Dfd to Structure

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DFD to Structure Charts

Georgia State University

DFDs to Structure Charts

First Level Data Flow Diagram
0 Library System

1

Book Catalog

3 2

Borrowers List

Georgia State University

DFDs to Structure Charts

Functional Decomposition
0 Library System 1 2 3

2.1

2.2

Georgia State University

DFDs to Structure Charts

Structure Chart
Structure Chart Example Q: Where do the modules come from?

Logical Design
Executive Module 'Update File' Top because it calls all others none call it at this level

Module 1 e.g. Read

Module 2.. e.g. Update

Module n e.g. Write

Open FIle
Georgia State University DFDs to Structure Charts

Structure Chart
Structure Chart Example Q: Where do the modules come from?

Physical Design

2
Executive Module 'Update File' Top because it calls all others none call it at this level

1

3
Module n e.g. Write

Module 1 e.g. Read

Module 2.. e.g. Update

Open FIle
Georgia State University DFDs to Structure Charts

DFD to Structure Charts?
0 Library System

1

Book Catalog

?
Module 1 e.g. Read Open FIle

Structure Chart Example Q: Where do the modules come from?

3

2

Executive Module 'Update File' Top because it calls all others none call it at this level

Module 2.. e.g. Update

Module n e.g. Write

Borrowers List

Georgia State University

DFDs to Structure Charts

Why Divide Things Up?
Logical Design
– Simplifies understanding
“divide and conquer”

– Allows parallel development

Physical Design
– Separately compilable modules
one change should not require a complete build

Georgia State University

DFDs to Structure Charts

Basic Design Goals
Fitness for purpose
– the system must work, and work correctly – it should perform all tasks as specified within the constraints of the resources

Robustness
– the design should be stable against changes to features such as file or data structures

Georgia State University

DFDs to Structure Charts

Three Design Principles
Simplicity
– the design should be simple as possible, but no simpler

Separation of Concerns
– the different concepts should be separated out

Information Hiding
– information about the detailed form of objects (e.g., data structures) should be kept local and “visible” to outside modules Georgia State University DFDs to Structure Charts

Specific Design Guidelines
Cohesion
– the degree modules are sufficient to carry out one, single, welldefined function. A measure of internal strength. Each module is a system unto itself.

Coupling
– Module independence preferred to tight interdependence. (e.g. less is better)

Information Hiding
only the data needed is made available to each module

Modularity
– small self-contained units for maintainability; each module is a system unto itself

Georgia State University

DFDs to Structure Charts

Specific Design Guidelines (cont’d)
Module Size
– reasonable size
Span of Control – calling of others: known and limited Scope of Effect/Control – clear path of relationships in the hierarchy

Georgia State University

DFDs to Structure Charts

Process-oriented Design Activities
Conduct Transformational and/or Transactional Analysis
– identify all transactions and see if there is common processing – do while also doing transformational analysis

•Develop Structure Charts •Design the Physical Database •Package Program Units •Write Program Specifications

Georgia State University

DFDs to Structure Charts

Partitioning the Data Flow Diagram
Transformation-centered Application
– – – – file handling and manipulation of data is central rather than I/O partition DFDs transforms data already in files by transformations

Transaction-centered Application
– multiple transaction types – the transaction is central – e.g. bank transfers in many forms, methods of conveyance – by wire, phone, voice, person-to-person, etc..

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