SDLC Phase II: Structuring
System Requirements
IS 582
Dr. Dania Bilal
Spring 2008
Last Week: SDLC Phase II
Determining user requirements
Structuring RFP based on gathered specifications
of user needs
– Wrote specifications for certain functions
– Presented written specifications in class
This Week: SDLC Phase II
continued
Structuring system requirements
Data Flow Diagrams (DFDs)
– Analysis tool to structure user requirements in
a system
– Graphically represents data movement or flow
in an information system and relationships
among data flow
DFDs
Show the processes that transform or
change data
Focus on movement of data between
processes
DFDs are called process models
– Process modeling shows data flow through the
system based on an organization’s rules,
decisions, and policies
DFDs
Increase software development
productivity by
– avoiding mistakes in representing user requirements
Saving money in software cost
Based on user requirements gathered
during planning stage
Evolve from the more general to the more
specific
DFDs
DFDs of current system
– Used to understand current system
DFDs of new logical system
– Used to show data flow, structure, and
functional requirements of new system
DFDs Symbols
Flow of data is represented by
– Data flow
– Data Store
– Process
– Source (external entities)
DFDs Symbols
Data Flow
– Data in motion from one place in a system to
another
Examples
– user query in a database
– Other examples???
DFDs Symbols
Data store
– Physical location or various locations of data
in an information system
– Example: a data store in a circulation system
contains data about students, faculty, staff,
community users, etc.
DFDs Symbols
Process
– Actions performed on data for transformation,
storage, and distribution
– May represent data in
File folder, notebook
DFDs Symbols
Source/sink
– Depicts the origin and/or destination of data
– Refers to a external entity
Another organization or unit that sends and receives
information from the system
A person who interacts with the system, inside or outside
Another information system that exchanges information
with the system under analysis
DFDs Symbols
Source/sink
– Defines the system boundaries
– Data originates outside a system from one or
more sources, and system produces
information to one or more sinks
DFD Mechanics
How the four symbols are represented?
– Data flow: drawn as an arrow
– Data store: drawn as a rectangular
– Process: drawn as a circle
– Source/sink: drawn as a square
DFD Definitions
Level - 0 diagram
– Represents a system’s major processes, data flow,
and data stores at the highest level of detail
Context diagram
– Shows major flow between entities and the system
(too general)
Level N diagram
– A DFD that results from a decomposed Level-0
diagram
Types of DFDs
Current logical
– Depicts current system
New logical
– Has additional functions
– Inefficient data flows are reorganized
DFDs
New physical
– Depicts physical implementation of the new
system
Examples
– Visit
http://www.umsl.edu/~sauter/analysis/dfd/dfd.htm
Group or Individual Class Activity
Do a Level 0 DFD for an automation
function that is of interest to you using
adequate symbols. You may use a
drawing software to perform this activity.
Do a Level N DFD based on Level 0
diagram