基于CORBA的自适应负载均衡服务的设计与实现

【作者】 张庆；

【导师】 谷建华；

【作者基本信息】 西北工业大学 ， 计算机应用技术， 2004， 硕士

【摘要】 实施负载均衡是提高分布式系统的整体性能，改善其可扩展性和可用性的一个有效手段，也是分布式应用程序不断发展的必然要求。负载均衡可以在系统中的不同层次上得到实现，其中，基于中间件的负载均衡技术与基于网络、基于操作系统以及基于应用层的负载均衡技术相比，在整合异构系统、透明访问和扩展能力等方面具有很大的优势。本文的主要研究内容就是基于CORBA中间件的负载均衡，具体的研究工作有以下几点： 探讨了系统负载的衡量问题，借鉴前人的研究成果，采用以资源利用率和节点的调和性能平均值来综合衡量系统负载的方法，量化了异构系统中各节点的性能，使得负载具有可比性。 系统地论述了动态负载均衡的四个组成要素：转移策略、选择策略、定位策略和信息策略，这些内容是进行负载均衡研究的基础。 详细分析了CORBA规范中可以用来实现负载均衡的基本原理：ServantLocator机制、ForwardRequest机制和Portable Interceptor机制。伺服程序定位器和可移植拦截器都能够引发ForwardRequest异常，指示ORB将客户请求转发到其它对象上，并且这种转发是透明的和可移植的。 针对现有的CORBA负载均衡解决方案的不足，提出了若干设计目标，并在CORBA的对象服务层上设计和实现了一个负载均衡服务。该服务具有较好的可扩展性、适应性和透明性，是一种预防型的负载均衡方法并具有一定的容错能力。 提出了一个自适应的负载均衡算法ALLRR，避免了有突发式的请求时，将大量请求绑定到同一个服务器对象上和系统重载情况下的任务迁移，并把这个算法应用到所设计的负载均衡服务中。通过实验数据表明，该算法能够有效地提高系统性能特别是在系统负载比较重的情况下。更多还原

【Abstract】 Load balancing is an increasingly popular and effective technique that can improve the overall performance of the distributed system and fulfill the needs of applications in high demand of scalability and availability. It can be implemented at various levels in a distributed system. Middleware-based load balancing has more advantages over network-based, OS-based and application level load balancing, such as shielding heterogeneousness and providing transparency and scalability.This thesis focuses on middleware-based load balancing supported by CORBA, and has made the following contributions:(1) Based on the previous research, we give a method using resource utilization and harmonic mean performance to quantify the load of each node in the heterogeneous environment, which makes the loads comparable. (2) We systematically review the main components of dynamic load balancing: transfer policy, location policy, information policy and selection policy that are the basic points of load balancing researches. (3) We analyze the mechanisms mandated by the CORBA specification to develop load balancing. Both the ’servant locator and the portable interceptor can throw ForwardRequest exception indicating that the ORB should forward the client request to other object transparently and portably. (4) In view of the insufficiencies of existing solutions, we design a load balancing service with certain fault tolerance at CORBA service level, striving for more scalability, adaptivity and transparency. (5) We present an adaptive load balancing algorithm which avoids binding a large number of bursting requests to the same object and dispatching tasks under the over-loaded situation. Experimental results demonstrate that our algorithm performs more effectively than conventional approaches especially when the system has heavy burden.更多还原