一种新的面向广域Web服务器集群的动态负载平衡算法

由于地理意义上的分散性和系统的动态变化性,群集间的负载分配成为了一个棘手的问题.首先给出了一种多量纲参数的融合策略,然后提出了一种动态负载平衡算法.通过对广域Web服务器集群环境建立数学模型,设计并实现了试验平台.仿真结果表明,该算法可以满足系统的负载平衡需求,因此较大程度地减少重定向引起的延时以及提高系统的准入概率.     

多Web服务器系统的建模、分析与控制

本文基于WWW应用,将服务器和通信访问网络统一考虑为一个整体,提出了Web服务系统的性能分析模型,并将模型扩展到了多服务器系统,分析了各模型参数对服务系统性能的影响,证明了增加新服务器的速率原则,并对比了几种提高服务器性能的方法在不同拥塞条件下的优劣,最后研究了多Web服务器系统负载平衡的实现问题,这些结论在网络管理,规划设计和网络升级都具有实际意义。     

OpenWebServer: an adaptive Web server using software patterns

The explosive growth of the Web requires servers to be extensible and configurable. This article describes our adaptive Web server, OpenWebServer, which uses the Reflection architectural pattern. The server supports the dynamic adoption of functionality, such as introducing additional protocols, modifying execution policies, and tuning system performance. This is achieved by specifying and coordinating metaobjects that represent various aspects within the Web server. We present a Java version of OpenWebServer, and describe its design using Reflection and other design patterns: Singleton, Bridge, Mediator, Observer, and Decorator. These patterns provide a better-factored design and allow the Web server to evolve continually beyond static and monolithic servers     

基于HTTP的网络服务性能建模与分析

该文将服务器系统和通信访问网络考虑为一个整体建模,给出了一个基于HTTP的Web系统的服务性能模型,并将模型扩展到了多服务器系统;分析了各模型参数对网络服务性能的影响,提出了避免死锁状态的方法和增加服务器的速率原则;最后对比了几种提高服务性能的方法.仿真结果证实了模型的有效性.     

Performance benchmarking of wireless Web servers

The advent of mobile computers and wireless networks enables the deployment of wireless Web servers and clients in short-lived ad hoc network environments, such as classroom area networks. The purpose of this paper is to benchmark the performance capabilities of wireless Web servers in such an environment. Network traffic measurements are conducted on an in-building IEEE 802.11b wireless ad hoc network, using a wireless-enabled Apache Web server, several wireless clients, and a wireless network traffic analyzer. The experiments focus on the HTTP transaction rate and end-to-end throughput achievable in such an ad hoc network environment, and the impacts of factors such as Web object size, number of clients, and persistent HTTP connections. The results show that the wireless network bottleneck manifests itself in several ways: inefficient HTTP performance, client-side packet losses, server-side packet losses, network thrashing, and unfairness among Web clients. Persistent HTTP connections offer up to 350% improvement in HTTP transaction rate and user-level throughput, while also improving fairness for mobile clients accessing content from a wireless Web server.     

Web服务器的网络负载均衡研究

介绍了网络负载均衡的定义和总体指标。详细讨论了网络负载均衡技术的4种类型。针对不同Web服务器的架构,分析了服务器的吞吐量和网络延迟,其结论对于有效提高服务器的整体性能、服务质量和服务水平具有参考意义。     

提高Web服务器性能的方法

目前大多网络应用采用Browser/Server多层次结构,这种结构使得Web服务器承受的压力过大,本文研究在此情况下提高Web服务器性能的方法。通过一些理论研究和实际应用,在多个可以提高Web服务器性能的方法中,主要阐述了多个Web服务器并行工作、负载均衡、Web交换机、Web缓存和流量管理几个方法。这些方法能很好地改进和提高Web服务器的性能可在网络设计和管理中综合考虑运用。     

网页防篡改中分布式文件同步复制系统

为了解决Web服务器核心内嵌防篡改系统中,文件的分布式发布以及文件篡改后的即时恢复的问题。文中基于J2EE技术架构以及MVC（模型一视图一控制）的软件开发模式,设计了一种分布式文件同步复制系统。该系统能将文件分布式的发送到多个Web服务器上;当检测到某个Web服务器上的文件被篡改了,系统能迅速从原始库同步复制到相应的Web服务器上,以达到网页防篡改的目的。系统通过对文件进行比对,只传输差异部分,提高了系统资源利用率。     

基于随机高级Petri 网的主动自调度集群系统的性能分析

首先在体系结构上对主动自调度集群系统(ASACS)与传统Web集群服务器系统进行了比较;然后提出了主动自调度集群系统的随机高级Petri网(SHLPN)模型,并设计了模型的精化方案;接着为传统Web服务器集群系统中2种负载均衡调度策略和ASACS的主动自调度策略进行了建模;最后利用SPNP工具对服务器集群系统中的3种调度策略在吞吐量、响应时间及拒绝概率等性能上作了数值分析,发现采用主动自调度策略实现的集群系统能更好地满足QoS的要求。     

Application-layer anycasting: a server selection architecture anduse in a replicated Web service

Server replication improves the ability of a service to handle a large number of clients. One of the important factors in the efficient utilization of replicated servers is the ability to direct client requests to the “best” server, according to some optimality criteria. In the anycasting communication paradigm, a sender communicates with a receiver chosen from an anycast group of equivalent receivers. As such, anycasting is well suited to the problem of directing clients to replicated servers. This paper examines the definition and support of the anycasting paradigm at the application-layer, providing a service that uses an anycast resolver to map an anycast domain name and a selection criteria into an IP address. By realizing anycasting in the application-layer, we achieve flexibility in the optimization criteria and ease the deployment of the service. As a case study, we examine the performance of our system for a key service: replicated Web servers. To this end, we develop an approach for estimating the response time that a client will experience when accessing given servers. Such information is maintained in the anycast resolver that clients query to obtain the identity of the server with the best estimated response time. Our performance collection technique combines server push with resolver probes to estimate the expected response time without undue overhead. Our experiments show that selecting a server using our architecture and estimation technique can improve the client response time by a factor of two over nearest server selection and by a factor of four over random server selection     

Performance modeling of an Apache Web server with a dynamic pool of service processes

In the current Internet the performance of service delivery crucially depends on the proper and efficient operation of Web servers. It is determined by their software architecture and characterized by the applied processing model. Here we consider the Unix software architecture of an Apache Web server with its non-threaded multi-processing module Prefork. We propose a tractable multi-server model to approximate the performance of the load-dependent dynamic behavior of Apache’s resource pool of available HTTP service processes, which has not been done before. Furthermore, we show that this Markovian queueing model can be solved by advanced matrix-geometric methods. Then the efficiently computed performance results of this analytic model are compared with measurements of a real Apache Web server. The outcome clearly indicates that our analytic model can very accurately predict the mean-value performance of Apache under the Prefork policy.     

DHTTP: an efficient and cache-friendly transfer protocol for the web

Today's HTTP carries Web interactions over client-initiated TCP connections. An important implication of using this transport method is that interception caches in the network violate the end-to-end principle of the Internet, which severely limits deployment options of these caches. Furthermore, while an increasing number of Web interactions are short, and in fact frequently carry only control information and no data, TCP is often inefficient for short interactions We propose a new transfer protocol for the Web, called Dual-Transport HTTP (DHTTP), which splits the traffic between UDP and TCP channels. When choosing the TCP channel, it is the server who opens the connection back to the client. Through server-initiated connections, DHTTP upholds the Internet end-to-end principle in the presence of interception caches, thereby allowing unrestricted caching within backbones. Moreover, the comparative performance study of DHTTP and HTTP using trace-driven simulation as well as testing real HTTP and DHTTP servers showed a significant performance advantage of DHTTP when the bottleneck is at the server and comparable performance when the bottleneck is in the network.     

Atomic multicast protocol for intrusion-tolerance based on trusted timely computing base

Distributed architecture is often adopted for the intrusion-tolerance system currently. However, this distributed intrusion-tolerance system has a consensus problem. To solve this problem, this article explores a distributed intrusion-tolerance system of hybrid time model based on trusted timely computing base (TTCB) and implement an atomic multicast protocol using TTCB services. The TTCB is a trust secure real-time component inside the server, with a well defined interface and separated from the operation system. It is in the synchronous communication environment, while the application layer in the server works asynchronously. By the atomic multicast protocol, it can be achieved that when the servers are over twice the number of faulty servers, the consensus can be satisfied. The performance evaluations show that the proposed protocol can yield larger good throughput with a lower unavailability.     

A comparison of load balancing techniques for scalable Web servers

Scalable Web servers can be built using a network of workstations where server capacity can be extended by adding new workstations as the workload increases. The topic of our article is a comparison of different method to do load-balancing of HTTP traffic for scalable Web servers. We present a classification framework the different load-balancing methods and compare their performance. In addition, we evaluate in detail one class of methods using a prototype implementation with instruction-level analysis of processing overhead. The comparison is based on a trace driven simulation of traces from a large ISP (Internet Service Provider) in Norway. The simulation model is used to analyze different load-balancing schemes based on redirection of request in the network and redirection in the mapping between a canonical name (CNAME) and IP address. The latter is vulnerable to spatial and temporal locality, although for the set of traces used, the impact of locality is limited. The best performance is obtained with redirection in the network     

Using name-based mappings to increase hit rates

Clusters of identical intermediate servers are often created to improve availability and robustness in many domains. The use of proxy servers for the World Wide Web (WWW) and of rendezvous points in multicast routing are two such situations. However, this approach can be inefficient if identical requests are received and processed by multiple servers. We present an analysis of this problem, and develop a method called the highest random weight (HRW) mapping that eliminates these difficulties. Given an object name and a set of servers, HRW maps a request to a server using the object name, rather than any a priori knowledge of server states. Since HRW always maps a given object name to the same server within a given cluster, it may be used locally at client sites to achieve consensus on object-server mappings. We present an analysis of HRW and validate it with simulation results showing that it gives faster service times than traditional request allocation schemes such as round-robin or least-loaded, and adapts well to changes in the set of servers. HRW is particularly applicable to domains in which there are a large number of requestable objects, there is a significant probability that a requested object will be requested again, and the CPU load due to any single object can be handled by a single server. HRW has now been adopted by the multicast routing protocols PIMv2 and CBTv2 as its mechanism for routers to identify rendezvous points/cores     

一个面向Internet的分布式信息检索系统模型

随着Internet上的用户急剧增加,如何快速地处理查询请求已成为面向Internet的信息检索系统急需解决的问题.文中提出了一个面向Internet的分布式信息检索系统模型(Fastresponse).在该系统模型中,针对当前分布式信息检索系统在可获得性、可靠性、可扩展性、查询服务智能性等方面存在的一些问题,我们提出了对服务器和文档进行两级调度;系统需要时动态招募备份服务器;对子服务器群中和子服务器群间的服务器进行两级监控;根据用户的要求自动重组服务器等一些新的管理策略,有效地解决了上述方面存在的问题,从而可以更好地处理用户的查询请求.     

Estimation‐Based Load‐Balancing with Admission Control for Cluster Web Servers

The growth of the World Wide Web and web‐based applications is creating demand for high performance web servers to offer better throughput and shorter user‐perceived latency. This demand leads to widely used cluster‐based web servers in the Internet infrastructure. Load balancing algorithms play an important role in boosting the performance of cluster web servers. Previous load balancing algorithms suffer a significant performance drop under dynamic and database‐driven workloads. We propose an estimation‐based load balancing algorithm with admission control for cluster‐based web servers. Because it is difficult to accurately determine the load of web servers, we propose an approximate policy. The algorithm classifies requests based on their service times and tracks the number of outstanding requests from each class in each web server node to dynamically estimate each web server load state. The available capacity of each web server is then computed and used for the load balancing and admission control decisions. The implementation results confirm that the proposed scheme improves both the mean response time and the throughput of clusters compared to rival load balancing algorithms and prevents clusters being overloaded even when request rates are beyond the cluster capacity.     

防范SQL注入的应用分析

由于各种Web服务器的漏洞与程序的非严密性,导致针对Web服务器的脚本攻击事件日益增多,其大多是通过ASP或PHP等脚本注入作为主要的攻击手段,Web站点迅速膨胀的今天,基于两者的SQL注入也慢慢成为目前攻击的主流方式。文中简要介绍了SQL注入攻击的概念和原理,以及SQL注入攻击的特点和实现过程,并在此基础上叙述了如何检测SQL注入攻击,总结了一般的SQL注入攻击的防范方法.     

Maximum Availability Server Selection Policy for Efficient and Reliable Session Control Systems

There has been a rapid growth of services based on session control. Session-based services comprise multimedia conferences, Internet telephone calls, instant messaging, and similar applications consisting of one or more media types such as audio and video. Deployment examples include session control services as part of the IP multimedia subsystem (IMS), in the third-generation mobile networks. High service dependability in session control systems is achieved by introducing redundancy, e.g., through reliable server pooling (RSerPool) or clustering. Namely, session control servers are multiplied in server sets. Performance of such replicated session control servers is quantified by transaction control time. Thus, reducing transaction control time enhances performance. Server selection policies (SSP) are crucial in achieving this goal. The maximum availability (MA) SSP is proposed to improve session control performance in scenarios with server and communication failures. Based on a status vector, MA aims at maximizing the probability of successful transaction with the current transmission, thereby minimizing the average number of attempted servers until success. MA is applicable in a broad range of IP-based systems and services, and it is independent of the fault-tolerant platform. A simple protocol extension is proposed in order to integrate MA into the RSerPool fault-tolerant architecture. In addition, an analytic model is derived based on certain system model assumptions. Analytic and simulation results show that transaction control time is considerably reduced with MA as opposed to when using traditional round robin