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<h2 class="title"><a id="ext"></a>Chapter 22. Berkeley DB Extensions </h2>
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<b>Table of Contents</b>
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<dl>
<dt>
<span class="sect1">
<a href="ext.html#ext_mod">Using Berkeley DB with Apache</a>
</span>
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<span class="sect1">
<a href="ext_perl.html">Using Berkeley DB with Perl</a>
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<h2 class="title" style="clear: both"><a id="ext_mod"></a>Using Berkeley DB with Apache</h2>
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<p>
A mod_db4 Apache module is included in the Berkeley DB
distribution, providing a safe framework for running Berkeley
DB applications in an Apache 1.3 environment. Apache natively
provides no interface for communication between threads or
processes, so the mod_db4 module exists to provide this
communication.
</p>
<p>
In general, it is dangerous to run Berkeley DB in a
multiprocess system without some facility to coordinate
database recovery between processes sharing the database
environment after application or system failure. Failure to
run recovery after failure can include process hangs and an
inability to access the database environment. The mod_db4
Apache module oversees the proper management of Berkeley DB
database environment resources. Developers building
applications using Berkeley DB as the storage manager within
an Apache module should employ this technique for proper
resource management.
</p>
<p>
Specifically, mod_db4 provides the following
facilities:
</p>
<div class="orderedlist">
<ol type="1">
<li>
New constructors for <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> and <a href="../api_reference/C/db.html" class="olink">DB</a> handles, which
install replacement open/close methods.
</li>
<li>
Transparent caching of open <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> and <a href="../api_reference/C/db.html" class="olink">DB</a>
handles.
</li>
<li>
Reference counting on all structures, allowing the
module to detect the initial opening of any managed
database and automatically perform recovery.
</li>
<li>
Automatic detection of unexpected failures
(segfaults, or a module actually calling exit() and
avoiding shut down phases), and automatic termination of
all child processes with open database resources to
attempt consistency.
</li>
</ol>
</div>
<p>
mod_db4 is designed to be used as an alternative interface
to Berkeley DB. To have another Apache module (for example,
mod_foo) use mod_db4, do not link mod_foo against the Berkeley
DB library. In your mod_foo makefile, you should:
</p>
<pre class="programlisting">#include "mod_db4_export.h"</pre>
<p>
and add your Apache include directory to your
CPPFLAGS.
</p>
<p>
In mod_foo, to create a mod_db4 managed <a href="../api_reference/C/env.html" class="olink">DB_ENV</a> handle, use
the following:
</p>
<pre class="programlisting">int mod_db4_db_env_create(DB_ENV **dbenvp, u_int32_t flags);</pre>
<p>
which takes identical arguments to <a href="../api_reference/C/envcreate.html" class="olink">db_env_create()</a>.
</p>
<p>
To create a mod_db4 managed <a href="../api_reference/C/db.html" class="olink">DB</a> handle, use the
following:
</p>
<pre class="programlisting">int mod_db4_db_create(DB **dbp, DB_ENV *dbenv, u_int32_t flags);</pre>
<p>
which takes identical arguments to <a href="../api_reference/C/dbcreate.html" class="olink">db_create()</a>.
</p>
<p>
Otherwise the API is completely consistent with the standard
Berkeley DB API.
</p>
<p>
The mod_db4 module requires the Berkeley DB library be
compiled with C++ extensions and the MM library. (The MM
library provides an abstraction layer which allows related
processes to share data easily. On systems where shared memory
or other inter-process communication mechanisms are not
available, the MM library emulates them using temporary files.
MM is used in several operating systems to provide shared
memory pools to Apache modules.)
</p>
<p>
To build this apache module, perform the following
steps:
</p>
<pre class="programlisting">% ./configure --with-apxs=[path to the apxs utility] \
--with-db4=[Berkeley DB library installation directory] \
--with-mm=[libmm installation directory]
% make
% make install</pre>
<p>
Post-installation, modules can use this extension via the
functions documented in
$APACHE_INCLUDEDIR/mod_db4_export.h.
</p>
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