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<h1 class="topictitle1">Cluster applications</h1>
<div><p><span><img src="./delta.gif" alt="Start of change" />Application resilience is one of the key elements
in a clustered environment. If you are planning to write and use highly
available applications in your cluster you should be aware that these applications
have specific availability specifications. <img src="./deltaend.gif" alt="End of change" /></span></p>
<div class="section"><p>By taking advantage of resilient applications in your cluster,
an application can be restarted on a different cluster node without requiring
you to reconfigure the clients. In addition, the data that is associated with
the application will be available after switchover or failover. This means
that the application user can experience minimal, or even seamless, interruption
while the application and its data switch from the primary node to the backup
node. The user does not need to know that the application and data have moved
on the back end.</p>
<p><img src="./delta.gif" alt="Start of change" />In order to achieve application resiliency
in your cluster, applications that meet certain availability specifications
must be used. Certain characteristics must be present in the application in
order for it to be switchable, and therefore always available to the users
of the application in the cluster. See <a href="http://www-03.ibm.com/servers/eserver/iseries/ha/" target="_blank">High Availability and Clusters </a> for details on these
application traits. Because these requirements exist, you have the following
options for using a switchable application in your cluster:<img src="./deltaend.gif" alt="End of change" /></p>
</div>
<ol><li class="stepexpand"><span><span class="uicontrol">Purchase a cluster-enabled software application</span></span> <p>Software products that are cluster-enabled meet certain high-availability
requirements.</p>
</li>
<li class="stepexpand"><span><span class="uicontrol">Write or change your own application to make it highly
available</span> </span> <p>Independent software vendors and application
programmers can customize applications to allow them to be switchable in an iSeries™ clustered
environment.</p>
</li>
</ol>
<div class="section"><p>Once you have a resilient application, it must be managed within
your cluster.</p>
</div>
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<ul class="ullinks">
<li class="ulchildlink"><strong><a href="rzaigapplicationsclusterproven.htm">i5/OS architecture for cluster-enabled applications</a></strong><br />
Additional end-user value is provided by any application that is highly available, recognizing applications that continue to be available in the event of an outage, planned or unplanned.</li>
<li class="ulchildlink"><strong><a href="rzaigapplicationswritehighlyavailable.htm">Writing a highly available cluster application</a></strong><br />
A highly available application is one that can be resilient to a system outage in a clustered environment.</li>
<li class="ulchildlink"><strong><a href="rzaigapplicationscrg.htm">Application CRG considerations</a></strong><br />
An application cluster resource group manages application resiliency.</li>
</ul>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzaigconcepts.htm" title="Get a complete understanding of how clusters work. Read about the benefits of clusters and how they can be important to you, as well as information on important clustering concepts and how they fit together.">Cluster concepts</a></div>
</div>
<div class="relconcepts"><strong>Related concepts</strong><br />
<div><a href="rzaigconceptsresilientapplications.htm" title="A resilient application is an application that can be restarted on a different cluster node without requiring you to reconfigure the clients.">Resilient applications</a></div>
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