In this blog post, the first of a series, Christopher Zolotas attempts to demonstrate the cornerstone technology used in S-CASE for automation, namely: Model Driven Engineering (MDE) using the analogy of Victorian architecture.
Object Management Group (OMG) has introduced MDE since 2000 and pretty much aspires to change the software design and construction paradigm. Its principal concept is the transition of code-centric software engineering to a model-based one, so as to achieve higher consistency, increased level of automation and productivity. In a nutshell, it aspires to deliver software in less time and at lower cost.
That said, the abstract nature of MDE concepts often makes it difficult to fully grasp the whole procedure. Therefore, in this blog post we will use an extensive metaphor taken from civil engineering domain through which we will gradually build up the concepts of MDE. Once this is done, in part two of this blog post we will examine the way S-CASE embeds MDE in order to automate the building process of RESTful services.
For the sake of this metaphor, we will use Victorian era buildings, which have a lot of distinct design characteristics and properties. Although there are quite many variations of Victorian Style buildings, we will examine the ones found in figure 1.
Just by taking a look at the exterior of a Victorian house, we observe some common properties and patterns that differentiate it from all the others. For example, they have steep dark colored roofs and a lot of ornamentation details. Additionally, they are built with bricks and have a lot of fancy sash-windows with large glass panes.
Moreover, Victorian houses share a lot of common characteristics in their interior. Figures 2 and 3 illustrate two such examples. The wallpapers and the carpets are rich in patterns with either of them dyed in rich dark ruby reds or forest greens. The interior also exhibits heavily upholstered furniture that is usually overstuffed, while the woodwork is usually dark colored. Finally, when such houses were introduced, during the reign of Queen Victoria, they had gas-fuelled ceiling luminaries, gradually replacing the ones with candles.
Of course, the point of this blog post is not to fully examine the Victorian style or even be absolutely accurate about it. The purpose is to clearly demonstrate that this specific style, as any other one, embeds some distinct characteristics and properties that differentiate it from the other ones. As a consequence, once you decide to buy such a house you expect it to have the aforementioned properties. For the shake of this metaphor, let’s assume that we are hidden spectators of a conversation between some 19th century customer, who wants to buy a house, and a civil engineer. We will also assume that this civil engineer designs only Victorian houses. Furthermore, he will also play the role of a Victorian style decorator of the building interior. Therefore, since this customer picked this specific engineer, it means that he knows that what he is going to buy is a Victorian house with the aforementioned properties.
During this conversation, the customer would probably explain his needs to the engineer. For instance he would probably want a spacious drawing room, a kitchen, a lavatory, a luminous dining room, one master bedroom and two child bedrooms, one for a boy and one for a girl. During this early stage of this collaboration however, no one considers any design issues or any specific materials that should be used for the building. They are focused on concepts related to the domain of house buildings. Thus, they are discussing only about the type and number of rooms it should have and probably some more details and properties of them. That’s it!
This is actually what happens during the first stage of MDE, which is the formulation of the Computational Independent Model (CIM). Nothing that has to do with the design of the actual piece of software is discussed, let alone specific implementation technologies or coding. It is only a discussion on what should be included in that house which is going to be built and nothing more.
Once the civil engineer is given the desired CIM concepts from his customer, he patiently and meticulously starts to design the building having in mind that the Victorian style must be applied to it. This is a step-by-step process that this civil engineer has to follow in order to create the design of the house in such a way that all the concepts his customer asked for in the CIM (3 bedrooms, 1 drawing room etc.) are taken care of. In particular, he will design a boy bedroom by applying to it Victorian properties like heavily patterned wallpapers and carpets, dark-coloured wood furniture for any closets/chest of drawers etc. and of course fancy sash-windows with big glass panes and a gas-fuelled ceiling luminaire!
However, the engineer does not yet bother to pick the specific materials with which the house will be built. He merely writes on his design that for the house exterior there will be used some sort of bricks. Additionally, the interior walls will be covered with some sort of patterned wallpapers, whilst the furniture will be of some type of dark-colored wood and the lighting will be of some sort of ceiling gas-luminaire. Full stop! Once he is done applying the Victorian style to every concept his customer asked for in CIM, he knows that he is done with the second phase of MDE, which is the Platform Independent Model (PIM) formulation or more appropriately the transformation of the CIM to its according PIM. In other words, in this second phase the design of the whole house that satisfies the customer needs, which were prescribed in the CIM, is introduced. Full stop!
Once the engineer is done with the PIM and before he calls his construction team to build the house, he still has to decide the specific materials to be used for that specific house. How does he decide that? This information originates principally from his customer. The customer will have to pick a specific combination of material brands that his engineer is able to handle. He is the one who pays after all!
Once the engineer knows his customer selection, he is able to proceed to the next phase. At this point he knows that the parts of the building that consist of bricks should be build with bricks of make X, the wallpapers should be of make Y, the ceiling gas lighting should be a specific 1897 model of make Z etc. In fact, when this point is reached, the engineer knows that he has transformed every single component of the house PIM to its specified counterpart, which is the Platform Specific Model (PSM). In other words, the engineer has specialized the materials with which the building of that specific design described in PIM is going to be implemented according to one of the available material combinations. Other customers could possibly pick other material combinations though (other PSMs). Thus, there is a relationship of one PIM to many PSMs. One PIM that embeds a specific design may be transformed to any number of different PSMs (different combinations of specific materials).
The final step is to employ a specific construction team that knows how to construct the Victorian buildings with the specific kind of materials that were selected. For example, he will have to hire builders that know how to handle bricks of make X following the Victorian Style. Similarly, he will hire an interior decorator that knows how to apply on the walls make Y wallpapers that are heavily patterned etc. Of course, it is quite probable that some parts of the building may be partially completed or even empty. For example, parts like a garden (figure 4) will need specialized personnel, like a gardener, to get completed. This final step is the analogous phase in MDE of the code generation from a PSM. Some parts of the code will be fully implemented, whilst others will need programmer intervention to get completed.
This is pretty much MDE. It comprises of four phases through which the MDE engine (civil engineer) has to go through. The CIM formulation with the high level domain concepts, then the transformation to the PIM in order to introduce the appropriate design, followed by the transformation to a PSM where specific materials are picked up for every aspect of the subject that is going to be built. The final step is the code generation (the action of building) and the revision by developers (specialized experts). It must be stressed though, that this procedure is automated. That’s the point! For instance, if a customer was able to speak to an imaginary MDE engine that knows how to build Victorian houses, he would say his needs to it and then the MDE engine would return to him the building automatically and instantly! In other words, the transformations, transitions from phase to phase, are automated.
In the following blog post, the second part of the MDE introduction, we will build upon the concepts demonstrated in this one through this metaphor. Thus, we will attempt to show how S-CASE embeds MDE in order to build RESTful web services or in other words, pieces of software that follow the REST architectural style.