The 4 pillars of SysML

You can already read a lot about SysML - the Systems Modeling Language - on this blog. But today I would like to discuss an important concept that is often used to characterize SysML.

These pillars are not limited to SysML. However, SysML provides model elements and views (diagrams) that drastically simplify systematic work. The four pillars are structure, behavior, requirements and parameters.

The following picture from A Practical Guide to SysML conceptually shows the four pillars:

Let's take a closer look at these four pillars:

Requirements

Requirements often represent the entry point for a development, but they also continue to emerge during the course of development. Among other things, they are used to establish relationships with other requirements, design elements and tests, and they also play an important role in the analysis.

Even though requirements are part of SysML, specialized tools are often used for requirements engineering. This is because requirements play an important role in stakeholder communication, and many users are reluctant to work with modeling tools.

Structure

The structure is about the structural composition of the system, but also its connection and classification. Here we first think of blocks, but also interfaces and ports.

The structure also has another function: it gives us a vocabulary with which we can describe the system. Modeling the structure makes our language precise.

Behavior

When we describe a system, the focus is usually on behavior. The best-known element of SysML is the use case. Behavior can take different forms, depending on what kind of system is being developed. In addition to the function-based description, the system can also be described on a message-based or state-based basis. And the forms of description can also be mixed.

Parameters

The behavior of systems is restricted and further specified by parameters and takes the form of constraints, performance values or physical properties.

In lightweight modeling, I sometimes see that non-functional requirements or quality requirements are used instead of parameters, which is legitimate. The main thing is that this aspect of the system description is not forgotten.

Four pillars even without SysML?

Anyone who has read carefully will realize that these four pillars are not actually specific to SysML. And in fact, these four aspects also exist in paper-based systems engineering and other approaches.

However, SysML makes it easier for us to work with these pillars because these aspects can be viewed and processed separately more easily and because the relationships between them are clearly defined.