Use of the Concept of Transparency in the Design of Hierarchically Structured Systems

Motivation(s)

Existing software designs emphasize a outside-in approach, which has severe limitations.

Proposed Solution(s)

The author propose a combination of outside-in and inside-out design via a transparency criterion.

Evaluation(s)

Two examples were offered to illustrate the inside-out approach. This overall scheme serves as a guidepost for software designers because the design search space has been illuminated not reduced.

Future Direction(s)

• Have the proposed criteria been data mined to generate suggestions or detectors?

Question(s)

• Is the transparency problem for coordination and synchronization solved?

• What other features are there for software engineering?

Analysis

Transparency is another indicator to extract more signals from software design. The insights are brilliant, but the author should have covered a larger variety of examples.

Notes

Top-Down/Outside-In Design Approach

• Focus on a precise description of the desired system and deriving internal structure from it.

• This should not be confused with Dijkstra’s Level Hierarchy.

  • Higher levels do not correspond to being closer to the outside.

  • Low level features may appear on the outside.

• Difficulties

  • Description is difficult to obtain and may restrict internal design decisions.

  • The set of possible derivations from a description tend to be in an exponential search space, which requires constraints (e.g. hardware) to make search feasible.

  • Choosing the desired outside design may make the internal implementation impractical.

  • It is difficult to apply this technique to design a set of systems whose only description is general purpose.

  • Applying this technique may result in unnecessarily inflexible software.

  • The inside (e.g. hardware, OS) is already fixed, so one needs to design around it.

Transparency of an Abstraction

• The base machine is either a lower level (e.g. hardware) or an intermediate level in the software stack.

• The virtual machine is an abstraction over the base or possibly current virtual machine.

• A virtual machine is completely transparent when any base machine state and sequence of base machine states can be obtained by programming the virtual machine.

  • Missing state sequences is defined as the loss of transparency.

Bottom-Up/Inside-Out Design Approach

• Examine the loss of transparency at each level.

  • The goal is to avoid losing any desirable features and discarding all the undesirable features.

Tradeoff between Transparency and Flexibility

A design which would increase the transparency for one machine may pose great implementation difficulties or inefficiencies for another base machine.

Suggestive Transparency

Allow the virtual machine to take user suggestions (e.g. virtual memory mechanisms).

Misleading Transparency

A virtual machine should be designed to make expensive base machine sequences impossible or difficult to evoke.

References

PS75

David Lorge Parnas and Daniel P. Siewiorek. Use of the concept of transparency in the design of hierarchically structured systems. Communications of the ACM, 18(7):401–408, 1975.