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<td bgcolor="ffffff" width="15">&nbsp;</td><td bgcolor="ffffff" width="12">&nbsp;</td><td valign="top" align="left" bgcolor="ffffff" width="*">
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<td width="90%"><font size="4" face="Verdana, Arial, Helvetica"><b>架构特性（质量属性）</b></font></td><td width="200" align="right"><font size="1" face="Verdana, Arial, Helvetica"><nobr>   第 2 页（共6 页）</nobr></font></td>
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<p>如前所述，系统的架构应当解决功能性、业务需要和非功能或者服务级需要。在初始阶段，架构师需要定义每一个服务级需求的服务质量度量。通常需要在这些需求之间寻找平衡。例如，要实现更好的扩展性，架构师可能引入包含许多对象的、模块化的架构，因而增加了内存要求，这会影响性能。最重要的服务级需求为：
		<ul>
<li>可伸缩性</li>
<li>可维护性</li>
<li>可靠性</li>
<li>可用性</li>
<li>可扩展性</li>
<li>性能</li>
<li>可管理性</li>
<li>安全性</li>
</ul>
		
</p>
<p>
<b>可伸缩性</b>
</p>
<p>可伸缩性是当事务负荷增加时，在保证服务质量的条件下容纳更多用户的能力。在负荷增加时，可伸缩的系统可以在可接受的限度内作出响应。要扩大容量已满的系统，可以垂直或者水平增加硬件，因而有以下两种伸缩：
		<ul>
<li>
<b>垂直伸缩（Vertical scaling）</b>
<br>
垂直伸缩通过在现有服务器中增加能力（内存、CPU 等）。这样对系统的架构只需要做很少改变或者不需要做任何改变。这种伸缩可帮助增加能力，为更多客户机提供服务，并且需要的资源管理较少。这种伸缩相对来说比水平伸缩更容易、更廉价。<br>
<br>
</li>
<li>
<b>水平伸缩（Horizontal scaling）</b>
<br>
水平伸缩通过在系统加增加服务器实现。这增加了服务器的可靠性和灵活性。它有时还会提高性能，这取决于负载平衡算法。但是水平伸缩增加了系统架构的复杂性，从而降低了可管理性。
		<p>J2EE 架构支持水平和垂直伸缩。在水平伸缩中，服务器可以管理更多组件。在垂直伸缩中，在负载平衡和群集的帮助下，可以为更多客户机提供服务。</p>
<br>
<br>
</li>
</ul>
		
</p>
<p>
<b>可维护性</b>
</p>
<p>可维护性是在不会影响任何其他组件的能力的条件下，修复现有功能缺陷的能力。要增强可维护性，系统设计应当是模块化的，并且系统应该有适当的文档。在部署系统时，无法对这项要求进行度量。</p>
<p>
<b>可靠性</b>
</p>
<p>可靠性是保证应用程序及其所有事务的完整性和一致性的能力。可以通过使用水平伸缩，如增加更多服务器，来增加可靠性。提高可靠性也会提高可用性。</p>
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