Abstract: Scalability is critical to the most Internet enterprises.Maintaining sufficient resources just to meet peak requirements can be costly.Cloud computing provides a powerful computing model that allows users to access resources on-demand.This paper describes a model for the dynamic scaling of Web applications based on thresholds in a virtualized cloud computing environment.It illustrates the scaling approach with a front-end load-balancer for routing and balancing user requests to Web applications deployed on Web servers installed in virtual machine instances.A dynamic scaling algorithm for automated provisioning of virtual machine resources based on threshold number of active sessions will be introduced.According to the proposed model and algorithm，when sudden load surges，the response time is not increased significantly，which may meet the actual demand.

Key words: cloud computing, scalability, virtualization, load-balance

摘要： 可扩展性对于很多互联网企业而言是非常重要的。如果按照访问量峰值需求配置资源，则成本很高，资源利用率很低。云计算提供了一个强大的计算模式，允许用户按需访问资源。基于虚拟云计算环境中阈值，提出一个动态可扩展的Web应用模型，该模型通过一个前端负载平衡器，将用户的访问请求路由安装在云计算环境中虚拟机上的Web服务器上。又提出了一个动态扩展算法用于自动扩展虚拟服务器的数量。根据提出的模型和算法，通过实验模拟，当用户访问量激增时，系统的响应时间不会显著延长，完全可以满足实际的需求。

关键词: 云计算, 扩展性, 虚拟化, 负载平衡