SIF8035. Events and System Requirements

Transcription

1 SIF8035 Lecture 4 DFD and PrM Events and System Requirements Events Occurrences at a specific time and place Trigger all system processing Requirement definition Determine relevant events External events first Temporal events second Decompose system into manageable units 1

2 Types of events External events are initiated from outside the system by external agents or actors A customer orders products A customer cancels an order Temporal events are triggered by a point of time The manager requires a monthly production statement Invoices must be generated at 3pm State events trigger processes based on a system s change from one state or condition to another something from inside the system Manager changes production schedule Quantity of item falls below re-order level Naming an Event The event name should reveal the system nature of the event External event External agent name followed by a reason CUSTOMER REQUESTS ACCOUNT BALANCE Temporal event Time to followed by action that must be taken TIME TO BILL CUSTOMER ACCOUNTS 2

3 Events Affecting a Charge Account Processing System Satzinger et al. Fig 5-6 External Event Checklist Satzinger et al. Fig 5-7 3

4 Temporal Event Checklist Satzinger et al. Fig 5-8 Actions Leading to Only One Event Affecting the System Satzinger et al. Fig 5-9 4

5 Transactions for One Customer Resulting in Many Events Satzinger et al. Fig 5-10 An Event Table Also see Satzinger et al. Fig 5-15 for complete event table Satzinger et al. Fig

6 DFD Fragment for the event: Look up item availability Satzinger et al. Fig 6-3 DFD Integrates Event Table and ERD Satzinger et al. Fig 6-4 6

7 DFD Fragments Represents system response to one event within a single process symbol Self contained model Focuses attention on single part of system Shows only data stores required to respond to events Two DFD Fragments for Order- Entry System Satzinger et al. Fig

8 Combining DFD Fragments Satzinger et al. Fig 6-11 Detailed Diagram for Create New Order Satzinger et al. Fig

9 Decomposing DFD Fragments Usually, DFD fragments need to be explored in more detail Broken into sub-processes with additional detail 9

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11 Documentation of DFD Components: Rule Modeling Lowest level processes need to be described in detail Data flow contents need to be described Data stores need to be described Various options for process definition exist Structured English Method of writing process specifications that combines structured programming techniques with narrative English Well suited to lengthy sequential processes or simple control logic Ill-suited for complex decision logic or few sequential processing steps 11

12 Structured English Process Description Satzinger et al. Fig Sequence A sequence of simple, declarative sentences one after another. Each sentence uses strong, action verbs such as GET, FIND, DELETE, CREATE, READ, UPDATE, etc A formula may be included as part of a sentence CALCULATE grosspay = hoursworked * hourlywage 12

13 2. A selection (conditional or decision structure ) indicates that a process must perform different actions under specified conditions. The IF-THEN-ELSE allows a choice between two alternatives The CASE construct is used when there are more than two sets of actions to choose from. For logic that is deeply nested use a decision table 3. An iteration (or repetition) structure specifies that a set of actions should be repeated based on some stated condition. The DO-WHILE construct indicates that certain actions are repeated zero, one, or more times based on the value of the stated condition. The REPEAT-UNTIL constructs indicates that certain actions are repeated one or more times based on the value of the stated condition. 13

14 DO WHILE there are claims remaining IF claimant has not sent in a claim set up a new claimant record ELSE continue ADD claim to YTDClaim IF claimant has policya THEN subtract excess of $400 from claim ELSE subtract excess of $100 from claim ENDIF.. ENDDO Decision Tables and Decision Trees Can summarize complex decision logic better than structured English Incorporates logic into the table or tree structure to make descriptions more readable 14

15 Will I get up and go to class? Get up decision time to get up? Y Y Y Y N tired? Y N Y Y - anything planned? Y - N Y - able to ignore? Y - - N - sleep get up X X X X X Decision Trees Decision trees are also useful when complex branching occurs, in a particular order, in a structured decision process Condition 1 1 Condition Action 1 Action 4 Condition 3 3 Condition Action 2 Action 3 15

16 Can I purchase goods on credit? Purchase >$200 - refuse credit Credit limit exceeded Credit limit not exceeded Good payment history Bad payment history Purchase < $200 - refer to boss Refuse credit Allow credit Data Dictionary A Data Flow Diagram shows an overview of an information system without the clutter of details A Data Dictionary is used to document the detailed descriptions of the data that flows through an information system and the data that is stored by that system 16

17 Data Dictionary... an organised listing of all the data elements that are pertinent to the system, with precise, rigorous definitions so that both user and systems analyst will have a common understanding of all inputs, outputs, components of stores, and intermediate calculations. (Yourdan 1989, p189) A Data Dictionary contains metadata ; Data Flows Data Records Data Elements Data Stores Data Dictionaries can vary from a stand alone dictionary to those that are fully automated (CASE tools) 17

18 Data Dictionary Notation Symbol = + ( ) { } [ ] * (or underline) Meaning is composed of and optional iteration selection comment identifier (key field) Examples; StudentName = * A students name* CourtesyTitle + FirstName + (MiddleName) +LastName CourtesyTitle = [Mr. Mrs. Ms. Miss Dr. Professor] FirstName = {legal-character} MiddleName = {legal-character} LastName = {legal-character} legal character = [A-Z a-z] Order = * A customer order* OrderNo + OrderDate + Department + CustomerNo + {[Item Service] + Cost} + TotalCost 18

19 Hours worked Wage Information = RateofPay + Overtime Wage Information 5.3 Compute Current Pay Amounts Current Pay Amounts Current Pay Amounts = GrossPay + TaxDeduction + SuperDeduction + NetPay Structured Program Design How do you use DFD diagram to implement software? Structured programming modules, subprograms (subroutines) subprogram structure diagrams transform analysis link between DFD and structure diagrams 19

20 Structure Diagram A calling subprogram A optional B called subprogram B Example with communication arrows 20

21 Transform analysis map functional specification to program modules include control relationships, communication, Read order from db. Enter & Edit new Enter & Verify order Edit Order Verify Cust credit Read cust. data note optional procedure call (DFD?).. Reject order Software design high subprogram cohesion low subprogram coupling good structure hierarchical simple interfaces.. 21

22 PPP Phenomenon, Process & Program better integration of notations more expressive notations 3 submodels Phenomenon model (PhM) [ER++] Process Model (PrM) [DFD++] Process Life Description (PLD) more comprehensive code generatrion DFD: missing detail temporal specification start/stop sequence computing input/output relations marking of resources T A2.1 T T project leader tool report 22

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24 PLD Process Life Description PrM to PLD 24

25 Constructivity A system s properties can be formally derived from statements of components structural relationships and components properties 25

26 PPP elements 26