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width="400"><tr><td valign="top" align="left"><br><img border="0" height="34" hspace="0" src="images/castor.gif" vspace="0" width="115"><br><img border="0" height="10" hspace="0" src="images/dotTrans.gif"></td><td width="120" height="5" valign="top" align="right"><a href="http://www.exolab.org"><img src="images/logo_exolab.gif" hspace="0" vspace="10" width="77" height="20" border="0"></a></td></tr></table><p></p><p></p><br><span class="bodyGrey"><a href="#Introduction">Introduction</a><br></span><span class="bodyGrey"><a href="#Bounded-dirty-checking">Bounded dirty checking</a><br></span><br><a name="Introduction"><h2>Introduction</h2></a> <p><span class="bodyGrey">The usual Castor transactions are called here short transactions: an object is read, modified and written within the bounds of one transaction. Long transactions consist of two Castor transactions: an object is read in the first and written in the second. Between them the object is sent "outwards" and modified there. For example, the object may be send to a client application and dispayed to a user, or it may be sent to a servlet engine and is displayed on a web page. After that the modified object returns back and is written to the database in the second transaction. At this point the object is usually not the same physical instance as one that was read in the first transaction. The example code for writing the object in the second Castor transaction follows: <span class="bodyBlack"><pre>
<span class="bodyGrey"><font color="red">
// a customer go to a webpage to review her personal information.
// The servlet then call this server side function: getCustomerInfo</font></span>
public CustomerInfo getCustomerInfo( Integer customNum ) {
<span class="bodyGrey"><font color="red">
// in most case, users simply review information and
// make no change. Even if they make changes, it often
// takes time for them to decide. We don't want to
// lock the database row, so commit right after we load.</font></span>
db.begin();
CustomerInfo info = (CustomerInfo)
db.load( CustomerInfo.class, customNum );
<span class="bodyGrey"><font color="red">
// we also want to keep track of customers patterns
// well...it helps us provide better service.</font></span>
info.setLastVisit( today );
db.commit();
return info;
}
<span class="bodyGrey"><font color="red">
// Three days passed, the indecisive customer finally agrees to
// marry Joe. She changes her last name in the webpage and
// clicked the "Submit" button on the webpage.
</font></span>
<span class="bodyGrey"><font color="red">
// The servlet then calls updateCustomerInfo to update the
// last name for the indecisive customer.</font></span>
public void updateCustomerInfo( CustomerInfo info ) {
db.begin();
db.update(info);
db.commit();
}
</pre></span> Note, that it is natural to read the object in the first transaction in the read-only mode. </span></p> <p><span class="bodyGrey">Since the time interval between the first and the second transaction is relatively big, it is desirable to perform dirty checking, i.e. to check that the object has not been modified in the database during the long transaction. For that the object must hold a timestamp: it is set by Castor during the first Castor transaction and is checked during the second one. In order to enable the dirty checking for long transactions, the object should implement the interface org.exolab.castor.jdo.TimeStampable having two methods: <span class="bodyBlack"><pre>long jdoGetTimeStamp()</pre></span> and <span class="bodyBlack"><pre>void jdoSetTimeStamp(long timeStamp)</pre></span> </span></p> <a name="Bounded-dirty-checking"><h2>Bounded dirty checking</h2></a> <p><span class="bodyGrey">The advantage of the bounded dirty checking is that it doesn't require any changes to the database schema. It uses the Castor cache to store object timestamps. The disadvantage of this algorithm is that it is bounded by a lifetime of the cached copy of the object. After the cached copy has been purged, db.update() causes ObjectModifiedException. </span></p> <p><span class="bodyGrey">Thus, parameters of the cache define dirty checking capabilities. The cache-type attribute is part of the <a href="jdo-mapping.html#The-<class>-element"> <class> element</a> in the XML mapping. Consider the existing cache types: <table border="0" cellpadding="2" cellspacing="2"><tr><td colspan="2" height="5"></td></tr><span class="bodyGrey"> <tr><td align="left" valign="top" width="10">-</td><td align="left" valign="top"><span class="bodyGrey">none - the bounded dirty checking is impossible </span></td></tr> <tr><td align="left" valign="top" width="10">-</td><td align="left" valign="top"><span class="bodyGrey">count-limited - the count limit for the cache is a count limit for the objects of this class that can participate in long and short transactions simultaneously. </span></td></tr> <tr><td align="left" valign="top" width="10">-</td><td align="left" valign="top"><span class="bodyGrey">time-limited - the time limit for the cache is a time limit for the long transaction. </span></td></tr> <tr><td align="left" valign="top" width="10">-</td><td align="left" valign="top"><span class="bodyGrey">unlimited - the bounded dirty checking gives correct results while the cache exists, i.e. until the crash of the server. </span></td></tr> </span></table> </span></p> </td></tr><tr height="5"><td width="10" height="5" bgcolor="#7270c2" valign="top" align="left"> </td><td width="115" height="5" bgcolor="#7270c2" valign="top"><img src="images/dotTrans.gif" width="1" height="15" border="0"><br><img src="images/line_sm.gif" width="105" height="3" border="0" align="right"></td><td width="7" height="5" bgcolor="#a9a5de" valign="top" align="left"> </td><td width="70" height="5" valign="top" align="left"> </td><td width="120" height="5" valign="top" align="left"> </td></tr><tr><td width="10" height="5" bgcolor="#7270c2" valign="top" align="left"> </td><td width="115" bgcolor="#7270c2" valign="top" align="left"></td><td width="7" bgcolor="#a9a5de" valign="top" align="left"><img src="images/dotTrans.gif" width="1" height="25" border="0"></td><td width="70" valign="top" align="left"><img src="images/dotTrans.gif" width="1" height="25" border="0"></td><td width="120" valign="top" align="left"> </td></tr><tr height="5"><td width="10" rowspan="2" height="100%" bgcolor="#7270c2" valign="bottom" align="left"><img src="images/stripes1.gif" width="10" height="125" border="0"></td><td width="115" rowspan="2" height="100%" bgcolor="#7270c2" valign="bottom" align="left"><img src="images/stripe105.gif" width="105" height="125" border="0"></td><td width="7" rowspan="2" height="100%" bgcolor="#a9a5de" valign="top" align="left"> </td><td width="70" height="100%" valign="top" align="left"> </td><td width="120" height="100%" valign="top" align="left"> </td></tr><tr height="5"><td width="70" height="25" valign="top" align="left"> </td><td width="400" height="25" valign="bottom" align="left"><br><br><img src="images/line_light.gif" border="0" width="400" height="3"><br><p></p><span class="bodyGrey"><small><notice> Copyright ) 1999-2003 <a href="http://www.exolab.org">ExoLab Group</a>. All rights reserved. </notice><br> <br></small><small><notice> Java, EJB, JDBC, JNDI, JTA, Sun, Sun Microsystems are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and in other countries. XML, XML Schema, XSLT and related standards are trademarks or registered trademarks of MIT, INRIA, Keio or others, and a product of the World Wide Web Consortium. All other product names mentioned herein are trademarks of their respective owners. </notice><br> <br></small></span><p></p> </td><td width="120" height="25" valign="top" align="left"> </td></tr></table></body></html>
