Popular Web sites can neither rely on a single powerful server nor on independent mirroredservers to support the ever increasing request load. Scalability and availability can be provided by distributed Web-server architectures that schedule client requests among the multiple server nodes in a user-transparent way. In this paper we will review the state of the art in load balancing techniques on distributed Web-server systems. We will analyze the efficiency and limitations of the various approaches and their tradeoff.

This paper considers policies for distributing requests in clustered Web servers, wherein multiple server machines are configured to function as a single high(er) performance Web server. We evaluate various load distribution policies with respect to both their ability to achieve good load balance (the primary goal) and also to their impact on the effectiveness of per-machine caching. Trace-driven simulation is employed, with workload traces from two heavily-loaded (3-8 million requests per day) commercial Web servers. Our results show that use of current state information is necessary in achieving good load balance only when the achievable per-request bandwidth is not strongly network or client limited. Use of current state information is not found to be necessary with respect to achieving good cache behaviour. Load distribution based on a static hashed assignment of the URL space is found to yield very similar cache performance to load distribution based on current cache contents. We also find that it is possible to achieve both good cache behaviour and good load balance, but it requires use of policies that take both objectives into consideration and that make use of information concerning current server loads.

Given the increasing traffic on the World Wide Web (Web), it is difficult for a single popular Web server to handle the demand from its many clients. By clustering a group of Web servers, it is possible to reduce the origin Web server’s load significantly and reduce user’s response time when accessing a Web document. A fundamental question is how to allocate Web documents among these servers in order to achieve load balancing? In this paper, we are given a collection of documents to be stored on a cluster of Web servers. Each of the servers is associated with resource limits in its memory and its number of HTTP connections. Each document has an associated size and access cost. The problem is to allocate the documents among the servers so that no server’s memory size is exceeded, and the load is balanced as equally as possible. In this paper, we show that most simple formulations of this problem are NP-hard, we establish lower bounds on the value of the optimal load, and we show that if there are no memory constraints for all the servers, then there is an allocation algorithm, that is within a factor 2 of the optimal solution. We show that if all servers have the same number of HTTP connections and the same memory size, then a feasible allocation is achieved within a factor 4 of the optimal solution using at most 4 times the optimal memory size. We also provide improved approximation results for the case where documents are relatively small. 1.

. Popular 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....

In this paper, we present a gateway-assisted QoS adaptation framework which satisfies high-level QoS application guarantees. We validate this framework via a distributed tracking system and show provisioning of critical QoS guarantees despite the best effort underlying resource management. Keywords: Visual tracking, resource adaptation, adaptive middleware, gateway 1.

is necessary to support high request rates to popular Web sites. A clustered Web-server organization is preferable to multiple independent mirrored-servers because it maintains a single interface to the users and has the potential to be more scalable, fault-tolerant and better load balanced. In this paper we propose a Web cluster architecture in which the Domain Name System server (DNS), that dispatches the user requests among the servers through the URL-name to IP-address mapping mechanism, is integrated with a redirection request mechanism based on the HTTP protocol. This should alleviate the side effect of caching the IP-address mapping at intermediate name servers. We compare many alternative mechanisms, including synchronous vs asynchronous activation, and centralized vs distributed decision on redirection. Moreover, we analyze reassignment of entire domains or individual client requests, different types of status information, and different server selection policies for redirecting requests. Our results show that the combination of centralized and distributed dispatching policies allows the Web-server cluster to handle the high load skews in the WWW environment.

Most implementations of HTTP servers do not distinguish among requests to different pages. This has the implication that requests for popular pages have the tendency to overwhelm the requests for other pages. In addition, HTTP servers do not allow a site to specify policies for server resource allocation. This paper presents a notion of quality of service that enables a site to customize how an HTTP server should respond to external requests by setting priorities among page requests and allocating server resources. It also describes a design and an implementation of a distributed HTTP server, QoS Web Server, that enforces the quality of service constraints. The performance analysis of the prototype server indicates that the server provides the desired quality of service with minimal overhead.