云软件是：云应用、按需应用还是SaaS应用？
写一篇关于开发云计算应用程序的文章，以及 我不知道如何调用云软件，标题文章是架构和 设计（例如按需应用程序），我可以如何使用定义？我对这个定义很困惑想知道这个概念的区别： 云应用、按需应用和SaaS应用。
谢谢
writing a article about development cloud computing applications, and
i dont know how a call cloud software, the title article is architeture and
design of (eg. On demand Application), how definition i can use??i am very confused about this definition wanna know the difference of this concepts:
Cloud Application,On demand application and SaaS application.
thx
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这些是我个人认为有用的定义：
云应用程序是在云上运行的任何应用程序，即它不依赖于任何特定的基础设施，而是完全在虚拟化服务上运行。这是建筑学上的区别。
按需应用程序是一种可以扩大或缩小需求的应用程序（例如，能够获得额外的处理能力来应对峰值需求）。这是一个功能区别，它可以在云上运行，但同样可以在物理基础设施上运行。例如，我相信 IBM 提供了大型机，您可以根据需要打开额外的处理器。
SaaS应用程序是指提供应用程序的业务模型，即以服务/订阅的方式从提供持续基础设施和维护的另一方租用。 salesforce.com 就是一个例子。这通常通过互联网完成，但也可以通过其他模型（例如专用服务器设备）来交付。
因此，这三个术语的含义和含义略有不同。一个给定的应用程序有可能同时满足这三个类别，但许多应用程序也只会属于这些类别中的零个、一个或两个类别。
These are the definitions I personally find helpful:
A cloud application is any application that runs on the cloud, i.e. it is not tied to any specific infrastructure but runs fully on virtualised services. This is an architectual distinction.
An on demand application is one where you can scale up or down the requirements (e.g. able to get extra processing capacity to deal with peak demand). This is a functional distinction which may run on a cloud but could equally well run on physical infrastructure. For example I believe IBM provides mainframes where you can turn on extra processors as needed.
A SaaS application refers to the business model by which the application is provided, i.e. rented on a service / subscription basis from another party which provides the ongoing infrastructure and maintenance. An example would be salesforce.com. This is typically done over the internet, but could be delivered through other models e.g. a dedicated server appliance.
Hence all three terms have slightly different meanings and implications. It's possible for a given application to be all three, but many will also only fall into zero, one or two of these categories.