Model interpretation approach is grasping attention of the model driven community. Industrial experiences of company Mendix has shown some very promising results. A recent post at a popular “model-minded” blog triggered a discussion about code generation versus model interpretation.

Model interpretation in itself is not a new concept and there exist well known interpreters for generic and mainstream domains (e.g., Ptolemy and Simulink). The novelty in model interpretation today is that model driven methods provide efficiency and flexibility, which enable application of this concept to arbitrary problem domains.

In a series of blogs we will illustrate this novel aspect and provide an example of model interpretation. Specifically this article will illustrate 1) how a custom modeling language (DSL) is developed for an arbitrary problem domain and 2) how a system behavior can be specified with the DSL and directly interpreted without any intermediate transformation steps. In a followup article we will show how a custom model interpreter can be efficiently built using a model driven method.

Model Interpretation As System

Traditional generative approaches like Model Driven Architecture (MDA) prescribe an (automated) code generation process that takes a system model as input and eventually produces code that implements the specified system. The system comes to existence when the code is executed.

Alternatively, the code generation process can be skipped and a system model be executed directly.  Model Interpretation achieves such direct execution by means of a model interpreter. In this case the system comes to existence when the model is being interpreted. Thereby system behavior is completely defined by the model being interpreted.

Fig. 1 illustrates a possible approach to model interpretation of event-driven systems.  An event-driven system exhibits behavior by generating (external) events in reaction to incoming external events. Therefore, the interpreter should support two-way event communication with the context. An example of an incoming external event is arrival of a positive signal from a motion sensor for an automated door. An outgoing external event could be a command to an actuator to open the door.

Figure 1: An approach to system as model interpretation

In the figure, entities are shown as boxes and their roles w.r.t.  each other are shown in italic. Given that a domain-specific language (DSL) and an interpreter already exist, a domain expert uses the DSL to specify a system and its events at development time. Moving to the run-time, the same model (system configuration) represents the system and its events. During model execution, the interpreter reads system state from the model and interprets system events according to the semantics of the events. Interpretation may change the state of the system by changing the system configuration at run time, and communicate external events to the system’s context.

Typically a sequence of external events is provided by the context of the system. Alternatively, these events can be specified in the system model and consequently generated by the interpreter itself (in this case, system behavior is simulated).

Domain

Today model interpretation can be applied to an arbitrary problem domain. To reflect this freedom, we chose a minor and uncommon weighbridge domain, whose purpose is to measure weight of vehicles.

The following is a typical weighbridge scenario: One or more delivery vans arriving (at a factory) must pass over a weighbridge on entry. A weighbridge accepts one van at a time and each weighing operation takes a certain amount of time. If the weighbridge is busy, arriving vans join the waiting queue to the bridge. When the weighbridge becomes available again, the first van in the waiting queue drives over the bridge.

This domain is characterized by a number of inherent variations, such as number of weighbridges, weighbridge capacity, weighing operation duration, number of arriving vans, arrival times of vans, etc.. The result is that a multitude of weighbridge system configurations are possible and per configuration a multitude of dynamic van arrival and weighing scenarios can play out.

Figure 2: A weighbridge system modeled in a DSL

Figure 2 shows a simplified weighbridge system configuration, originally described by Birtwistle and Tofts [1]. Yellow boxes are vans. The large blue box is a weighbridge and green entities are a van arrival queue (EL) at the factory and a van waiting queue (Delay) at a weighbridge. As you can see the factory’s configuration has a single weighbridge W, which is free at this time. Finally, three delivery vans V1, V2 and Main have arrived (external events). An execution of this model is illustrated further in the article. An AToM3 implementation of a DSL for the domain is briefly described next.

Weighbridge DSL

The earlier mentioned freedom of application depends on flexibility and efficient adaptation of model interpreters to new domains. Model driven methods achieve this flexibility through metamodeling. If you are not familiar with metamodeling, you can skip this section as it is not required for understanding the demo.

A DSL is defined with abstract syntax, concrete syntax, static semantics and dynamic semantics. (Such a definition is known as metamodel.) Behind every DSL is a modeling paradigm that gives fundamental guidelines for metamodeling. In case of the weighbridge domain, a proper modeling paradigm is Process Interaction [2].

Figure 3: PI Metamodel

For the purposes of this demo, a PI modeling language will suffice  and we will reuse and extend a PI metamodel developed by Juan de Lara [3]. We just have to keep in mind that Process and Resource in Juan’s metamodel correspond to van and weighbridge concepts in the demo domain. The abstract syntax of the PI DSL is illustrated in Figure 3. The concrete syntax of this DSL is illustrated in Figure 2. We skip static semantics (in other words, business rules) as the focus of the domain is interpretation, not domain modeling. The following is a brief description of the key PI concepts:

Resource is a synonym for the Weighbridge concept. A weighbridge has the following attributes:

Name is a unique identifier of Weighbridge: String

State denotes availability of Weighbridge: Enum{idle, busy}

Tproc is typical duration of weighing service: Time (used in simulated execution)

Capacity denotes capability to weigh multiple vans at the same time: [1..N]

Load denotes weighbridge’s capacity occupied with served vans: [0..Capacity]

Process is a synonym of the Van concept. A van has the following attributes:

Name is a unique identifier of Van: String

Tcreation is time-stamp of Van’s arrival event: Time

Tinitproc is the start time of weighing operation: Time

Tendproc is the end time of weighing operation: Time

Body is a sequence of tasks: sequence{task} (tasks examples are bridge access, van weighing, bridge exit, etc.)

EVnext is the iterator for tasks in body: [0..N]

For simulation purposes, additional concepts are defined:

Time is a clock for simulated time.

ProcIntGenerator specifies time intervals between van arrivals.

Finally, to assist visualization of system state, the original metamodel was extended with additional relationship:

manageElement specifies an operation (append, insert or remove) on an element (target end of this relationship) of a sequence (source end of this relationship).

The final touch of DSL definition is dynamic semantics (meaning of DSL concepts). In the model interpretation approach, such semantics is defined in an interpreter. In case of a DSL and a pure interpretive approach there is a good chance that an interpreter exactly matching the DSL will need to be developed. More so if the interpreter has to meet additional specific requirements. In our case, such requirements were run-time visualization of system behavior and interpreter integration with the factory context (not covered in this article). In a followup article we will show how a custom model interpreter can be developed. Incidentally our development approach is also based on model interpretation.

Run Time Example

A picture is worth a thousand words. With that in mind an illustration of model interpretation is best done with a video. The following screencast shows execution of the weighbridge system configuration introduced earlier (see Figure 2). For the sake of visualization, execution is carried out in the step-by-step mode and displays how the state of the weighbridge configuration changes in response to events.

Conclusions

In our experience model interpretation is characterized by very fast development times. In fact it did not even feel like development at all as domain experts are completely shielded from all incidental technical details.  I believe that Birtwistle and Tofts, the scientists that coined the weighbridge benchmark, would feel at home with the demonstrated DSL and the interpreter in no time. With incidental complexity out of the equation and direct involvement of domain experts, I think we’ve come very close to the essential complexity of the domain and development times cannot be drastically improved any more. That said, I feel that those interested in this approach should be aware of run-time performance penalty due to interpreter indirection. Whether this will pose a problem, depends on the application domain.

What are your experiences with model interpretation? What is your domain?

References

[1] Graham Birtwistle and Chris Tofts. An operational semantics of process-oriented simulation languages: Part 1 πDemos. ACM Transactions on Modeling and Computer Simulation, 10(4):299–333, December 1994.

[2] Jerry Banks, editor. Handbook Of Simulation. Principles, Methodology, Advances, Applications, and Practice, pages 813 – 833. Wiley-Interscience Publication, New York, September 1998.

[3] Juan de Lara. Simulacio ́n educativa mediante meta-modelado y grama ́ticas de grafos. Revista de Ensen ̃anza y Tecnolog ́ıa, 23, Mayo-Agosto 2002.