SDLC Phase III: Structuring System Requirements Dr. Dania Bilal Spring 2009

SDLC Phase III 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 Saves money in software cost Based on user requirements gathered in previous phase Evolves from the more general to the more specific

DFDs DFDs of current system DFDs of new logical system Used to understand current system DFDs of new logical system Used to show data flow, structure, and functional requirements of new system

DFD Symbols Flow of data is represented by Data flow Data Store Process Source (external entities)

DFD Symbols: Data Flow Data in motion from one place in a system to another Example: Querying a database Students: Describe how this activity flows from an interface by a user? Give another example.

DFD 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 and staff Community and other users

DFDs Symbols: Process Actions performed on data for Transformation Storage Distribution Data may be represented in File folder Notebook Tables Other formats

DFD 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

DFD Symbols: Source/Sink Defines the system boundaries Data originates outside a system from one or more sources 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 Context diagram Level N 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 DFD Current logical New logical Depicts current system Has additional functions Inefficient data flows are reorganized

DFDs New physical Examples Depicts physical implementation of the new system Examples Visit http://www.umsl.edu/~sauter/analysis/dfd/dfd.htm