Recently I had a very heated argument with a customer representative about how a material grade should be modelled in ISA 95 model. To make question clear let's have a look at ISA 95 standard for material class and definition. It's basically a class diagram in UML for classes without method or property.
As you should've expected. there is nothing domain specific there and that is open to interpretation. And this is how it should be. But ISA 95 standard gives an example that models Aluminium as a Material Class and A Grade Aluminium as Material Definition. This example is provided out of any solution context.
We were discussing how different grain grades such as APH2-Wheat or FEED-Wheat should be modelled. To my colleague it was obvious that they should be modelled as different material definitions to adhere to ISA 95 standard. These are few arguments supporting this model
1. We can model commodities like 'Wheat' as material classes,
2. The example from ISA 95 shows this is probably how standard is meant to be used
3. The ISA 95 does not have any other pre-defined structure close to grade concept
So why I even doubted that? Maybe because I'm also a software engineer and I might be able to read some implicit info out a UML diagram. While it's not very complicated, but understanding OO concepts is crucial to understand the above diagram and any ISA 95 model.
In my opinion (Which I will defend next) , 'Grain' is a material class and 'Wheat' is a material definition. What is the 'Grade'? It's simply a quality target or quality criteria. The 'Grade' concept should not and can not be forged into above object model. And it's not even required for ISA 95 compliance. Extensibility is the beauty of Object Oriented design.
What are my reasons for thinking like that?
1. In contrast to something like aluminium, grain can be mixed with minimum effort. let's say A-Grade and C-Grade wheat get mixed and form B-Grade which is still Wheat. modelling grades as materials according to above UML chart means objects of type A-Grade and C-Grade are replaced with an object of type B-Grade. Knowing that no business entity has been deleted or created and instead they are just sitting next to each other makes such a model un-necessarily irrelevant to physical reality. E.g. if you put enough of A-Grade then type of result object can change or even become undetermined. This does not match Object Oriented principals where type of objects are defined and can not change.
If you model grade as quality criteria, then two objects of type 'Wheat' one fulfilling quality criteria of 'A-Grade' and the other one fulfilling quality criteria of 'C-Grade' form a set that now fulfils quality criteria of 'B-Grade'. I see this model closer to reality. I know if I put too much of A-Grade then resulting set will tend to satisfy 'A-Grade' and my model supports that.
2 Overlapping Grades
We know wheat can be categorized to two different grades. For example APH2 wheat can also be assumed to be Feed Wheat. Same material falls into different grades depending on specification and standard used. different countries on different years have different specifications.
looing closely at above chart shows every lot can only have one material associated to it. This means either we will have to ignore this real-life fact or our model will not comply with ISA 95 standard!
But if we have grades as separate objects, every lot can get associated with multiple grades and we won't need to break ISA 95 model
I probably could list few more reasons, but after all it's a reality of an engineer's life that sometimes designs are imposed to him. I wish we had more education on ISA 95 before actually starting to design the solution
We were discussing how different grain grades such as APH2-Wheat or FEED-Wheat should be modelled. To my colleague it was obvious that they should be modelled as different material definitions to adhere to ISA 95 standard. These are few arguments supporting this model
1. We can model commodities like 'Wheat' as material classes,
2. The example from ISA 95 shows this is probably how standard is meant to be used
3. The ISA 95 does not have any other pre-defined structure close to grade concept
So why I even doubted that? Maybe because I'm also a software engineer and I might be able to read some implicit info out a UML diagram. While it's not very complicated, but understanding OO concepts is crucial to understand the above diagram and any ISA 95 model.
In my opinion (Which I will defend next) , 'Grain' is a material class and 'Wheat' is a material definition. What is the 'Grade'? It's simply a quality target or quality criteria. The 'Grade' concept should not and can not be forged into above object model. And it's not even required for ISA 95 compliance. Extensibility is the beauty of Object Oriented design.
What are my reasons for thinking like that?
1. In contrast to something like aluminium, grain can be mixed with minimum effort. let's say A-Grade and C-Grade wheat get mixed and form B-Grade which is still Wheat. modelling grades as materials according to above UML chart means objects of type A-Grade and C-Grade are replaced with an object of type B-Grade. Knowing that no business entity has been deleted or created and instead they are just sitting next to each other makes such a model un-necessarily irrelevant to physical reality. E.g. if you put enough of A-Grade then type of result object can change or even become undetermined. This does not match Object Oriented principals where type of objects are defined and can not change.
If you model grade as quality criteria, then two objects of type 'Wheat' one fulfilling quality criteria of 'A-Grade' and the other one fulfilling quality criteria of 'C-Grade' form a set that now fulfils quality criteria of 'B-Grade'. I see this model closer to reality. I know if I put too much of A-Grade then resulting set will tend to satisfy 'A-Grade' and my model supports that.
2 Overlapping Grades
We know wheat can be categorized to two different grades. For example APH2 wheat can also be assumed to be Feed Wheat. Same material falls into different grades depending on specification and standard used. different countries on different years have different specifications.
looing closely at above chart shows every lot can only have one material associated to it. This means either we will have to ignore this real-life fact or our model will not comply with ISA 95 standard!
But if we have grades as separate objects, every lot can get associated with multiple grades and we won't need to break ISA 95 model
I probably could list few more reasons, but after all it's a reality of an engineer's life that sometimes designs are imposed to him. I wish we had more education on ISA 95 before actually starting to design the solution
