1,720,999 research outputs found
Geographic load balancing for scalable distributed web systems
No full textUsers of highly popular Web sites may experience long delays when accessing information. Upgrading content site infrastructure from a single node to a locally distributed Web cluster composed by multiple server nodes provides a limited relief, because the cluster wide-area connectivity may become the bottleneck. A better solution is to distribute Web clusters over the Internet by placing content nodes in strategic locations. A geographically distributed architecture where the Domain Name System (DNS) servers evaluate network proximity and users are served from the closest cluster reduces network impact on response time. On the other hand, serving closest requests only may cause unbalanced servers and may increase system impact on response time. To achieve a scalable Web system, we propose to integrate DNS proximity scheduling with an HTTP request redirection mechanism that any Web server can activate. We demonstrate through simulation experiments that this further dispatching mechanism augments the percentage of requests with guaranteed response time, thereby enhancing the Quality of Service of geographically distributed Web sites. However, HTTP request redirection should be used selectively because the additional round-trip increases network impact on latency time experienced by users. As a further contribution, this paper proposes and compares various mechanisms to limit reassignments with no negative consequences on load balancing
Efficient state estimators for load control policies in scalable Web server clusters
No full textReplication of information across a server cluster provides a promising way to support popular Web sites. However a Web server cluster requires some mechanism for directing requests to the best server. One common approach is to use the Domain Name Server (DNS) as a centralized schedule. However address caching mechanisms and the non-uniformity of the load from different client domains complicate the load balancing issue and make existing scheduling algorithms for traditional distributed systems not applicable to Web server clusters. We consider the theoretical DNS policies that require some system state information. We extend them to realistic situations where state information needs to be estimated with low computation and communication overhead. We show that by incorporating these estimators into the DNS policies, load balancing improves substantially, even if the DNS control is limited to a small portion of client request
High Performance Web-Server Systems
No full textPopular Web sites cannot rely on a single powerful server nor on independent mirrored-servers to support ever increasing request load. Scalability and availability can be provided by distributed Webserver architectures that distribute client requests among the multiple servers in a user-transparent way. In this paper we will review the state of the art in distributed Web-server systems. We will analyze the efficiency and limitations of the various approaches and their tradeoff. 1 Introduction The explosive growth of traffic on the World Wide Web is causing a rapid increase in the request rate to popular Web sites. These sites can suffer from severe congestion, especially in conjunction with special events. One approach to handle popular Web sites is based on the replication of information across an independent mirrored-server architecture. This solution has a number of disadvantages, including the not usertransparent architecture, and the lack of control on the request distribution...
Impact of workload models in the evaluating the performance of distributed Web-server systems
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Dynamic Load Balancing in Geographically Distributed Heterogeneous Web Servers
No full textWith ever increasing Web traffic, a distributed multi-server Web site can provide scalability and flexibility to cope with growing client demands. Load balancing algorithms to spread the requests across multiple Web servers are crucial to achieve the scalability. Various domain name server (DNS) based schedulers have been proposed in the literature, mainly for multiple homogeneous servers. The presence of heterogeneous Web servers not only increases the complexity of the DNS scheduling problem, but also makes previously proposed algorithms for homogeneous distributed systems not directly applicable. This leads us to propose new policies, called adaptive TTL algorithms, that take into account of both the uneven distribution of client request rates and heterogeneity of Web servers to adaptively set the time-to-live (TTL) value for each address mapping request. Extensive simulation results show that these strategies are robust and effective in balancing load among geographically distributed heterogeneous Web servers
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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