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Section: Scientific Foundations

Keywords : Objects, design patterns, software components, contracts, aspects, models, UML, product lines.

Model Driven Engineering for Distributed Software

Software Product Lines

It is seldom the case nowadays that we can any longer deliver software systems with the assumption that one-size-fits-all. We have to handle many variants accounting not only for differences in product functionalities (range of products to be marketed at different prices), but also for differences in hardware (e.g.; graphic cards, display capacities, input devices), operating systems, localization, user preferences for GUI (“skins”). Obvioulsy, we do not want to develop from scratch and independantly all of the variants the marketing department wants. Furthermore, all of these variant may have many successive versions, leading to a two-dimensional vision of product-lines.

Object-Oriented Software Engineering

The object-oriented approach is now widespread for the analysis, the design, and the implementation of software systems. Rooted in the idea of modeling (through its origin in Simula), object-oriented analysis, design and implementation takes into account the incremental, iterative and evolutive nature of software development [57] , [55] : large software system are seldom developed from scratch, and maintenance activities represent a large share of the overall development effort.

In the object-oriented standard approach, objects are instances of classes. A class encapsulates a single abstraction in a modular way. A class is both closed , in the sense that it can be readily instanciated and used by clients objects, and open , that is subject to extensions through inheritance  [59] .

Design Pattern

Since by definition objects are simple to design and understand, complexity in an object-oriented system is well known to be in the collaboration between objects, and large systems cannot be understood at the level of classes and objects. Still these complex collaborations are made of recurring patterns, called design patterns. The idea of systematically identifying and documenting design patterns as autonomous entities was born in the late 80's. It was brought into the mainstream by such people as Beck, Ward, Coplien, Booch, Kerth, Johnson, etc. (known as the Hillside Group). However the main event in this emerging field was the publication, in 1995, of the book Design Patterns: Elements of Reusable Object Oriented Software by the so-called Gang of Four (GoF), that is E. Gamma, R. Helm, R. Johnson and J. Vlissides  [56] . Today, design patterns are widely accepted as useful tools for guiding and documenting the design of object-oriented software systems. Design patterns play many roles in the development process. They provide a common vocabulary for design, they reduce system complexity by naming and defining abstractions, they constitute a base of experience for building reusable software, and they act as building blocks from which more complex designs can be built. Design patterns can be considered reusable micro-architectures that contribute to an overall system architecture. Ideally, they capture the intent behind a design by identifying the component objects, their collaborations, and the distribution of responsibilities. One of the challenges addressed in the Triskell project is to develop concepts and tools to allow their formal description and their automatic application.

Component

The object concept also provides the bases needed to develop software components , for which Szyperski's definition  [62] is now generally accepted, at least in the industry:

A software component is a unit of composition with contractually specified interfaces and explicit context dependencies only. A software component can be deployed independently and is subject to composition by third party.

Component based software relies on assemblies of components. Such assemblies rely in turn on fundamental mechanisms such as precise definitions of the mutual responsability of partner components, interaction means between components and their non-component environment and runtime support (e.g. .Net, ejb , Corba Component Model ccm , OSGI or Fractal).

Components help reducing costs by allowing reuse of application frameworks and components instead of redeveloping applications from scratch (product line approach). But more important, components offer the possibility to radically change the behaviors and services offered by an application by substitution or addition of new components, even a long time after deployment. This has a major impact of software lifecycle, which should now handle activities such as the design of component frameworks, the design of reusable components as deployment units, the validation of component compositions coming from various origins and the component life-cycle management.

Empirical methods without real component composition models have appeared during the emergence of a real component industry (at least in the Windows world). These methods are now clearly the cause of untractable validation and of integration problems that can not be transposed to more critical systems (see for example the accidental destruction of Ariane 501  [58] ).

Providing solutions for formal component composition models and for verifiable quality (notion of trusted components ) are especially relevant challenges. Also the methodological impact of component-based development (for example within the maturity model defined by the sei ) is also worth attention.

Contracts

Central to this trusted component notion is the idea of contract . A software contract captures mutual requirements and benefits among stake-holder components, for example between the client of a service and its suppliers (including subcomponents). Contracts strengthen and deepen interface specifications. Along the lines of abstract data type theory, a common way of specifying software contracts is to use boolean assertions called pre- and post-conditions for each service offered, as well as class invariants for defining general consistency properties. Then the contract reads as follows: The client should only ask a supplier for a service in a state where the class invariant and the precondition of the service are respected. In return, the supplier promises that the work specified in the post-condition will be done, and the class invariant is still respected. In this way rights and obligations of both client and supplier are clearly delineated, along with their responsibilities. This idea was first implemented in the Eiffel language  [60] under the name Design by Contract , and is now available with a range of expressive power into several other programming languages (such as Java) and even in the Unified Modeling Language (UML) with the Object Constraint Language (OCL)  [63] . However, the classical predicate based contracts are not enough to describe the requirements of modern applications. Those applications are distributed, interactive and they rely on resources with random quality of service. We have shown that classical contracts can be extended to take care of synchronization and extrafunctional properties of services (such as throughput, delays, etc)  [54] .

Models and Aspects

As in other sciences, we are increasingly resorting to modelling to master the complexity of modern software development. According to Jeff Rothenberg,

Modeling, in the broadest sense, is the cost-effective use of something in place of something else for some cognitive purpose. It allows us to use something that is simpler, safer or cheaper than reality instead of reality for some purpose. A model represents reality for the given purpose; the model is an abstraction of reality in the sense that it cannot represent all aspects of reality. This allows us to deal with the world in a simplified manner, avoiding the complexity, danger and irreversibility of reality.

So modeling is not just about expressing a solution at a higher abstraction level than code. This has been useful in the past (assembly languages abstracting away from machine code, 3GL abstracting over assembly languages, etc.) and it is still useful today to get a holistic view on a large C++ program. But modeling goes well beyond that.

Modeling is indeed one of the touchstone of any scientific activity (along with validating models with respect to experiments carried out in the real world). Note by the way that the specificity of engineering is that engineers build models of artefacts that usually do not exist yet (with the ultimate goal of building them).

In engineering, one wants to break down a complex system into as many models as needed in order to address all the relevant concerns in such a way that they become understandable enough. These models may be expressed with a general purpose modeling language such as the Unified Modeling Language (UML), or with Domain Specific Languages when it is more appropriate.

Each of these models can be seen as the abstraction of an aspect of reality for handling a given concern. The provision of effective means for handling such concerns makes it possible to establish critical trade-offs early on in the software life cycle, and to effectively manage variation points in the case of product-lines.

Note that in the Aspect Oriented Programming community, the notion of aspect is defined in a sligthly more restricted way as the modularization of a cross-cutting concern. If we indeed have an already existing “main” decomposition paradigm (such as object orientation), there are many classes of concerns for which clear allocation into modules is not possible (hence the name “cross-cutting”). Examples include both allocating responsibility for providing certain kinds of functionality (such as loggin) in a cohesive, loosely coupled fashion, as well as handling many non-functional requirements that are inherently cross-cutting e.g.; security, mobility, availability, distribution, resource management and real-time constraints.

However now that aspects become also popular outside of the mere programming world  [61] , there is a growing acceptance for a wider definition where an aspect is a concern that can be modularized. The motivation of these efforts is the systematic identification, modularization, representation, and composition of these concerns, with the ultimate goal of improving our ability to reason about the problem domain and the corresponding solution, reducing the size of software model and application code, development costs and maintenance time.

Design and Aspect Weaving

So really modeling is the activity of separating concerns in the problem domain, an activity also called analysis . If solutions to these concerns can be described as aspects, the design process can then be characterized as a weaving of these aspects into a detailed design model (also called the solution space). This is not new: this is actually what designers have been effectively doing forever. Most often however, the various aspects are not explicit , or when there are, it is in the form of informal descriptions. So the task of the designer is to do the weaving in her head more or less at once, and then produce the resulting detailled design as a big tangled program (even if one decomposition paradigm, such as functional or object-oriented, is used). While it works pretty well for small problems, it can become a major headache for bigger ones.

Note that the real challenge here is not on how to design the system to take a particular aspect into account: there is a huge design know-how in industry for that, often captured in the form of Design Patterns (see above). Taking into account more than one aspect as the same time is a little bit more tricky, but many large scale successful projects in industry are there to show us that engineers do ultimately manage to sort it out.

The real challenge in a product-line context is that the engineer wants to be able to change her mind on which version of which variant of any particular aspect she wants in the system. And she wants to do it cheaply, quickly and safely. For that, redoing by hand the tedious weaving of every aspect is not an option.

Model Driven Engineering

Usually in science, a model has a different nature that the thing it models ("do not take the map for the reality" as Sun Tse put it many centuries ago). Only in software and in linguistics a model has the same nature as the thing it models. In software at least, this opens the possibility to automatically derive software from its model. This property is well known from any compiler writer (and others), but it was recently be made quite popular with an OMG initiative called the Model Driven Architecture (MDA). This requires that models are no longer informal, and that the weaving process is itself described as a program (which is as a matter of facts an executable meta-model) manipulating these models to produce a detailled design that can ultimately be transformed to code or at least test suites.

The OMG has built a meta-data management framework to support the MDA. It is mainly based on a unique M3 “meta-meta-model” called the Meta-Object Facility (MOF) and a library of M2 meta-models, such as the UML (or SPEM for software process engineering), in which the user can base his M1 model.

The MDA core idea is that it should be possible to capitalize on platform-independent models (PIM), and more or less automatically derive platform-specific models (PSM) –and ultimately code– from PIM through model transformations. But in some business areas involving fault-tolerant, distributed real-time computations, there is a growing concern that the added value of a company not only lies in its know-how of the business domain (the PIM) but also in the design know-how needed to make these systems work in the field (the transformation to go from PIM to PSM). Reasons making it complex to go from a simple and stable business model to a complex implementation include:

This wider context is now known as Model Driven Engineering.


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