An Introduction to Software Architecture Case Studies

KWIC
Key Word In Context (KWIC)
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Search index
searching for keywords with context sensitive display
provides the user with more information

Example
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Clouds are white Ottawa is beautiful
Input: Titles Output: Index
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are white Clouds beautiful Ottawa is Clouds are white is beautiful Ottawa Ottawa is beautiful white Clouds are

Solution 1
Main Program/Subroutine with Shared Data
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Functional decomposition Components are subroutines
Master Control
Input
Circular Shift
Output
Alphabetizer
Characters
Index
Alphabetized Index
Input Medium
Output Medium
Subprogram Call Direct Memory Access System I/O

Solution 1
Strengths
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Centralized data
– efficient representation of data
Modular decomposition
Weaknesses
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Resistant to change
consider the impact of data storage format
difficult to enhance the overall functionality
reuse of component is difficult

Solution 2
Abstract Data Types
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Similar to one with data encapsulation
data access via component interface invocation
no direct data access
Components similar to solution 1

Solution 2
Abstract Data Types
Master Control
Input
Characters
Output
Alphabetic Shifts
Input Medium
Output
Medium
Subprogram Call
System I/O
Circular Shift
setchar
char
word
word
setup
setchar char
alph
i-th

Solution 2
Advantages
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Handles change well
algorithm and data are encapsulated in individual modules
Reuse
modules interact via defined interfaces
Disadvantages
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Evolution still a problem
– to add new features may require changes to existing or addition of new components

Solution 3
Implicit Invocation
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Similar to solution 1
shared data
Two main differences
data is more abstract
underlying storage is not exposed to components
components are invoked implicitly
e.g. when a line is added

Solution 3
Master Control
Input
Circular Shift
Output
Alphabetizer
Input Medium
Output Medium
Subprogram call Implicit invocation System I/O
Lines
insert
delete
i -th
Lines
insert
delete
i -th
Calls to circular shift and alphabetizer are implicit, and are the result of inserting lines

Solution 3
Advantages
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Strong evolution path
functional enhancements are easy
new components can be attached and removed
components are shielded from data storage representation
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REALLY WHY?
Minimal component coupling/dependency
– data events are the source of all interactions

Solution 3
Disadvantages
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Difficult to control the ordering of processing Requires more storage capacity
– IS THIS REALLY A DISADVANTAGE?

Solution 4
Pipes & Filters
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Four filters
input, shift, alphabetize, output
each filter can compute when data is available at the input
data sharing is restricted by pipes
Input
Circular Shift
Output
Alphabetizer
Input Medium
Output Medium
pipe
System I/O

Solution 4
Advantages
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Intuitive flow of processing Reuse
Evolution
– new filters can be easily added

Solution 4
Disadvantage
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Virtually impossible to support an interactive system
Is this a true pipes & filters?
– consider the data flow
What is the LCD data unit?

Comparison