Web强缓存一致性的研究

Web对象缓存技术是一种减少web服务器访问通信量和访问延迟的重要手段。Web缓存的引入虽然大大减轻了服务器负载,降低了网络拥塞,减少了客户端访问的延迟等优点,但同时也带来缓存的一致性问题,这样使客户端获得web的数据可能不是最新的版本。该文通过分析现有的缓存一致性方针,提出了一个应适于web的强缓存一致性算法。     

集中管理式Web缓存系统及性能分析

共享缓存文件是减少网络通信量和服务器负载的重要方法，本文在介绍Web Caching技术及流行的Web缓存通信协议ICP的基础上，提出了一种集中管理式Web缓存系统，该系统通过将用户的HTTP请求，按照一定的算法分发到系统中某一合适的缓存服务器上，从而消除了缓存系统内部服务器之间庞大的通信开销及缓存处理负担，减少了缓存内容的冗余度．通过分析，证明了集中管理式Web缓存系统比基于ICP的简单缓存系统具有缓存效率高、处理开销低、延迟小等优点，并且该系统具有良好的可扩展性．     

A locality aware cache diffusion system

Web caching is a widely deployed technique to reduce the load to web servers and to reduce the latency for web browsers. Peer-to-Peer (P2P) web caching has been a hot research topic in recent years as it can create scalable and robust designs for decentralized internet-scale applications. However, many P2P web caching systems suffer expensive overheads such as lookup and publish messages, and lack locality awareness. In this paper, we present the development of a locality aware cache diffusion system that makes use of routing table locality, aggregation, and soft state to overcome these limitations. The analysis and experiments show that our cache diffusion system reduces the amount of information processed by nodes, reduces the number of index messages sent by nodes, and improves the locality of cache pointers.     

一种基于分散协作的Web缓存集群体系结构

Web对象缓存技术是一种减少Web访问通信量和访问延迟的重要手段，该文通过分析现有的各种Web缓存系统，提出了一种基于分散协作的Web缓存集群体系结构。该体系结构克服了集中式系统需要额外配备一台管理服务器的缺陷，消除了管理服务器瓶颈失效造成系统瘫痪的危险，减少由于管理服务器带来的延迟；同时消除了分散系统的缓存不命中情况下的多级转发的延迟和缓存内容重叠，提高了资源利用率和系统效率，具有良好的可扩展性和健壮性。     

ART2 Clustering of Multiple Web Objects for Qualitative Web Prefetching

The web resources in the World Wide Web are rising, to large extent due to the services and applications provided by it. Because web traffic is large, gaining access to these resources incurs user-perceived latency. Although the latency can never be avoided, it can be minimized to a larger extent. Web prefetching is identified as a technique that anticipates the user’s future requests and fetches them into the cache prior to an explicit request made. Because web objects are of various types, a new algorithm is proposed that concentrates on prefetching embedded objects, including audio and video files. Further, clustering is employed using adaptive resonance theory (ART)2 in order to prefetch embedded objects as clusters. For comparative study, the web objects are clustered using ART2, ART1, and other statistical techniques. The clustering results confirm the supremacy of ART2 and, thereby, prefetching web objects in clusters is observed to produce a high hit rate.     

代理Web Cache性能分析

采用WebCache技术提高当前Internet性能已成为一个主流的研究领域,其功能原理就象处理器和文件系统中的多级高速缓存一样。大规模Web高速缓存系统已成为许多国家Internet基础设施的重要组成部分。该文从三个不同访问规模的代理WebCache的跟踪日志出发,分析了WebCache的用户访问模式、Cache命中率、Cache服务器处理延迟等统计特征,提出基于分布式共享RAM和外存储结合的两级协同WebCache集群技术,可以提供可扩展的高性能并行Web高速缓存服务。     

Proxy Cache Replacement Algorithms: A History-Based Approach

Accesing and circulation of Web objects has been facilitated by the design and implementation of effective caching schemes. Web caching has been integrated in prototype and commercial Web-based information systems in order to reduce the overall bandwidth and increase system's fault tolerance. This paper presents an overview of a series of Web cache replacement algorithms based on the idea of preserving a history record for cached Web objects. The number of references to Web objects over a certain time period is a critical parameter for the cache content replacement. The proposed algorithms are simulated and experimented under a real workload of Web cache traces provided by a major (Squid) proxy cache server installation. Cache and bytes hit rates are given with respect to different cache sizes and a varying number of request workload sets and it is shown that the proposed cache replacement algorithms improve both cache and byte hit rates.     

基于片段缓存的动态网页传送模型*

片段缓存机制是加速动态网页分发的有效解决方案之一,但是实施片段缓存需要有效的共享片段检测机制。针对这种情况,提出了一种高效的共享片段检测算法,介绍了基于片段缓存的动态网页传送模型。该模型能够自动识别共享片段和有效的缓存单元,更好地消除冗余数据,提高缓存命中率。实验和分析表明,与现有方案ESI和Silo相比,该模型能够有效节约带宽,缩短用户请求的响应时间。     

Evaluation of Strong Consistency Web Caching Techniques

The growth of the World Wide Web (WWW or Web) and its increasing use in all types of business have created bottlenecks that lead to high network and server overload and, eventually, high client latency. Web Caching has become an important topic of research, in the hope that these problems can be addressed by appropriate caching techniques. Conventional wisdom holds that strong cache consistency, with (almost) transactional consistency guarantees, may neither be necessary for Web applications, nor suitable due to its high overhead. However, as business transactions on the Web become more popular, strong consistency will be increasingly necessary. Consequently, it is important to have a comprehensive understanding of the performance behavior of these protocols. The existing studies, unfortunately, are ad hoc and the results cannot be compared across different studies. In this paper we evaluate the performance of different categories of cache consistency algorithms using a standard benchmark: TPC-W, which is the Web commerce benchmark. Our experiments show that we could still enforce strong cache consistency without much overhead, and Invalidation, as an event-driven strong cache consistency algorithm, is most suitable for online e-business. We also evaluate the optimum deployment of caches and find that proxy-side cache has a 30–35% performance advantage over client-side cache with regard to system throughput.     

Form-based proxy caching for database-backed web sites: keywords and functions

Web caching proxy servers are essential for improving web performance and scalability, and recent research has focused on making proxy caching work for database-backed web sites. In this paper, we explore a new proxy caching framework that exploits the query semantics of HTML forms. We identify two common classes of form-based queries from real-world database-backed web sites, namely, keyword-based queries and function-embedded queries. Using typical examples of these queries, we study two representative caching schemes within our framework: (i) traditional passive query caching, and (ii) active query caching, in which the proxy cache can service a request by evaluating a query over the contents of the cache. Results from our experimental implementation show that our form-based proxy is a general and flexible approach that efficiently enables active caching schemes for database-backed web sites. Furthermore, handling query containment at the proxy yields significant performance advantages over passive query caching, but extending the power of the active cache to do full semantic caching appears to be less generally effective.     

Predictive Prefetching on the Web and Its Potential Impact in the Wide Area

The rapid increase of World Wide Web users and the development of services with high bandwidth requirements have caused the substantial increase of response times for users on the Internet. Web latency would be significantly reduced, if browser, proxy or Web server software could make predictions about the pages that a user is most likely to request next, while the user is viewing the current page, and prefetch their content.In this paper we study Predictive Prefetching on a totally new Web system architecture. This is a system that provides two levels of caching before information reaches the clients. This work analyses prefetching on a Wide Area Network with the above mentioned characteristics. We first provide a structured overview of predictive prefetching and show its wide applicability to various computer systems. The WAN that we refer to is the GRNET academic network in Greece. We rely on log files collected at the network's Transparent cache (primary caching point), located at GRNET's edge connection to the Internet. We present the parameters that are most important for prefetching on GRNET's architecture and provide preliminary results of an experimental study, quantifying the benefits of prefetching on the WAN. Our experimental study includes the evaluation of two prediction algorithms: an n most popular document algorithm and a variation of the PPM (Prediction by Partial Matching) prediction algorithm. Our analysis clearly shows that Predictive prefetching can improve Web response times inside the GRNET WAN without substantial increase in network traffic due to prefetching.     

Evolutionary Techniques for Web Caching

Web caching has been proposed as an effective solution to the problems of network traffic and congestion, Web objects access and Web load balancing. This paper presents a model for optimizing Web cache content by applying either a genetic algorithm or an evolutionary programming scheme for Web cache content replacement. Three policies are proposed for each of the genetic algorithm and the evolutionary programming techniques, in relation to objects staleness factors and retrieval rates. A simulation model is developed and long term trace-driven simulation is used to experiment on the proposed techniques. The results indicate that all evolutionary techniques are beneficial to the cache replacement, compared to the conventional replacement applied in most Web cache server. Under an appropriate objective function the genetic algorithm has been proven to be the best of all approaches with respect to cache hit and byte hit ratios.     

Enhanced Proxy Caching with Content Management

In this paper, we propose a novel approach to enhancing web proxy caching, an approach that integrates content management with performance tuning techniques. We first develop a hierarchical model for management of web data, which consists of physical pages, logical pages and topics corresponding to different abstraction levels. Content management based on this model enables active utilization of the cached web contents. By defining priority on each abstraction level, the cache manager can make replacement decisions on topics, logical pages, and physical pages hierarchically. As a result, a cache can always keep the most relevant, popular, and high-quality content. To verify the proposed approach, we have designed a content-aware replacement algorithm, LRU-SP+. We evaluate the algorithm through preliminary experiments. The results show that content management can achieve 30% improvement of caching performance in terms of hit ratios and profit ratios (considering significance of topics) compared to content-blind schemes. Received 19 March 2001 / Revised 17 April 2001 / Accepted in revised form 25 May 2001     

Value complete,column complete,predicate complete

Caching is a proven remedy to enhance scalability and availability of software systems as well as to reduce latency of user requests. In contrast to Web caching where single Web objects are accessed and kept ready somewhere in caches in the user-to-server path, database caching uses full-fledged database management systems as caches, close to application servers at the edge of the Web, to adaptively maintain sets of records from a remote database and to evaluate queries on them. We analyze a new class of approaches to database caching where the extensions of query predicates that are to be evaluated are constructed by constraints in the cache. Starting from the key concept of value completeness, we explore the application of cache constraints and their implications on query evaluation correctness and on controllable cache loading called cache safeness. Furthermore, we identify simple rules for the design of cache groups and their optimization before discussing the use of single cache groups and cache group federations. Finally, we argue that predicate completeness can be used to develop new variants of constraint-based database caching.     

Web Caching Replacement Algorithm Based on Web Usage Data

Web caching is one of the fundamental techniques for reducing bandwidth usage and download time while browsing the World Wide Web. In this research, we provide an improvement in web caching by combining the result of web usage mining with traditional web caching techniques. Web cache replacement policy is used to select which object should be removed from the cache when the cache is full and which new object should be put into the cache. There are several attributes used for selecting the object to be removed, such as the size of the object, the number of times the object was used, and the time when the object was added into the cache. However, the flaw in these previous approaches is that each object is treated separately without considering the relation among those objects. We have developed a system that can record users’ browsing behavior at the resources level. By using information gathered from this system, we can improve web cache replacement policy so that the number of cache hits will increase, resulting in a faster web browsing experience and less data bandwidth, especially at lower cache storage environments such as on smart phones.     

一种基于有限记忆多LRU的Web缓存替换算法

Web缓存的核心是缓存内容的替换算法.在动态不确定的网络环境下,本文提出一种基于有限记忆的多LRU (LH-MLRU)Web缓存替换算法,它是一种低开销、高性能和适应性的算法.LH-MLRU综合考虑各项因素对Web对象使用多个LRU队列进行分类管理,引入Web对象最近被访问的历史作为缓存内容替换的一个关键因素,来预测对象可能再次被访问的概率.通过周期性的训练参数可以适应动态不确定的网络环境.轨迹驱动的仿真实验表明LH-MLRU在各项性能指标上均优于其他算法,可以显著的提高Web缓存的性能.     

Preferential treatment for short flows to reduce web latency

In this paper, we propose short flow differentiating (SFD) algorithm to reduce the user-perceived web latency. This algorithm gives short flows preferential treatment. We implement SFD algorithm as a simple differentiated services policy and evaluate its performance in simulation. We find that SFD algorithm reduces the transmission latency of short flows and the response time to retrieve representative web pages by about 30%. Using web traces, we demonstrate that 99% web pages would be transferred faster. SFD penalizes long flows, but the penalty is well bounded. We further evaluate how different schemes trade-off the performance between short and long flows.     

Efficient data distribution in a Web server farm

High-performance Web sites rely on Web server `farms', hundreds of computers serving the same content, for scalability, reliability, and low-latency access to Internet content. Deploying these scalable farms typically requires the power of distributed or clustered file systems. Building Web server farms on file systems complements hierarchical proxy caching. Proxy caching replicates Web content throughout the Internet, thereby reducing latency from network delays and off-loading traffic from the primary servers. Web server farms scale resources at a single site, reducing latency from queuing delays. Both technologies are essential when building a high-performance infrastructure for content delivery. The authors present a cache consistency model and locking protocol customized for file systems that are used as scalable infrastructure for Web server farms. The protocol takes advantage of the Web's relaxed consistency semantics to reduce latencies and network overhead. Our hybrid approach preserves strong consistency for concurrent write sharing with time-based consistency and push caching for readers (Web servers). Using simulation, we compare our approach to the Andrew file system and the sequential consistency file system protocols we propose to replace     

Neuro-fuzzy system in partitioned client-side Web cache

This paper proposes a novel contribution in Web caching area, especially in Web cache replacement, so-called intelligent client-side Web caching scheme (ICWCS). This approach is developed by splitting the client-side cache into two caches: short-term cache that receives the Web objects from the Internet directly, and long-term cache that receives the Web objects from the short-term cache. The objects in short-term cache are removed by least recently used (LRU) algorithm as short-term cache is full. More significantly, when the long-term cache saturates, the neuro-fuzzy system is employed efficiently in managing contents of the long-term cache. The proposed solution is validated by implementing trace-driven simulation and the results are compared with least recently used (LRU) and least frequently used (LFU) algorithms; the most common policies of evaluating Web caching performance. The simulation results have revealed that the proposed approach improves the performance of Web caching in terms of hit ratio (HR), up to 14.8% and 17.9% over LRU and LFU. In terms of byte hit ratio (BHR), the Web caching performance is improved up to 2.57% and 26.25%, and for latency saving ratio (LSR), the performance is better with 8.3% and 18.9% over LRU and LFU, respectively.     

企业环境P2P模型下的合作式Web缓存系统研究

提出了一种高性能的合作式Web缓存系统(WebRing),包括一种基于连续哈希的Web对象路由模式,保证了对任意Web请求经过一次哈希计算且至多经过一次转发就可到达目标节点。同时,基于节点状态标记切割哈希空间的系统负载均衡算法大大提高了系统的吞吐量。解决了传统合作式缓存系统中多级转发和多重哈希计算造成的高时延和单点失效问题。