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Presentation at Dagstuhl Seminar 1999 |
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Improving the Performance of Media Servers Providing Physical Data
IndependenceProblems, Concepts, and Challenges
Ulrich Marder
University of Kaiserslautern
P.O. Box 3049, 67653 Kaiserslautern, Germany
e-mail: marder@informatik.uni-kl.de
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Abstract
This presentation deals with media servers providing physical data independence.
Todays media serversespecially continuous media serversusually do not
provide physical data independence at all. Oneif not the mainreason for this
is performance. Physical data independence without optimization costs a lot of
performance. Therefore, we are looking for a solution of this optimization problem.
The following is a typical scenario where physical data independence would be highly
beneficial: There is global media dataoften called media assetsstored in an
MM-DBMS (which might be a part of a digital library). Lots of heterogeneous clients with
different capabilities of storing, processing, and presenting media data are willing to
access this MM-DBMS. Some only want to retrieve media objects for presentation or possibly
printing. Others create or modify media objects. And again others create media objects by
editing and combining existing ones. Assuming a sort of unbalance between these
applications sounds reasonable: Usually there will be many applications of the
presentation type but few of the other types. Thus, one could think of optimizing the
system for presentation. But unfortunately, the other applications often have much
stronger quality and performance demands.
We show that there are considerable problems when attempting to provide physical data
independence with a media server (or MM-DBMS). Such systems tend to require frequent
format conversions inevitably resulting in bad performance. They may inadvertently lose
data due to irreversible updates. And hiding the internal data representation from the
client obviously means that all the strongly necessary optimization is to be accomplished
by the server, which is both more difficult and more promising than leaving optimization
to the applications.
Our proposed media server concept is based on a generalization of data independence
called transformation independence. This abstraction reduces the creation,
retrieval, and modification of media objects to what can be called the "pure
application semantics". The consequence are multiple optimization dimensions being
left for exploitation by the server. The VirtualMedia concept realizes transformation
independence based on virtual media objects being described by filter graphs. With this
concept, optimization can be basically characterized as the process of optimally matching
transformation request graphs and materialization graphs.
Since we are just at the beginning of developing these concepts, there are still a lot
of challenges to be mastered, like formalizing and evaluating the graph transformation
algorithms and realizing the concept using available DBMS-technology and media server
components.
Presented at: Seminar 99351
"Multimedia Database Support for Digital Libraries", Schloss Dagstuhl, Sept. 1st,
1999.
Published in: Bertino, E., Heuer, A., Ozsu, T., Saake, G. (eds.): Multimedia
Database Support for Digital Libraries, Dagstuhl-Seminar-Report 249, IBFI gGmbH,
Schloss Dagstuhl, Germany, Sept. 1999.
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