Using multiplicity in BBD (e.g., car with 4 wheels, system board with eight HW processors )

[avi-mak]

Hi all,

I saw an old post entitled "Towards understanding the UML analogs of SysML BDD and IBD diagram types" (link: https://groups.google.com/g/sysmlforum/c/RmKCp6Rd55k),

 

and a particular digression attracted my interest. It is about multiplicity defined in a BDD and what it looks like in IBDs.

 

Somebody had asked:

"So if I would like to model a car that has 4 wheels how would I model that?

I would use a bed and draw a composition relation where the car is the whole and wheel is the part and specify a multiplicity of 4."

 

So somebody gave the answer:

"Actually , you’ve highlighted a quirk in the language. If you defined a Car with a single composition of Wheel with a multiplicity of 4, the IBD would have a single part (of type Wheel) representing a collection of Wheels, with a multiplicity of 4 (not 4 individual wheels). What you probably want is four compositions each to Wheel, the IBD would then have 4 parts on. "

 

I remember I once decided to do exactly that, or similar. I.e., I created a BDD diagram to define an electronics board with eight HW processors, i.e., a multiplicity of 8, so I thought. Then I created an IBD and expected eight processors to appear, and was very disappointed when that did not happen.

So how would a car look with "a single composition of Wheel with a multiplicity of 4" and the "IBD would have a single part (of type Wheel) representing a collection of Wheels, with a multiplicity of 4"

?

Thanks,

Avi

[James Towers]

You can’t mix them. The IBD has one part element per compositional relationship on the BDD 

The BDD needs 4 separate compositional relationships in order to show 4 separate parts on the IBD 

Sent from my iPhone

On 9 Aug 2022, at 16:33, avi-mak <amak...@gmail.com> wrote:



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[Editor, SysML Forum]

 > You can’t mix them. The IBD has one part element per compositional

> relationship on the BDD ...

Nor should you mix them in practice! If you seek to specify four (4) separate wheels as four (4) separate Part "prototypical instances" on an IBD, where each Part has a distinctive AssociationEnd reference (e.g., "driverFrontWhl", "passFrontWhl", "driverRearWhl", "passRearWhl") you can readily accomplish this with the current generation of SysML tools by specifying four (4) separate PartAssociations ("black diamond" Composition) relationships on the BDD used to define the subject IBD.

However, I concur with the implication that SysML tools (cf. SysML language spec) should provide this as a clickology option in order to make it easier for noobs to learn and master the language. Stated otherwise, SysML as a UML2 profile is a relatively old language (17+ years depending on how you count), the SysML 2 effort looks half-baked, and UML2/SysML vendor tool innovation has been largely non-existent for almost two decades.

/ Cris

Editor, SysML Forum

[Sent from my Android Phone; please excuse thumb-typing typos!]

[James Towers]

Sorry, having reread your question I don’t think I answered it. Actually I’m not sure what your question means “how would a car look”? 

A) Like a car? 

Sent from my iPhone

On 9 Aug 2022, at 16:41, James Towers <jtowe...@gmail.com> wrote:

You can’t mix them. The IBD has one part element per compositional relationship on the BDD 

[avi-mak]

Hi James,

>> Sorry, having reread your question I don’t think I answered it. Actually I’m not sure what your question means “how would a car look”? 

>> A) Like a car? 

 

I mean how would the wheels look, i.e., I understand they are not 4 x wheel parts; rather they are one wheel part representing a collection of 4 x Wheels.

(To quote: "…the IBD would have a single part (of type Wheel) representing a collection of Wheels, with a multiplicity of 4 (not 4 individual wheels)."

So I imagine such car would look not like a car with 4 wheels in the right places, but as a car propped up on a pile of 4 wheels situated at one of it axel end points. Maybe as a joke I could sketch it and add the caption "This is a SysML Car".

In order to get my company to accept the MBSE approach I have to show that MBSE is good, and always better than what they have now. (Btw, I am doing this bottom-up.)

Blessings 

Avi

[Stephan Roth]

Hi Avi,

as always, it depends on the abstraction level of your model. If you have no requirements or reasons to treat the four wheels differently, then a single composition relationship with a multiplicity of 4 at the association end touching the «block» Wheel may be quite sufficient. If you want to use the four wheels differently as parts in the IBD (Remember: The IBD is a diagram on the usage level!), then you must also assign them to the owning block in the BDD with four composition relationships and different role names (e.g., "frontLeft", "frontRight", etc.).

There is no wrong or right here. As mentioned above, it depends on the abstraction level you choose for your model, i.e. the level of detail.

Best regards,

Stephan

[avi-mak]

Hi Stephan,

Sorry for not responding.  Thank you for your answer.

Best

Avi