V. Cardellini, M. Colajanni, P.S. Yu,
``DNS dispatching algorithms with state estimators for scalable Web-server clusters'',
World Wide Web Journal, Baltzer Science, Vol. 2, No. 3, pp. 101-113, Aug. 1999.
Replication of information across
a server cluster provides a promising
way to support popular Web sites.
However, a Web-server cluster requires some mechanism for the scheduling
of requests to the most available server.
One common approach
is to use the local Domain Name Server (DNS) as
a centralized dispatcher. The main problem is that WWW
address caching mechanisms (although reducing network traffic) only
let this DNS dispatcher
control a very small fraction of the requests
reaching the Web-server cluster. The non-uniformity
of the load from different client domains, and the high variability
of real Web workload introduce additional degrees
of complexity to the load balancing issue.
These characteristics make existing scheduling
algorithms for traditional distributed systems
not applicable to control the load of Web-server clusters
and motivate the research on entirely new DNS
policies that require some system state information.
We analyze various DNS dispatching policies under
realistic situations where state information needs to be estimated
with low computation and communication overhead
so as to be applicable to a Web cluster architecture.
In a model of realistic scenarios for the Web cluster,
a large set of simulation experiments shows that, by incorporating
the proposed state estimators
into the dispatching policies, the effectiveness of the DNS scheduling
algorithms can improve substantially, in
particular if compared to the
results of DNS algorithms not using adequate state information.
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