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<h1>Storing MarshalByRef Objects</h1><div id="TOC"><div id="TOCinner"><span class="TOCtitle">Contents</span><div class="TOCcontents"><ul><li><a href ="#Problem ">Problem </a></li><li><a href ="#Reason">Reason</a></li><li><a href ="#Solution">Solution</a></li></ul></li></ul></div></div></div><p><font color="#990000">This topic applies to .NET version only</font><br></p><a name="Problem "></a><h2>Problem </h2>
<p>MarshalByRef objects from .NET Remoting can not be stored by
db4o.<br></p>
<a name="Reason"></a><h2>Reason</h2>
<p>MarshalByRef objects are not really objects. They are proxy
objects to instances, which live on another machine. There are 2 cases to
distinguish:</p>
<ol><li>if the
object instance is local, then the object is storable without an exception
unless a remote client has a lifetime lease on the object instance;</li><li>if the
object instance is remote, then you're actually dealing with a DCOM proxy,
which cannot be safely stored in db4o, since the remote object lifetime
would expire as soon as the object was garbage collected. </li></ol>
<a name="Solution"></a><h2>Solution<br></h2>
<p>
1. Use marshal by value technology for the persistent object. In this case the
object implements ISerializable interface and can be stored by db4o without any
problems. The object is created on the client and is passed to the server using
MarshalByRef proxy. </p>
<p>In the example, TestValue class should be stored to db4o.
TestValue implements ISerializable. In order to make TestValue class available
to the server and to the client it is declared in a separate class library
RemotingClasses. </p>
<p>TestValueServer class extends MarshalByRefObject and plays
the role of a carrier between the client and the server. TestValue instance is
created on the client and passed to the TestValueServer, which runs as a
service on the server. TestValueServer then stores TestValue object to the
database and retrieves its value on request.</p>
<p>Download the example solution: <a href="http://developer.db4o.com/Resources/view.aspx/Reference/Usage_Pitfalls/Storing_MarshalByRef_Objects/RemotingExampleValue.zip">c#</a>, <a href="http://developer.db4o.com/Resources/view.aspx/Reference/Usage_Pitfalls/Storing_MarshalByRef_Objects/RemotingExampleValueVB.zip">VB</a></p>
<p>2. If using MarshalByRef object is mandatory for you, use
the following configuration:</p><p><code>configuration.ObjectClass("System.Runtime.Remoting.ServerIdentity,
mscorlib").Translate(new TTransient()); </code><br>
<code>configuration.ObjectClass("System.Threading.TimerCallback,
mscorlib").Translate(new TTransient()); </code><br>
<br>
The TTransient translator will prevent instances of ServerIdentity and
TimerCallback from being stored within a db4o database. These classes are
protected internal classes within the .NET Framework. When retrieving your
MarshalByRef objects from db4o, you will need to re-marshal them.</p>
<p>In the following example Test object extends
MarshalByRefObject. Test object lives on the server and can store itself to
db4o when a request is received from the client. The configuration above is
used to make it storable with db4o.</p>
<p>Download the example solution: <a href="http://developer.db4o.com/Resources/view.aspx/Reference/Usage_Pitfalls/Storing_MarshalByRef_Objects/RemotingExampleRef.zip">c#</a>, <a href="http://developer.db4o.com/Resources/view.aspx/Reference/Usage_Pitfalls/Storing_MarshalByRef_Objects/RemotingExampleRefVB.zip">VB</a>.</p>
Note: in many cases using <a href="../client-server.html" class="wikiLink">db4o
client-server</a> version can be a better option for a remote persistence.</div>
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This revision (7) was last Modified 2007-05-23T09:52:55 by Tetyana.
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