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* metacat-architecture.html
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* Authors: Michael Daigle
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* Copyright: 2009 Regents of the University of California and the
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* National Center for Ecological Analysis and Synthesis
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* For Details: http://www.nceas.ucsb.edu/
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* Created: 2009 January 19
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* Version:
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* File Info: '$ '
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-->
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<html>
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<head>
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<title>Metacat Service Based Architecture</title>
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<!-- unfortunately, we have to look for the common css file in the
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user docs directory. This is because the user docs deploy to
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the top level of the metacat docs on the knb web server -->
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<link rel="stylesheet" type="text/css" href="../user/common.css">
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<link rel="stylesheet" type="text/css" href="./default.css">
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<body>
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<table class="tabledefault" width="100%">
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<tr>
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<td rowspan="2"><img src="./images/KNBLogo.gif"></td>
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<td colspan="7"><div class="title">KNB Software Development Guide: Metacat Service Based Architecture</div></td>
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<tr>
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<td><a href="/" class="toollink"> KNB Home </a></td>
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<td><a href="/data.html" class="toollink"> Data </a></td>
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<table width="100%">
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<tr>
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<td class="tablehead" colspan="2"><p class="label">Metacat Service Based Architecture</p></td>
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<td class="tablehead" colspan="2" align="right">
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<a href="./query-caching.html">Back</a> | <a href="./index.html">Home</a> |
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<a href="./utilities-classes.html">Next</a>
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</td>
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</tr>
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</table>
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<div class="header1">Table of Contents</div>
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<div class="toc">
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<div class="toc1"><a href="#Overview">Overview</a></div>
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<div class="toc1"><a href="#BaseService">Extending BaseService Class</a></div>
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<div class="toc1"><a href="#ServiceService">Controling Services With ServiceService Class</a></div>
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</div>
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<a name="Overview"></a><div class="header1">Overview</div>
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<p>There is a goal to migrate Metacat to a service based architecture. This
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provides several advantages: </p>
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<ul>
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<li>Cached values can be refreshed via the configuration utility without
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requiring a restart of Metacat.</li>
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<li>Services can be gracefully shut down when Metacat is stopped. For
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instance, the database service can make sure all db requests have completed
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before continuing with shutdown.</li>
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<li>Services can have a common interface to expose MBeans for JMX monitoring
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(see the <a href="http://java.sun.com/javase/technologies/core/mntr-mgmt/javamanagement/">Java Management Extensions</a> page).</li>
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</ul>
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<p>From a
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<p>There are a few basic questions that can be asked when you are creating a
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class to decide if it should be a service.</p>
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<ul>
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<li>Does this class perform some action repeatedly throughout the lifetime of
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the Metacat application?</li>
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<li>Does it make sense for this class to cache certain information? This is
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typically information that is accessed often and seldom changes. For example,
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system properties rarely change and are constantly being accessed. (Thus the
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PropertyService class.)</li>
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<li>Would it make sense for the information in the class to be refreshable? Note
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that this is not a strict requirement, since, as we will discuss, a class can
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declare itself to be non-refreshable. A good example of a non-refreshable
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class might be a database service. If core database information changes,
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that usually requires a system restart, and not a service refresh.</li>
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</ul>
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<p>Any of these could signify the need for a service. Note that another option
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is to create a utility class. Utility classes are singletons that have static
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methods and less state requirements. See the
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<a href="./utilities-classes.html">Utilities Classes page</a> for more information.</p>
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<a name="BaseService"></a><div class="header1">Extending BaseService Class</div>
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<p> All Services extend the BaseService class except the ServiceService class (discussed
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next). </p>
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<p>Currently, BaseService defines three methods:</p>
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<ul>
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<li> refreshable() - a package level abstract method that reports whether the class can be
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refreshed. This has package level access because we only want ServiceService to be
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able to access this information. You will need to decide at implementation time
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whether it makes sense to refresh your service and return the appropriate boolean from
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this method.</li>
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<li> doRefresh() - a protected abstract method that performs the service refresh. It is
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protected because it should only be called by the refresh() method (see next). Typically,
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an implementation of the doRefresh() method updates any cached values in the service and
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performs any appropriate state-specific service updates.</li>
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<li> refresh() - a package level method that calls refreshable() and doRefresh(). It has
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package level access because we only want the ServiceService to be able to refresh the
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service.</li>
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<li>stop() - Gracefully shut down the service, releasing any resources.</li>
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</ul>
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<a name="ServiceService"></a><div class="header1">Controling Services With ServiceService Class</div>
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<p> The ServiceService class is a exception to the BaseService extension rule. This service
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controls all other services. It maintains a registry of available services. </p>
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<p>Some of the methods available via ServiceService are: </p>
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<ul>
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<li>registerService() - register a service by sending the service name and an instance of
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the service. </li>
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<li>refreshService() - refresh a service by name.</li>
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</ul>
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<br>
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<a href="./query-caching.html">Back</a> | <a href="./index.html">Home</a> |
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