AUBURN COMP 7700 - Combining Stochastic Process Algebras (13 pages)

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Combining Stochastic Process Algebras



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Combining Stochastic Process Algebras

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13
School:
Auburn University
Course:
Comp 7700 - SOFTWARE ARCHITECTURE (3)
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Combining Stochastic Process Algebras and Queueing Networks for Software Architecture Analysis Simonetta Balsamo Marco Bernardo Marta Simeoni Dipartimento di Informatica Univ Ca Foscari di Venezia Centro per l Appl delle S T I Univ di Urbino Dipartimento di Informatica Univ Ca Foscari di Venezia balsamo dsi unive it bernardo sti uniurb it simeoni dsi unive it ABSTRACT We propose an integrated approach to the functional and performance analysis of Software Architectures SAs based on Stochastic Process Algebras SPAs and Queueing Networks QNs in order to combine their main advantages formal techniques for the verification of functional properties of systems for SPAs and efficient performance analysis for QNs We first introduce milia a SPA based architectural description language for the compositional graphical and hierarchical modeling of SAs which is equipped with suitable checks for the detection of architectural mismatches Then we present a systematic approach to derive QN models from milia specifications This is based on the identification of three different classes of QN basic elements arrival processes buffers and service processes and on syntactic restrictions to be imposed to milia specifications so that each architectural component directly falls into one of the three classes Although performance analysis could be carried out directly on the Markov chain MC underlying an milia specification having a QN model allows performance indices to be evaluated possibly by exact product form solutions or by well known approximate methods Furthermore unlike the underlying MC the high level of abstraction of the QN model should ease the interpretation of the performance results at the architectural description level 1 INTRODUCTION Software Architecture SA is an emerging field within software engineering aiming at describing both the structure and the behavior of software systems at a high level of abstraction 18 17 The static and behavioral descriptions characterize at an early stage of development the basic design choices on the system under consideration which clearly influence the subsequent development and deployment phases Appropriate languages and tools are then required in order to support SA with a suitable formalization of the architecture descriptions and the automatic analysis Permission to make digital or hard copies of part or all of this work or personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page To copy otherwise to republish to post on servers or to redistribute to lists requires prior specific permission and or a fee WOSP 02 July 24 26 2002 Rome Italy 2002 ACM ISBN 1 1 58113 563 7 02 07 5 00 190 of their functional properties There is moreover a growing interest in quantitative analysis of software systems and it has been recognized in the last years that performance analysis should be integrated in the software development life cycle since the early stages see e g 20 21 In particular SAs have been devised as the appropriate design level to conduct early predictive performance analysis thus allowing for a choice among alternative architectures on the basis of quantitative aspects In this paper we propose an integrated approach to the functional and performance analysis of SAs based on Stochastic Process Algebras SPAs and Queueing Networks QNs The idea is to combine the main advantages of the two frameworks formal techniques for system specification and verification of functional properties for SPAs and efficient performance analysis for QNs SPA see e g 11 10 6 is a well known formal specification technique for concurrent and distributed systems Its main features i e composability which allows system descriptions to be built in a modular and hierarchical way and abstraction which allows the internal details of a system description to be hidden at analysis time make SPA suited to work with at the architectural level of design Compared to classical process algebra see e g 16 besides the purely functional aspects with SPA it is possible to express activity durations by using random variables In addition to functional verification e g via model checking 7 this permits the quantitative analysis of the modeled system through the construction and solution of the underlying stochastic process QNs see e g 15 14 13 have been widely applied as system performance models Classical QNs represent resource sharing systems and can be solved by efficient algorithms which do not require the construction of the underlying stochastic process Moreover extensions of classical QNs have been introduced in order to represent other interesting features of real systems such as synchronization and concurrency constraints finite capacity queue and memory constraints and simultaneous resource possession Some approximate solution techniques have been defined for several types of extended QNs The presentation of our integrated approach starts with the introduction of milia an architectural description language ADL based on an expressive SPA called EMPAgr 6 milia provides a formal framework for the compositional graphical and hierarchical modeling of software systems which is equipped with some checks inspired by 3 4 for the detection of possible architectural mismatches The timing of the durational actions is mainly expressed by exponential random variables so that the underlying stochastic process yields a Markov Chain MC Although performance analysis could be carried on directly by milia this requires the construction and solution of the underlying MC whose state space explosion soon makes the analysis unfeasible Moreover the MC is a flat model that does not reflect the structure of the specified SA thus hampering the interpretation of the performance analysis results at the architectural description level In order to overcome these two drawbacks related to the efficient evaluation of performance indices and with the possibility of getting some feedback at the architectural description level in case of poor performance we propose an approach to derive QN models from milia specifications More precisely we define a mapping from milia to QNs based on the identification of three different classes of QN basic elements arrival processes buffers and service processes and on some restrictions to the milia specifications so that each architectural component


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