Emmerich, W

Skene, J

English

We present an approach to managing formal models using Model Driven Architecture (MDA) technologies that delivers analysis techniques through integration with the design tools and repositories that practitioners use. Expert modelling knowledge is captured in domain-specific languages and meta-model constraints. These are represented using UML and colocated with designs and analysis models, providing a flexible and visible approach to managing semantic associations. The approach relies on standards to permit deployment in multiple tools. We demonstrate our approach with an example in which queuing-network models are associated with UML design models to predict average case performance

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A Model-Driven Approach toNon-Functional Analysis of Software Architectures∗James Skene and Wolfgang EmmerichDepartment of Computer Science, University College LondonE-mail: {j.skene|w.emmerich}@cs.ucl.ac.ukAbstractWe present an approach to managing formal models usingModel Driven Architecture (MDA) technologies that deliv-ers analysis techniques through integration with the designtools and repositories that practitioners use. Expert mod-elling knowledge is captured in domain-speciﬁc languagesand meta-model constraints. These are represented usingUML and colocated with designs and analysis models, pro-viding a ﬂexible and visible approach to managing seman-tic associations. The approach relies on standards to permitdeployment in multiple tools. We demonstrate our approachwith an example in which queuing-network models are as-sociated with UML design models to predict average caseperformance.1 IntroductionFormal analysis is often the only way to determinewhether an architectural design will meet its non-functionalrequirements, such as performance and reliability. Formalanalysis is rarely performed partly because of difﬁcultiesinherent in its application, such as the need to employ un-usual high-level languages, to combat state space explosionand a lack of integrated tool support. This has motivatedresearch into the automated derivation of analysis modelsfrom design models, most commonly based on the UniﬁedModelling Language (UML), a widely adopted design nota-tion capable of capturing details pertinent to a broad rangeof analyses. Extensions to UML have also been deﬁned toimprove its amenity to analysis, most notably [6].This work suffers in several respects. Firstly, UML andits extensions do not have a formally deﬁned semantic,meaning that there can be no strong proof of the validityof a particular derivation, and this is unlikely to change asit has thus far proved impossible to build a consensus for∗This research is partly funded through the EU project TAPAS (IST-2001-34069)strong formality for UML. Secondly, the derivations pro-posed in the literature are deﬁned using a number of ad-hoctechniques ranging from graph-grammars to natural lan-guage descriptions. The lack of a standard representationhinders their deployment, and when deployed the techniquebecomes coupled to a particular tool. Thirdly, a completelyencapsulated derivation is unlikely to produce successfulanalyses every time because of the difﬁculty in deriving afeasible and valid model for all designs. This implies thattools are likely to be brittle if they cannot be adapted.In [10] we sketched an approach to analysing non-functional properties of distributed software architecturesbased on Model Driven Architecture (MDA) technologies.MDA is a development approach based on UML models,in which business knowledge (Platform Independent Mod-els - PIMs) is maintained seperately from technical arte-facts, such as design models (Platform Speciﬁc Models -PSMs) and source code. The successful application of theMDA approach depends on technologies and tools support-ing ﬂexible modelling of diverse semantic domains (PIMsand PSMs), and relationships and transformations betweenthem (deployment of PIMs to PSMs). We use the UMLproﬁle mechanism to deﬁne classes of analysis models, de-sign models and the mapping between the two. Proﬁles aredenoted using UML and may be injected into any conform-ing tool, reducing tool tie-in. The derivation is visible andmodiﬁable within a tool, adding ﬂexability, and its coloca-tion with design and analysis models reduces ambiguity dueto the relatively informal semantics of UML.The main contribution of this paper is a substantiationand evaluation of the ideas in [10], using an example inwhich queuing networks are associated with architecturaldesigns.The paper proceeds as follows: Section 2 describes ourapproach; Section 3 gives an example of the approach ap-plied to associate queuing networks with architectural de-signs; Section 4 evaluates the approach; Section 5 describesrelated work and Section 6 concludes.2 ApproachOur approach to delivering expert analysis techniques isto specify the derivation of a formal analysis model from aUML design using logical constraints. This is similar to thederivation of a PSM from a PIM in the MDA developmentapproach.The meaning and structure of UML is deﬁned by a meta-model (the ‘abstract syntax’) and a set of constraints overthe meta-model. The meta-model is object-oriented, con-taining classes that informally deﬁne the meaning of modelelements. This informal semantic is reinforced by con-straints expressed using the Object Constraint Language(OCL). These prevent the production of models that wouldrepresent an illogical situation in the real-world.A proﬁle is a collection of extensions to the meta-model.It contains ‘stereotypes’, which are labels for model ele-ments that indicate membership of a ‘virtual meta-class’.Virtual meta-classes extend meta-classes to provide a re-ﬁned meaning. This meaning is reinforced using constraintsattached to the stereotype. Model elements may also beadorned with ‘tagged values’ to specify new properties.In our approach we deﬁne a proﬁle which extends UMLto model speciﬁcations in some formal language. This lan-guage can be used for analysis, so these speciﬁcation mod-els can be operated upon to generate results. The MDAtechnologies include standard interfaces for operating onmodel data [4, 5], so there is the potential to closely inte-grate model solvers with design tools.We use a proﬁle for the design domain to direct thederivation of analysis models. Constraints ensure that de-sign models are reasonable and contain sufﬁcient detail topermit a derivation.The derivation itself is speciﬁed in a third proﬁle, al-lowing reuse of design and analysis proﬁles. The mappingproﬁle provides a stereotype on associations. The reﬁnedtype of association is constrained to be between two sub-models, one representing the design and the other the anal-ysis model. Additional constraints on the contents of thesub-models, ensuring that the analysis model correctly rep-resents the design.Proﬁles are repesented using UML and may be usedwherever necessary by including them in a model. There-fore, proﬁles speciﬁed according to our scheme can be im-portedintostandards-complianttoolstoenableformalmod-elling and the consistent association of formal models withdesigns.To provide a completely automated derivation of formalmodels it is necessary to implement an additional modeltransformation algorithm, perhaps using a tool-speciﬁcscripting language. The mapping constraints provide thecontract for such an algorithm, and can be used to test itsoperation. Figure 1 shows the overall approach.<<profile>> <<profile>> <<profile>>Design Mapping Analysisintegrationsemi-automatic constructioninterpretationtranslationdesignresultstool executionanalysisresultsmeta-model:model:Figure 1. ApproachResults can also be reintegrated into design models byapplying a mapping from a results domain to tagged-valuesor notes in the design domain. We do not consider this inthis paper.The approach is applicable to analyses relying on graphi-cal formalisms, as UML can easily be extended to resemblethese. For example, performance evaluation and functionalanalysis using Petri nets or Markov chains. [8] proposesto use the technique for reliability modelling, and Baysiannetworks or fault trees would be an appropriate formalism.Our example in the next section uses queuing networks toforecast performance and resource utilisation.3 Example: Analysing real-time UML usingqueuing networksIn this section we provide an example of our approachapplied to deﬁne a derivation from a class of architecturalmodels to queuing networks. To demonstrate the way thatthis derivation would be used in practice, we present arunning example based on a hypothetical website content-management system. The proﬁles and example packagesare shown in Figure 2.<<AnalysisContext>>Website(Figures 3 & 4)<<QueuingNetwork>>WebsiteQN(Figure 5)<<ACtoQN>><<profile>>PADesign<<profile>>ACtoQNMapping<<profile>>AnalysisContextmeta-model:model:Figure 2. Example proﬁlesThe design model proﬁle closely resembles the perfor-2mance subproﬁle of the standard Proﬁle for Schedulabil-ity, Performance and Time Speciﬁcation (the ‘real-time pro-ﬁle’) [6], which permits the modelling of systems contain-ing contended resources. Reuse of such standard proﬁlesenhances the applicability of our approach.To permit analysis the designer must model the environ-ment of the system in terms of populations engaging in use-cases, identiﬁed by a stereotype on an Actor. In our exam-ple there is a large <<OpenPopulation>> of site users,characterised by an exponential arrival rate, and also a asmall <<ClosedPopulation>> of editors who interactafter a think-time.The behaviour of the system is modelled using a se-quence diagram for each use-case, including <<step>>actions with resource demands. The ‘user’ use-case isshown in Figure 3.User LoadBalancer Webserver1 Webserver2 Database<<Step>>{resource=LB CPU,demand=10}<<Step>>{resource=Server 1 CPU,demand=250,reps=0.5}<<Step>>{resource=Server 2 CPU,demand=250,reps=0.5}<<Step>>{resource=Disk,demand=50,reps=10}<<Step>>{resource=Disk,demand=50,reps=10}Figure 3. Sequence diagram, describing de-mands associated with a use-caseThe structure and <<resource>>s of the system aremodelled using a deployment diagram, as shown in ﬁgure 4Webserver1<<resource>>Server 1 CPULoadBalancer<<resource>>LB CPUWebserver2<<resource>>Server 2 CPUCMS<<resource>>CMS CPUDatabase<<resource>>DiskFigure 4. Deployment diagram, describingsystem resourcesProﬁle constraints are presented here in natural lan-guage, but all have a counterpart deﬁned formally usingOCL. An example is provided later in the section. The fullOCL constraints are available on the web [9].The designproﬁle constraints are:1. An analysis context must contain at least one population.2. A population (stereotype on an actor) must be associatedwith a use-case.3. Every use-case must be realised by exactly one associatedinteraction (the dynamic part of a collaboration).4. Each interaction should include at least one message with aresource demand.5. Resources must be uniquely named.6. Every message with a resource demand must be sent to a roledeployed in a context where the resource is available.Analysis models are queuing networks. Figure 5 showsthe queuing network derived from our example design, in-cluding the demands due to editors.Analysis model constraints:1. The network must contain at least one workload class2. Classiﬁers can be either queues or workloads, not both3. All associations stereotyped as demands must start from aclassiﬁer stereotyped with a workload and end in one stereo-typed as a queue.4. All queues must be demanded by at least one workload class.5. All workloads must demand at least one queue.<<Queue>>LB CPU<<Queue>>Server 1 CPU<<Queue>>Disk<<Queue>>CMS CPU<<Queue>>Server 2 CPU<<OpenWorkload>>User{rate = 0.00001}<<ClosedWorkload>>Editor{population=10,thinktime=720000}{demand = 10}{demand = 125}{demand = 125}{demand = 250}{demand = 2000}{demand = 5000}Figure 5. Queuing network modelThe mapping is deﬁned in the context of a stereotypeACtoQN deﬁned on an association. Mapping constraintsare:1. The mapping must be between an analysis context model anda queuing network model.2. Every population must correspond to exactly one workloadclass in name, type and tagged values.3. Every workload must have exactly one corresponding popu-lation.4. All resources with resource demands in a use-case must berepresented by queues.35. Allresourcedemandspresentinaninteractionmustberepre-sented by demands on associations between the correspond-ingworkloadandqueue. Thedemandsmustequalthesumofthe products of action demands and action repetitions withinthe workload.Constraint 3 above is expressed in OCL as follows(this constraint relies on the previously deﬁned functions‘populations’ and ‘workloads’ that return the sets of theseelements in the associated models):package Foundation::Corecontext Abstraction inv:self.stereotype→exists(name = ”ACtoQN”)impliesself.populations→forAll(w : ModelElement |self.workloads→one(p : ModelElement |p.name = w.name and(w.stereotype→exists(name = ”OpenWorkload”) impliesp.stereotype→exists(name = ”OpenPopulation”)) and(w.stereotype→exists(name = ”ClosedWorkload”) implies(p.stereotype→exists(name = ”ClosedPopulation”))))4 EvaluationA lack of tool support for OCL currently hinderswidespread use of our approach. We have shown it to beworkable in our example, which we developed using a com-mercial UML editor, the Rocase OCL Evaluator [3], a re-search prototyped developed as part of the IST Neptuneproject, and an external queuing network evaluator. Theneed to develop models and then export them to an externaltools compromises the goal of tight integration of analysisand design tools. We have reason to be hopeful that the sit-uation will improve. The reliance of the MDA approach onmodel transformations, the use of OCL to describe domainrules, and an increased emphasis on the meta-modelling ofdisparate domains will lead to more standards-based toolscompatible with our approach.5 Related workOur work identiﬁes a standard means to describe thederivation of formal models from UML, so is related to andwe believe compatible with the large amount of work thatcovers particular derivations [2]. Our example uses a for-malisation of part of the real-time proﬁle and derives a sim-ple queuing network. It resembles the real-time proﬁle tolayered queuing network derivation presented using graphgrammars in [7].The Precise UML group [1] have proposed the use ofconstraints between language and domain meta-models todescribe the semantics of UML. We apply this approach tomappings, but using proﬁles instead, to enable tool support.6 ConclusionsWe have presented an approach to associating analy-sis models with designs using MDA technologies, includ-ing UML and OCL, and based on model-transformations,a concept intrinsic to the MDA. The approach addressesseveral problems with previous work that derives analysismodels from UML diagrams: It relies on standards so is notcoupled to a particular modelling tool; mappings are visi-ble and modiﬁable, increasing the ﬂexibility of tools; map-pings and analysis models are visible in the same contextas designs, reducing ambiguity due to the lack of a strictsemantic for the design notation.Despite an existing lack of tool support, we have conﬁ-dence that the emerging popularity of the MDA will leadto an increasing adoption of standards-compliant integrateddevelopment environments capable of supporting the kindof re-deployable automated analysis modelling that we de-scribe.References[1] The precise UML group. http://www.puml.org/.[2] WOSP 2002, Third International Workshop on Software andPerformance. ACM Press, 2002.[3] Computer Science Research Laboratory, “BABES-BOLYAI” University, Romania. OCL-Evaluator.http://lci.cs.ubbcluj.ro/ ocle/.[4] OMG document formal/02-01-01. XML Metadata Inter-change (XMI), version 1.2, 2002.[5] OMG document formal/02-04-03. Meta Object Facility(MOF), version 1.4, April 2002.[6] OMG document ptc/03-03-02. UML Proﬁle for Schedula-bility, Performance, and Time Speciﬁcation, 2003.[7] D. Petriu and H. Shen. Applying the UML performanceproﬁle: Graph grammar-based derivation of LQN modelsfrom UML speciﬁcations. In Proceedings of PerformanceTOOLS 2002, 2002.[8] G. N. Rodrigues, G. Roberts, W. Emmerich, and J. Skene.Reliability support for the model driven architecture. InWorkshop on Software Architectures for Dependable Sys-tems (ICSE-WADS). ACM Press, 2003.[9] J. Skene and W. Emmerich. Full example pro-ﬁles. http://www.cs.ucl.ac.uk/ staff/j.skene/ FSE-2003-profiles.[10] J. Skene and W. Emmerich. Model driven performance anal-ysis of enterprise information systems. In Workshop onTest and Analysis of Component Based Systems (ETAPS-TACoS), Electronic Notes in Theoretical Computer Science(ENTCS). Elsevier Science B. V., 2003.4

Applying the UML performance proﬁle: Graph grammar-based derivation of LQN models from UML speciﬁcations.

Full example proﬁles.

Model driven performance analysis of enterprise information systems.

OMG document formal/02-01-01. XML Metadata Interchange (XMI), version 1.2,

OMG document formal/02-04-03. Meta Object Facility (MOF), version 1.4,

OMG document ptc/03-03-02. UML Proﬁle for Schedulability, Performance, and Time Speciﬁcation,

Reliability support for the model driven architecture.

A model-driven approach to non-functional analysis of software architectures

A model-driven approach to non-functional analysis of software architectures

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