1. On a “Buzzword” Hierarchical Structure
D. L. Parnas

2. “Hierarchical” as a Buzzword
The term “hierarchical” has been used in many different ways to describe the design of operating systems
“Hierarchically” structured systems have a good connotation
Researchers consistently overload this term, using “hierarchical” in many different ways
Parnas argues that this term has been used in ways that have different meanings

3. Paper Objectives
Clearly define what is meant by a hierarchically structured system
Examine 3 operating system examples where this term was used, and provide better definitions
T.H.E
MULTICS
RC4000

4. Paper Objectives – Actually…
Parnas wrote this paper not only to point out the ambiguity in the term “hierarchically structured”, but also to point out that we should not use buzzwords in an engineering context
Consider “High Level” also…
Parnas is making a point!

5. Hierarchically Structured
Level i is the set of parts such that:
There exists a  on level i-1 such that R(, )
If R(,) then  is on level i-1 or lower
Level 0 is the set of  such given that there are no  such that R(, ) A B C
Formal D E F
Level i is the set of parts such that:
All nodes have at least 1 parent.
The graph has no cycles
Level 0 is a root in the hierarchy.
To have a hierarchy you need to define appropriate constraints on a relation…
Informal

6. What does “Hierarchically Structured” Mean?
Any program can be hierarchically structured
Everything is a single part
Easy to arbitrarily divide a program such that the “parts” are hierarchically structured
If the “parts” are not hierarchically structured, just redefine the “parts”
Thus, by itself the formal definition of a hierarchy carries no meaning at all when used in a software engineering context

7. How and When to use Hierarchies…
You need to define “which” hierarchy and clearly specify the “relation” used to define the hierarchy
You then need to validate that the “relation” used as a constraint actually forms a hierarchy
You need to validate that the hierarchical constraint provides some value – don’t make it an unnecessary constraint

8. Hierarchies 101 Hierarchies are based on mathematical formalism
Hierarchies make things easier for humans (divide and conquer)
Hierarchies have the connotation of being a “good thing”, so we strive to use them wherever possible
However, sometimes we say we have a hierarchy when we don’t, and sometimes forcing a hierarchy introduces unnecessary and unwanted constraints

9. Some Hierarchical Relations – “Maybe”
“uses” (Program Hierarchy)
A calling program should be able to ignore the internal workings of a called program
The called program should not make any assumptions about the internal structure of the calling program
“gives work to” (Resource Allocation)
A process can assign part of a workload to another process in the system

10. Some Hierarchical Relations – “Maybe”
“Part Of” (Program Structure)
Small modules recursively grouped into larger modules (subsystems)
Helpful for human comprehension
Need criteria for establishing this hierarchy, will be hard to do based on the program call structure
Sometimes the criteria is heuristic based
Software Architecture Example: High coupling, low cohesion…

11. Some Hierarchical Relations – “Maybe”
“can be accessed by” (visibility / security)
Adding layers of indirection – good for security
Core data at the center ring, protected by functions and interfaces
Each subsequent layer has data that it manages, but must call down to a lower level to work with more “sensitive” data
Forcing a hierarchy here may not be good, the “can be accessed by” relation has value in one direction but not the other…
Why can’t an inner ring use the services of an outer ring?

12. Some Hierarchical Relations – “Maybe”
Top-Down Design
Start with the user visible features of the system
“drill down” in verifiable steps to refine and implement these features
Is Top-Down design really possible?
Is it a hierarchy?
Is the design of a system hierarchical?
Can’t clearly define a relation for this one – might be trouble here…

13. Another Buzzword – “High Level”
Example – High-Level Language
What is the relation for this example?
“has fancier syntax”
“has stronger typing” …
Need a definition for “high level”, although it is used in practice often.

14. Summary: Hierarchy as a Buzzword
This paper is not about hierarchies, its about how buzzwords are used in research
This is just an example
Other example provided is “high level”
Buzzwords are dangerous because they convey no meaning at all, however authors who use buzzwords are trying to imply something…