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<h1>The Activation Pitfall</h1>
<p>In order to work effectively with db4o
you must understand the concept of
<a href="../basics/activation.htm">Activation</a>. <span class="MCPopup"><a href="javascript:void(0);" class="MCPopupSpot" onclick="FMCPopup( event, this ); return false;" MadCap:src="../basics/activation.htm">Activation<img style="border: none;margin-left: 5px;" src="../SkinSupport/ExpandingClosed.gif" MadCap:altsrc="../SkinSupport/ExpandingOpen.gif" class="MCExpandingIcon" onload="if ( typeof( FMCPreloadImage ) == 'function' ) { FMCPreloadImage( '../SkinSupport/ExpandingOpen.gif' ); }" /></a></span> controls the amount
of referenced objects loaded into the memory. There are two main
pitfalls that you must be aware about.</p>
<h2>Accessing Not Activated Objects</h2>
<p>One common pitfall is to access not activate objects. This usually results in null pointer exceptions or invalid values. This happens when you navigate beyond the activated object-graph area. For example, we have a complex relationships and follow them:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">Person jodie = QueryForJodie(container);
Person julia = jodie.Mother.Mother.Mother.Mother.Mother;
// This will print null
// Because julia is not activated
// and therefore all fields are not set
Console.WriteLine(julia.Name);
// This will throw a NullPointerException.
// Because julia is not activated
// and therefore all fields are not set
string joannaName = julia.Mother.Name;</pre>
<div class="codesnippet-meta">ActivationDepthPitfall.cs: Run into not activated objects
<div class="codedownload"><a href="../CodeExamples/pitfalls/activation/Example-pitfalls-activation-csharp.zip" class="codedownload" MadCap:conditions="Primary.Online">Download Code</a></div><div class="codedownload copylink-marker" MadCap:conditions="Primary.Online"><a href="#copy">Copy Code</a></div></div>
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim jodie As Person = QueryForJodie(container)
Dim julia As Person = jodie.Mother.Mother.Mother.Mother.Mother
' This will print null
' Because julia is not activated
' and therefore all fields are not set
Console.WriteLine(julia.Name)
' This will throw a NullPointerException.
' Because julia is not activated
' and therefore all fields are not set
Dim joannaName As String = julia.Mother.Name</pre>
<div class="codesnippet-meta">ActivationDepthPitfall.vb: Run into not activated objects
<div class="codedownload"><a href="../CodeExamples/pitfalls/activation/Example-pitfalls-activation-vb.zip" class="codedownload" MadCap:conditions="Primary.Online">Download Code</a></div><div class="codedownload copylink-marker" MadCap:conditions="Primary.Online"><a href="#copy">Copy Code</a></div></div>
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<p>This will result in a exception. Because by default db4o only activates object up the a depth of 5. This means that when you load a object, that object and all object which are reachable via 4 references are activated. </p>
<p>There are multiple solutions to this issue.</p>
<ul>
<li value="1">Activate the object explicitly as you dive deeper into the object graph.</li>
<li value="2">Increase the <a href="../configuration/common/activation_depth.htm">global activation-depth</a>.</li>
<li value="3">Increase the activation-depth <a href="../configuration/objectclass/minimum_activation_depth.htm">for certain types</a>.</li>
<li value="4">Use wisely the <![CDATA[ ]]><a href="../configuration/objectclass/cascade_on-activate.htm">cascading activation</a>.</li>
<li value="5">The most elegant solution is <a href="../basics/activation_concept/transparent_activation_framework.htm">transparent activation</a>. With transparent activation db4o takes care of activating object as you access them.</li>
</ul>
<p> </p>
<h2>To High Activation Depth Or Two Many Cascade Activation</h2>
<p>Having a high activation-depth makes working with db4o much easier. However activation can take a long time with deeper object graphs and become a serious performance bottleneck. The same applies when using cascade activation on almost all types. To reduce the time spend on activating objects, you need to be more selective about what to activate and what not.</p>
<ul>
<li value="1">Activate the object explicitly as you dive deeper into the object graph.</li>
<li value="2">The most elegant solution is <a href="../basics/activation_concept/transparent_activation_framework.htm">transparent activation</a>. With transparent activation db4o takes care of activating object as you access them.</li>
</ul>
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