<html>
<head>
<META http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Pluggable Typehandler Benefits</title>
<link rel="stylesheet" type="text/css" href="../../../style.css">
</head>
<body>
<div class="CommonContent">
<div class="CommonContentArea">
<h1>Pluggable Typehandler Benefits</h1>
<p>As the name suggests Pluggable Typehandler allows anybody to
write custom typehandlers, and thus control the way the class objects are
stored to the database and retrieved in a query. Why would you do this? There
can be various reasons:</p>
<ul class="unIndentedList"><li>
You know a more performant way to convert
objects to byte array or to compare them.</li><li>
You need to store only part of the object's
information and want to skip unneeded fields to keep the database smaller. You
can also do the same using <a href="../../tuning/selective_persistence.html" class="wikiLink">Transient</a>
marker, but this is only possible for classes with available code. Using custom
typehandler you can configure partial storage for any third-party class.</li><li>
You need to keep information that will allow you
to restore fields that cannot be stored as is, for example: references to
environmental variables (like time zone), proxy objects or variables of
temporary state (like current memory usage). Previously, this job was done by
<a href="../translators.html" class="wikiLink">Translators</a>, but certainly custom Typehandler gives you more control
and unifies the approach.</li><li>
You want to customize the way Strings are stored
(special encodings).</li><li>
You need to perform a complex refactoring
on-the-fly (use typehandler versioning)</li><li>
You want to cipher each object before putting it
into the database</li><li>
You want to implement cascade on delete through the
typehandler </li></ul>
<p>Other not so common and more
difficult in realization behaviours that can be realized with the new
Typehandler:</p>
<ul class="unIndentedList"><li>
Custom handling of platform-specific generic
collections</li><li>
Fast collection handlers that operate on a lower
level and scale for large collections </li><li>
Customary indexes</li><li>
Versioning of typehandlers (can be used for
refactoring and db4o version upgrades)</li></ul>
<p>Of course, writing typehandlers
is not totally simple, but once you understand how to do that - you will also
gain a much deeper understanding of db4o itself. You can start with a <a href="custom_typehandler_example.html" class="wikiLink">simple
example</a> provided in this documentation and continue by looking into existing
db4o typehandler implementations: StringHandler, VariableLengthTypeHandler,
IndexableTypeHandler etc.</p>
</div>
</div>
<div id="footer">
This revision (1) was last Modified 2008-02-03T09:36:04 by Tetyana.
</div>
</body>
</html>
