Software Architecture
CS 414, Software Engineering I
Mark Ardis, Rose-Hulman Institute
January 14, 2003
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Outline
Definitions of Architecture
Architectural Styles
Patterns
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Definitions of Architecture (1)
1. The art and science of designing and
erecting buildings.
2. Buildings and other large structures: “the low,
brick-and-adobe architecture of the
Southwest.”
American Heritage® Dictionary of the English Language: Fourth
Edition. 2000.
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Definitions of Architecture (2)
3. A style and method of design and
construction: “Byzantine architecture.”
4. Orderly arrangement of parts; structure:
“the architecture of the federal
bureaucracy; the architecture of a
novel.”
American Heritage® Dictionary of the English
Language: Fourth Edition. 2000.
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Definitions of Architecture (3)
5. Computer Science The overall design
or structure of a computer system,
including the hardware and the software
required to run it, especially the internal
structure of the microprocessor.
American Heritage® Dictionary of the English
Language: Fourth Edition. 2000.
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Software Architecture
Global control
Communication, synchronization and
data access
Assignment of functionality to design
elements
Composition of design elements
Scaling and performance
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Outline
Definitions of Architecture
Architectural Styles
Patterns
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Architectural Styles (1)
Dataflow


Batch sequential
Pipes and filters
Call-and-return



Main program and subroutine
OO systems
Hierarchical layers
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Architectural Styles (2)
Independent components


Communicating processes
Event systems
Virtual machines


Interpreters
Rule-based systems
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Architectural Styles (3)
Data-centered



Databases
Hypertext systems
Blackboards
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Blackboard Architecture
Knowledge sources

separate and independent
Blackboard data structure


problem-solving state
knowledge sources update it
Control

driven by state of blackboard
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Blackboard Example
ks1
ks2
ks7
ks3
Blackboard
(shared data)
ks6
ks4
ks5
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Pipe and Filter Architecture
Filters
Pipes
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Examples of Pipe-and-Filter
Unix shell scripts
Compilers

lexical analysis, parsing, semantic
analysis, code generation
Batch sequential

degenerate case where all input processed
before any output
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Layered Architecture
Applications
Utilities
Core
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Interpreter Architecture
Program
Data
Interpreter
Engine
Output
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Architecture Review Boards
Review Architectural Specifications
Provide shared knowledge across
projects
Prevent mistakes early
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First Cartoon of the Day
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Outline
Definitions of Architecture
Architectural Styles
Patterns
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Christopher Alexander
Architect who proposed
that people design their
own dwellings, using
accepted patterns of
design
Wrote a series of books
that influenced the OO
community
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A Place to Wait (1/3)
The process of waiting has inherent
conflicts in it.
On the one hand, whatever people are
waiting for---the doctor, an airplane--has built-in uncertainties, which make it
inevitable that they must spend a long
time hanging around, waiting, doing
nothing.
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A Place to Wait (2/3)
On the other hand, they cannot usually
afford to enjoy this time. Since they
never know when their turn will come,
they cannot even take a stroll or sit
outside.
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A Place to Wait (3/3)
Therefore:
In places where people end up waiting, create a
situation which makes the waiting positive.
Fuse the waiting with some other activity--newspaper, coffee, pool tables; something
which draws people in who are not simply
waiting. And also the opposite: make a place
which can draw a person waiting into a
reverie; quiet; a positive silence.
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History of Software Patterns
Kent Beck had read Alexander's work
while in college
Kent convinced Ward Cunningham that
ideas were good
They helped a group design a user
interface by letting users design it
(1987)
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Beck and Cunningham's
Original Patterns
Window Per Task
Few Panes
Standard Panes
Nouns and Verbs
Short Menus
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Window Per Task
Make a specific window for each task the
user must perform. All of the
information needed to complete a task
should be available in the Few Panes of
the window. Assume prerequisite tasks
have been completed (if they haven't,
the user will simply change windows).
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Few Panes
To understand complex things one often
must see it from several points of view.
Therefore: Provide these points of
view by dividing the area of your
Window Per Task into panes.
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Standard Panes
One must learn to operate each kind of
pane offered in the Few Panes of every
window. Therefore: Cast each pane
into the format offered by one of a few
standard panes. [Tektronix had 4.]
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Nouns and Verbs
Things exist while action happens.
Therefore: Put lists of things (nouns)
in a list pane (one of Few Panes) which
persists through interactions. Put
actions (verbs) in Short Menus which
pop up and then disappear as the
action commences.
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Short Menus
The elements of a pop-up menu must be
visually searched repeatedly.
Therefore: Make them short, fixed and
single-level.
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What is a Pattern?
"a named nugget of insight that conveys
the essence of a proven solution to a
recurring problem within a certain
context amidst competing concerns"
"a solution to a problem in a context"
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Pattern Parts
Name
Problem description
Context: when is it applicable?
Forces: what are the tradeoffs?
Solution
Consequences
...
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A Good Pattern
solves a problem
is a proven concept
describes a relationship
has a significant human component
provides a solution that isn't obvious
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Qualities of a Pattern
Encapsulation and abstraction
Openness and variability
Generativity and composability
Equilibrium
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Leaky Bucket Counters (1/2)
Problem: How do you deal with transient
faults?
Context: Fault-tolerant software that must
deal with failure events.
Forces:


You want a hardware module to exhibit hard
failures before taking drastic action.
Some failures come from the environment, and
should not be blamed on the device.
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Leaky Bucket Counters (2/2)
Solution: A failure group has a counter that is
incremented for each failure. Each counter is
decremented periodically. A hard failure is
not reported until the counter exceeds a
threshold.
Consequences: Hard errors are handled, but
transient errors are not as likely to trigger
error handling.
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Second Cartoon of the Day
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Writers' Workshops
Method of publishing patterns
Process:
1.
2.
3.
Author reads a short segment
Others provide constructive criticism
while author remains silent
Group thanks author (applause)
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Pattern Mining
Interview experts about their domain
Record patterns that are found
Share resulting set with experts
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Advantages of Patterns
Shorten discovery
Avoid inexpert design decisions
Reuse large chunks of design
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References
Gamma, Helm, Johnson and Vlissides Design Patterns,
Addison-Wesley 1995, ISBN 0-201-63361-2. (Gang of
Four)
Jim Coplien, Software Patterns Management Briefing,
1996, http://www.bell-labs.com/
user/cope/Patterns/WhitePaper
Appleton, Brad
Patterns and Software: Essential Concepts and
Terminology, 2000
http://www.cmcrossroads.com/
bradapp/docs/patterns-intro.html
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Books by Alexander
Notes on the Synthesis of Form, Harvard
University Press, 1964
The Oregon Experiment, Oxford University
Press, 1975
A Pattern Language: Towns, Buildings,
Construction, Oxford University Press, 1977
The Timeless Way of Building, Oxford University
Press, 1979
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