Data and Actions
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Two aspects of a product
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Actions which operate on data
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Data on which actions operate
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The two basic ways of designing a product
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Action-oriented design
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Data-oriented design
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Third way
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Hybrid methods
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For example, object-oriented design
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Design Activities
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Architectural design
Detailed design
Design testing
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Architectural Design
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Input: Specifications
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Output: Modular decomposition
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Abstraction
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Detailed Design
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Each module is designed
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Specific algorithms
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Data structures
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Action-Oriented Design Methods
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Data flow analysis
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When to use it
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With most specification methods (Structured
Systems Analysis here)
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Key point: Have detailed action information from DFD
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How to Perform Data Flow Analysis?
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Data Flow Analysis (DFA) is a design technique for achieving modules with high cohesion
Using the points of highest abstraction of input and output, the product is decomposed into three modules: input module , transform module and output module
This procedure is continued stepwise until each module performs a single action i.e., the design consists of modules with high cohesion
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Data Flow Analysis
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Product transforms input into output
Determine
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“Point of highest abstraction of input”
“Point of highest abstract of output”
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Data Flow Analysis (contd)
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Decompose into three modules
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Repeat stepwise until each module has high cohesion
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Minor modifications may be needed to lower coupling
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Data Flow Analysis (contd)
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Example
Design a product which takes as input a file name, and returns the number of words in that file (like UNIX wc )
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Data Flow Analysis Example(contd)
First refinement
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Refine modules of communicational cohesion
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Data Flow Analysis Example(contd)
Second refinement
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All eight modules have functional cohesion
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Multiple Input and Output Streams
Point of highest abstraction for each stream
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Continue until each module has high cohesion
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Adjust coupling if needed
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Transaction Analysis
DFA poor for transaction processing products
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Example: ATM (Automatic Teller Machine)
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Poor design
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Logical cohesion, control coupling
On the other hand it seems a waste of time and effort to have five very similar edit modules and five very similar update modules
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Corrected Design Using Transaction Analysis
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Software reuse (Section 8.1)
A basic edit module should be designed, coded, documented and tested, then instantiated five times. Each version will be slightly different but the differences will be small enough to make this worthwhile.
Similarly a basic update module can be instantiated five times and slightly modified to cater to the five different update types.
The resulting design has high cohesion and low coupling
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Data-Oriented Design
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Basic principle
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The structure of a product must conform to the structure of its data
Three very similar methods
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Warnier
Orr
Jackson
Data-oriented design
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Has never been as popular as action-oriented design
With the rise of OOD, data-oriented design has largely fallen out of fashion
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Design of Real-Time Systems
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Difficulties associated with real-time systems
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Inputs come from real world
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Software has no control over timing of inputs
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Frequently implemented on distributed software
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Communications implications
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Timing issues
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Problems of synchronization
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Race conditions
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Deadlock (deadly embrace)
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Major difficulty in design of real-time systems
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Determining whether the timing constraints are met by the design
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Real-Time Design Methods
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Usually, extensions of nonreal-time methods to real-time
We have limited experience in use of any real-time methods
The state-of-the-art is not where we would like it to be
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Testing during the Design Phase
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Design reviews
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Design must correctly reflect specifications
Design itself must be correct
Transaction-driven inspections
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Metrics for the Design Phase
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The five basic metrics
Cyclomatic complexity is problematic
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Data complexity is ignored
Little use with object-oriented paradigm
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Challenges of the Design Phase
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Design team should not do too much
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Detailed design should not become code
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Design team should not do too little
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It is essential for the design team to produce a complete detailed design
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Difficult to find ‘great designers’
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