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Defense Equipment”, Issue 2, 1997
UK Ministry of Defense. Defense Standard 00-56: “Safety Management Requirements for
Defense Systems”, Issue 2, 1996
International Electrotechnical Commission, IEC 61508, “Functional Safety of
Electrical/Electronic/Programmable Electronic Safety-Related Systems”, draft 61508-2 Ed 1.0,
1998
FAA System Safety Handbook, Appendix D
December 30, 2000
D - 1
Appendix D
Structured Analysis and Formal Methods
FAA System Safety Handbook, Appendix D
December 30, 2000
D - 2
D.1 Structured Analysis and Formal Methods
Structured Analysis became popular in the 1980’s and is still used by many. The analysis consists of
interpreting the system concept (or real world) into data and control terminology, that is into data flow
diagrams. The flow of data and control from bubble to data store to bubble can be very hard to track and
the number of bubbles can get to be extremely large. One approach is to first define events from the
outside world that require the system to react, then assign a bubble to that event, bubbles that need to
interact are then connected until the system is defined. This can be rather overwhelming and so the
bubbles are usually grouped into higher level bubbles. Data Dictionaries are needed to describe the data
and command flows and a process specification is needed to capture the transaction/transformation
information. The problems have been: 1) choosing bubbles appropriately, 2) partitioning those bubbles in
a meaningful and mutually agreed upon manner, 3) the size of the documentation needed to understand
the Data Flows, 4) still strongly functional in nature and thus subject to frequent change, 5) though “data”
flow is emphasized, “data” modeling is not, so there is little understanding of just what the subject matter
of the system is about, and 6) not only is it hard for the customer to follow how the concept is mapped
into these data flows and bubbles, it has also been very hard for the designers who must shift the DFD
organization into an implementable format.
Information Modeling, using entity-relationship diagrams, is really a forerunner for OOA. The analysis
first finds objects in the problem space, describes them with attributes, adds relationships, refines them
into super and sub-types and then defines associative objects. Some normalization then generally occurs.
Information modeling is thought to fall short of true OOA in that, according to Peter Coad & Edward
Yourdon:
1) Services, or processing requirements, for each object are not addressed,
2) Inheritance is not specifically identified,
3) Poor interface structures (messaging) exists between objects, and
4) Classification and assembly of the structures are not used as the predominate
method for determining the system’s objects.
This handbook presents in detail the two new most promising methods of structured analysis and design:
Object-Oriented and Formal Methods (FM). OOA/OOD and FM can incorporate the best from each of
the above methods and can be used effectively in conjunction with each other. Lutz and Ampo described
their successful experience of using OOD combined with Formal Methods as follows: “ For the target
applications, object-oriented modeling offered several advantages as an initial step in developing formal
specifications. This reduced the effort in producing an initial formal specification. We also found that
the object-oriented models did not always represent the “why,” of the requirements, i.e., the underlying
intent or strategy of the software. In contrast, the formal specification often clearly revealed the intent of
the requirements.”
D.2 Object Oriented Analysis and Design
Object Oriented Design (OOD) is gaining increasing acceptance worldwide. These fall short of full
Formal Methods because they generally do not include logic engines or theorem provers. But they are
more widely used than Formal Methods, and a large infrastructure of tools and expertise is readily
available to support practical OOD usage.
FAA System Safety Handbook, Appendix D
December 30, 2000
D - 3
OOA/OOD is the new paradigm and is viewed by many as the best solution to most problems. Some of
the advantages of modeling the real world into objects is that 1) it is thought to follow a more natural
human thinking process and 2) objects, if properly chosen, are the most stable perspective of the real
world problem space and can be more resilient to change as the functions/services and data &
commands/messages are isolated and hidden from the overall system. For example, while over the
course of the development life-cycle the number, as well as types, of functions (e.g. turn camera 1 on,
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