54
4.4
Application Server Models
In a cluster based data center or network server, all requests from clients to an
application server are first passed to a distributor from the webswitch, and then the
distributor forwards each request to one of the application servers according to its dis 
tribution policy. The distribution in the application server should be done differently
compared to the front tier Web server, in which a cache aware distribution like Locality 
Aware Request Distribution (LARD) [58] shows good performance. Especially due to
the high overhead of the SSL protocol, the distributor in an application server should
adopt a policy that minimizes the SSL overhead. Since the session reuse scheme, which
is widely used in single Web servers, is very effective to reduce the SSL overhead [8], we
plan to exploit the session reuse scheme for the cluster based application servers.
Currently, for cluster based Web servers, the Weighted Round Robin (WRR),
Round Robin (RR) and cache aware distribution policies [58] are widely used [29]. Since
these distribution schemes do not consider the SSL session, even though each server in an
application layer is configured to reuse the session information, the subsequent requests
from the same client are likely to be assigned to different nodes. Therefore, we need a
distributor which considers the SSL session.
In this research, we study three different distributor models (Round Robin, ssl with
session and ssl with bf (backend forwarding)) to analyze the performance of SSL enabled
application servers. Detailed explanation of these models is given next.
  Round Robin: In this model, a distributor does not consider the SSL session of
clients, and the requests are distributed by using the RR policy. Thus, requests












  

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