- Slides: 16
Structured system Analysis
Structured System Analysis-Definition • It focuses on specifying what the system or application is required to do. • The systematic and integrated use of tools and techniques to aid the analysis and design of information systems • Structured methodologies use one or more tools to define information flow and processes.
Structured terms • Data elements - lowest level of information on which a process can act i. e. DB attributes/record fields - e. g. unit price • Data stores - places where data are stored; e. g. files; microfiche, filing cabinets • Data flows - represent movement of data in a system; consist of data input and data output e. g. forms, reports, invoices, letters – show movement of data about a physical “thing”
Continues. . • Process specifications - definitions of how processes work • Data dictionary - document containing definitions of all system data; includes data elements, data structures, data stores, data flows, and process specifications
Elements of structured analysis • Petri nets • Data Dictionary
Petri nets • Consists of three types of components: places (circles), transitions (rectangles) and arcs (arrows): – Places represent possible states of the system; – Transitions are events or actions which cause the change of state; And – Every arc simply connects a place with a transition or a transition with a place.
Change of States • It is denoted by a movement of token(s) (black dots) from place(s) to place(s); and is caused by the firing of a transition. • The firing represents an occurrence of the event or an action taken. • The firing is subject to the input conditions, denoted by token availability.
Continue. . • A transition is firable or enabled when there are sufficient tokens in its input places. • After firing, tokens will be transferred from the input places (old state) to the output places, denoting the new state.
Example: Vending Machine • The machine dispenses two kinds of snack bars – 20 c and 15 c. • Only two types of coins can be used – 10 c coins and 5 c coins. • The machine does not return any change.
Normal requirement design Take 15 c snack bar c De po sit 15 cents c Deposit 5 c 0 cent Deposit 5 c po e D 5 sit Deposit 10 c 5 cents Deposit 10 c 20 cents t 5 i s o p De 10 cents Take 20 c snack bar
Vending Machine-Petri nets Take 15 c bar Deposit 10 c 5 c 15 c Deposit 5 c 0 c Deposit 10 c Take 20 c bar Deposit 5 c 20 c Deposit 10 c
Other Types of Petri Nets • High-level Petri nets • Tokens have “colours”, holding complex information. • Timed Petri nets • Time delays associated with transitions and/or places. • Fixed delays or interval delays. • Stochastic Petri nets: exponentially distributed random variables as delays.
Other Types of Petri Nets • Object-Oriented Petri nets • Tokens are instances of classes, moving from one place to another, calling methods and changing attributes. • Net structure models the inner behaviour of objects. • The purpose is to use object-oriented constructs to structure and build the system.
An O-O Petri Net Producer Consumer accepted ready produce send Storage accept consume ready Producer data: ITEM produce( ) void send(ITEM) Consumer data: ITEM accept( ) void consume(ITEM)
Data dictionaries • Data dictionaries are lists of all of the names used in the system models. Descriptions of the entities, relationships and attributes are also included • Advantages – Support name management and avoid duplication – Store of organisational knowledge linking analysis, design and implementation • Many CASE workbenches support data dictionaries
Data dictionary entries