc/aliases", undef) or die "no aliases?";
my $value = $ALI{"merlyn\0"}; # note appended NUL
$value =~ s/\0$//; # remove trailing NUL
print "Randal's mail is headed for: $value\n"; # show result</pre></blockquote>
<p>If your DBM files may be concurrently accessed by more than one
process (for example if they're being updated over the Web),
you'll generally need to use an <a name="INDEX-1047" /></a>auxiliary lock file. The details of this
are beyond the scope of this book; see <em class="emphasis"><a href="../cookbook/index.htm">The Perl Cookbook</a> </em> by Tom Christiansen and Nathan Torkington
(O'Reilly & Associates, Inc.).<a name="INDEX-1048" /></a> <a name="INDEX-1049" /></a>
</p>
</div>
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