and EventSim [
Architects can use the Palladio Component Model (PCM) to model and evaluate the performance
of software systems at design-time [
However, over the years both software performance simulators together with the PCM
language itself were enhanced with new features, resulting in large code base that is dificult
to extend and maintain. Although SimuLizar, for example, provides many (explicit) extension
points for diferent purposes, the monolithic architecture is resistant against changes, making it
complex to introduce new extension points. All in all, we introduce a new simulator for PCM
models that bases on the Event-Driven Architecture (EDA) [
First, we developed the core module of Slingshot after several architecture reviews. This module
contains one extension point in which new features can be plugged-in. Furthermore, core
metaevents are defined to describe the run-time state of the system, on which extensions can react
and behave accordingly. This allows for implementing specific simulation behaviors directly
afterwards, such as the standard interpreters (usage model, system and resource interpretation).
Finally, the implementation is compared to the other simulators by using specific software
metrics. Typically, the software metrics for analyzing extensibility are cohesion (such as
Lackof-Cohesion-in-Methods (LCOM)) and coupling (such as Coupling-Between-Objects (CBO)),
since highly cohesive, loosely coupled and modular systems tend to be extensible. Furthermore,
changeability also factors into extensibility. However, these software metrics are defined on a
class-level. Because modularity is more important for extensibility, we needed metrics such as
EXTCBO (external coupling between object classes) and CC (component cohesion) [
For changeability, we counted the number of lines that needed to be changed in the existing code base when adding a certain interpretation behavior in the simulators. For SimuLizar and EventSim, we hypothetically removed the lines responsible for that behavior and looked at what changes were needed.
The results show that EDA does improve extensibility without creating further explicit extension points. On one hand, Slingshot requires less configuration when adding new extensions. Furthermore, as long as the modules remain highly cohesive, Slingshot has looser coupling and is highly changeable. On the other hand, even though SimuLizar has rather low EXTCBO, it is hardly adaptable to new features since it provides only a finite set of extension points. Its changeability also sufered by the scenario. The results, therefore, suggest that extensibility is afected if the right software architecture was chosen, and that it can be improved by using EDA.
It is important to note that the current results only show the aspect of adding another
interpreter into the system. However, extensibility is subjective to the developers and their
skill-set, as well as documentation [
At the moment, Slingshot is still under development. As of now, only the standard interpretation is featured. Later, it will also be able to monitor and simulate self-adaptive systems.
We create a new PCM simulator named Slingshot to achieve high extensibility. The results suggest that Slingshot is extensible regarding adding of new simulation capabilities by simply listening to existing events and contributing new events. However, further analysis is needed to check whether extensibility is achieved in all aspects, and whether Slingshot sufers in areas where SimuLizar and EventSim are providing extension points. Also, other metrics such as performance and validity are still being investigated.