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\section{Conclusions}
\label{sec:conclusion}

Distributed systems implemented with standardized middleware present
several challenges with respect to the accidental complexities associated
with provisioning i.e., configuration and QoS validation.  In current
practice, provisioning of middleware are performed through
low-level, non intuitive and non reusable means.  The manual nature of
these techniques are error prone and tedious, and prohibit a system
provisioner from rapidly exploring various design alternatives.  To
address these challenges,  this paper presented POSAML, which is a
visual modeling language that addresses the provisioning problem at a
higher-level of abstraction.

From our experience, we have found that POSAML allows various
provisioning scenarios to be explored in a rapid manner that is
middleware-independent.  The concerns that are separated among the
various aspects in POSAML provide an ability to evolve the
configuration in a manner that isolates the effect to a single design
change.  When a choice is made for a pattern, POSAML removes all of
the inconsistent choices among  other patterns.  This allows the
provisioner to work with a narrowed search space and ignore  all 
incompatible configurations.  Furthermore, model interpreters
associated with POSAML assist in generating the artifacts needed to
perform QoS validation.  

We have applied POSAML to model several case studies implemented in
the ACE/TAO middleware.  Although our experience  in using POSAML to
configure and provision these case studies has been positive, there
are  still a few limitations that remain.  For example, we do not yet
have the means to let systems provisioners express the application QoS
requirements visually at a higher level of abstraction such that these
requirements can be mapped automatically to the appropriate
provisioning of the middleware.  Secondly, our generative techniques
are applied only for the TAO middleware.  In future we shall
investigate solutions to both these limitations.

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