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<title>Metacat Service Based Architecture</title>
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      <td colspan="7"><div class="title">KNB Software Development Guide: Metacat Service Based Architecture</div></td>
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      <td class="tablehead" colspan="2"><p class="label">Metacat Service Based Architecture</p></td>
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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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