Multitenancy is the fundamental technology that clouds use to share IT resources cost-efficiently and securely.
多租户支持是所有云服务提供商都需要着力实现的一种技术，用于云基础设施上在成本可控和保证安全性的前提下进行IT资源分享。
多租户支持面临的四大挑战：
- how to keep tenant-specific data secure in a shared database so that one tenant can’t see another tenant’s data?
如何保证共享数据库内不同租户间的数据隔离，通俗的说，一个租户不应该看到其他租户的数据。
- 当一个租户对其拥有的schema对象或者应用编程接口进行调整时，不应影响到其他租户的功能和整个系统的可用性。
How can one tenant customize various schema objects and an application’s user interface in real time without affecting the functionality or availability of the system for all other tenants?
- How can the system’s code base be patched or upgraded without breaking tenant-specific schemas?
系统基层实现发生变动，比如代码版本升级，或者新的补丁进来时，不应该破坏每个租户私有的对象。
- And how will the system’s response time scale as tens of thousands of tenants use the service?
随着系统租户数量的增加，整个系统的响应时间不应该急剧下降。
Force.com’s core technology uses a runtime engine that materializes all application data from metadata—data about the data itself. In Force.com’s well-defined metadata-driven architecture, there is a clear separation of the compiled runtime database engine (kernel), tenant data, and the metadata that describes each application. These distinct boundaries make it possible to independently update the system kernel and tenant-specific applications and schemas, with virtually no risk of one affecting the others.
Salesforce应对这些挑战的设计是，采取了一种运行时引擎的思路，所有的应用数据都通过元数据生成，即所谓元数据驱动的架构(metadata driven architecture)。这样一来，模型的元数据作为输入，交给运行时引擎加工，生成运行时使用的模型和数据，三者各司其职，有严格的区分，实现了一种高度动态的内核。
注：本文的英文文字和图片来自Salesforce官网：https://developer.salesforce.com/page/Multi_Tenant_Architecture
中文文字为本文作者原创内容。
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