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SysML Support For Requirements Modeling

“To communicate requirements, someone has to write them down.” – Scott Berkun

Prolific author Gerald Weinberg once said something like: “don’t write about what you know, write about what you want to know“. With that in mind, this post is an introduction to the requirements modeling support that’s built into the OMG’s System Modeling Language (SysML). Well, it’s sort of an intro. You see, I know a little about the requirements modeling features of SysML, but not a lot. Thus, since I “want to know” more, I’m going to write about them, damn it! 🙂

SysML Requirements Support Overview

Unlike the UML, which was designed as a complexity-conquering job performance aid for software developers, the SysML profile of UML was created to aid systems engineers during the definition and design of multi-technology systems that may or may not include software components (but which interesting systems don’t include software?). Thus, besides the well known Use Case diagram (which was snatched “as is” from the UML) employed for capturing and communicating functional requirements, the SysML defines the following features for capturing both functional and non-functional requirements:

  • a stereotyped classifier for a requirement
  • a requirements diagram
  • six types of relationships that involve a requirement on at least one end of the association.

The Requirement Classifier

The figure below shows the SysML stereotyped classifier model element for a requirement. In SysML, a requirement has two properties: a unique “id” and a free form “text” field. Note that the example on the right models a “non-functional” requirement – something a use case diagram wasn’t intended to capture easily.

One purpose for capturing requirements in a graphic “box” symbol is so that inter-box relationships can be viewed in various logically “chunked“, 2-dimensional views – a capability that most linear, text-based requirements management tools are not at all good at.

Requirement Relationships

In addition to the requirement classifier, the SysML enumerates 6 different types of requirement relationships:

A SysML requirement modeling element must appear on at least one side of these relationships with the exception of  <<derivReqt>> and <<copy>>, which both need a requirement on both sides of the connection.

Rather than try to write down semi-formal definitions for each relationship in isolation, I’m gonna punt and just show them in an example requirement diagram in the next section.

The Requirement Diagram

The figure below shows all six requirement relationships in action on one requirement diagram. Since I’ve spent too much time on this post already (a.k.a. I’m lazy) and one of the goals of SysML (and other graphical modeling languages) is to replace lots of linear words with 2D figures that convey more meaning than a rambling 1D text description, I’m not going to walk through the details. So, as Linda Richman says, “tawk amongst yawselves“.

References

1) A Practical Guide to SysML: The Systems Modeling Language – Sanford Friedenthal, Alan Moore, Rick Steiner

2) Systems Engineering with SysML/UML: Modeling, Analysis, Design – Tim Weilkiens

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