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Methods of Software Development
Problem Frames 1
(This lecture is largely based on material graciously
provided by Professor Mary Shaw)
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Are there any questions?
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Overview: This course is about
deciding what to design.
Problem
Engineering approach
Knowledge of past solutions
Business & market constraints
Here a
Miracle
Happens
Regulations
Solution
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Course contents
 Identifying
the problem
 Problem
frames is for representing problems.
 Contextual inquiry is for studying & representing the
context of use.
 Use cases are for describing generalized scenarios that
the software will support.
 Designing
a solution
 Prototyping
identifies ways to refine software designs.
 Usability deals with making the software easy to use.
 Business and regulatory issues constrain choices about
which markets to enter, and how to enter them.
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Moving on to today’s lesson…
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A question to get you thinking…

Which of these problems is not like the others?
1.
Writing a report for school
Creating a spreadsheet for a business
Browsing the web
2.
3.
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A question to get you thinking…

Which of these problems is not like the others?
1.
Making a heart pacemaker beat once per second
Steering a nuclear missile into a city
Encrypting a bank database
2.
3.
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The key idea in problem frames is
recurring problem types
 Different
problems share characteristics.
 If
you stand back from the details of your current
problem, you may recognize it as a known problem.
 Many
known types of problems are already solved.
A
problem frame represents a “well-known” type of
software problem.
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Problem frames are a way of representing
certain software-related expertise.
 The
nature of expertise
 Experts
have about 50,000 chunks of knowledge
 It takes them about 10 years to become experts
 This is true across many domains
 Experts recognize these chunks instead of deriving them
 What passes for insight or intuition is often recognition
 The
idea in problem frames
 Frames
represent chunks of knowledge
 Knowing more variations will allow you to recognize
more pre-solved problems and apply prior art
 This is easier and less risky than analyzing from scratch
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Basic steps in applying problem frames
1.
2.
3.
4.


Break the context into pieces (called domains).
Identify the shared phenomena (called interfaces)
among the domains.
Represent the domains and their interfaces in a
context diagram
Add the conditions (called requirements) that the
software must bring about.
A context diagram that has been augmented with
requirements is called a problem diagram.
A problem diagram that recurs a lot is called a
problem frame.
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A sample problem diagram:
editing a “periods & ranges” database
n
Periods &
ranges
X
o
PREdit
machine
Data entry
m
Medical
staff
m: MS! (EnterPeriod, EnterRange,
EnterPatientName, etc) [C1]
m
B
n: PM! (EditOperations) [C2]
o: PR! (DataValues) [Y3]
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The different types of domains
 Causal
(C) – has predictable relationships among
physical phenomena
 The
machine domain, which has a double stripe, is
always a causal domain.
 Biddable
(B) - physical but unpredictable
 Humans
are the most common biddable domain.
 Lexical
(X) – physical representation of data and
symbolic phenomena
 Designed
domains, which have a single stripe, are
usually lexical domains.
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Examples of each type of domain
 Causal
(C)
 LCD
screens
 Heart pacemakers
 Nuclear missiles
 Biddable
(B)
 Humans
 Lexical
(X)
 Word
documents
 Spreadsheets
 HTML documents
 Bank databases
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The different types of phenomena
 Causal
(C or E) – the events that one domain
initiates in order to influence or control another
domain
 Causal
phenomena deal with what happens.
 Symbolic
(Y) – values, truths, and states
 Symbolic
phenomena are encodings of other phenomena.
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Examples of each type of phenomenon
 Causal
(C or E)
 DrawLine(x1,y1,x2,y2),
supported by LCD screens
 MakeElectricalPulse(voltage), supported by pacemakers
 FireThruster(duration), supported by nuclear missiles
 InsertRow(), supported by Microsoft Excel
 ClickHyperlink(url), supported by humans
 Symbolic
(Y)
 ListOfParagraphs(),
supported by Word documents
 GridOfCells(), supported by spreadsheets
 LevelOfEncryption(), supported by bank database
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A sample problem diagram:
editing a “periods & ranges” database
n
Periods &
ranges
o
X
Databases are lexical.
PREdit
machine
All machine domains
are causal.
Data entry
m
Medical
staff
m
B
Humans are biddable.
Sending SQL to the database is causal.
m: MS! (EnterPeriod, EnterRange,
EnterPatientName, etc) [C1]
The act of giving inputs to the machine is causal.
n: PM! (EditOperations) [C2]
o: PR! (DataValues) [Y3]
The values are symbolic.
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There are 5 basic problem frames.
 Required
behavior
Control part of the physical world to satisfy a condition.
 Commanded
behavior
Control part of the physical world according to operator
instructions.
 Information
display
Obtain state/behavior information from the physical world
and present it as required.
 Simple
workpieces
Build a tool to create & edit persistent information objects
 Transformation
Transform information inputs to required outputs
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Required behavior
 Machine
sends commands (C1) to controlled
domain, which may provide feedback (C2)
Control
Machine
CM ! C1
CD ! C2
Controlled
Domain
C
CD ! C3
Required
Behavior
 The
requirement is that the controlled domain must
demonstrate some behavior (C3).
 Note that the required behavior (C3) is different
from the commands (C1) that are sent to try and
cause that behavior.
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Commanded behavior
 Now
a human operator sends certain events (E4)
that implicitly specify the required behavior (C3).
Control
machine
CM ! C1
CD ! C2
OP ! E4
Controlled
Domain
CD ! C3
Obedience
C
OP ! E4
Operator
B
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Information display
 The
real world (or sometimes a lexical domain)
sends some phenomena (C1) to the machine, which
sends commands (E2) to the display device.
Information
Machine
RW ! C1
IM ! E2
Real
World
RW ! C3
C
Show
Status
DD ! Y4
Display
Device
C
 Note
that the real state of the world (C3) might not
be entirely visible to the machine, yet the symbolic
state (Y4) of the display should still be right!
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Workpieces
A
user’s editing commands (E3) implicitly specify
what the document’s symbolic state (Y4) should be.
Editing
Tool
ET ! E1
WP ! Y2
US ! E3
Work
pieces
WP ! Y4
X
Command
effects
US ! E3
User
B
 In
many cases, the symbolic state seen by the
machine (Y2) matches the real symbolic state (Y4).
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Transformation
 The
machine reads the input document’s symbolic
state (Y1) and actually constructs a totally new
symbolic state (Y2) in an output document.
Transform
Machine
In ! Y1
TM ! Y2
Input
IN ! Y3
X
IO
Relation
OU ! Y4
Output
x
 Usually
the symbolic states seen by the machine (Y1
and Y2) match the true states (Y3 and Y4).
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Things to think about on your own:
Problem Frames and some sample problems.
 What
should the problem diagram look like for
each of the following example problems?
 Writing
a report for school
 Creating a spreadsheet for a business
 Browsing the web
 Making a heart pacemaker beat once per second
 Steering a nuclear missile into a city
 Encrypting a bank database
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Things to think about on your own:
Problem frames uses separation of concerns.
 Separation
of Concerns is a classic technique for
managing complexity
 Focus
on one thing at a time, allocate responsibility
 Assumes that partial solutions can be combined
 In
problem frames,
 Different
domains capture different concerns
 Domains scope the problem (ie: if it’s not in the
diagram, then the developers can ignore it)
 What
other mechanisms do we have in software
engineering for separation of concerns?
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Things to think about on your own:
Problem Frames is a language.
A
language has syntax - what the language looks like
 Primitive
elements – boxes, lines, and letters in this case
 Primitive operations – hooking boxes, lines, etc together
 Encapsulation, abstraction, and composition
mechanisms – we’ll discuss this in another lecture
 To
be useful, it also has semantics – what it means
 You
need rules that describe the relationship between
things in your notation and things in the real world.
 Boxes
are domains, lines are phenomena interfaces, etc.
 Problem
frames is a problem description language.
 Can you think ways to improve the language?
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Things to think about on your own:
How would you apply this in a business?
 Status
of Problem Frames: a thoughtful proposal
 Not
actually in common use yet, no tools, not complete
 But there’s a growing community of interest
 So at present, it’s a good conceptual tool
 How
would you need to modify Problem Frames so
that you could use it in a software development
organization?
 What kind of information would you need from
your clients so that you could apply Problem
Frames?
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Are there any questions?
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