STRUCTURED SYSTEMS ANALYSIS & DESIGN

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Julia Logan
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1 Lecture 5 STRUCTURED SYSTEMS ANALYSIS & DESIGN Objectives Explain SSADM Describe tools of SSADM 2 1

2 SSADM It is a data (logical) oriented approach to system analysis and design that focuses on nature and flow of data through the system APPROACH Uses models (graphics & pictorial) to show relationships between data & business processes throughout the stages of analysis & design 3 Advantages Gives a pictorial representation that can be understood by both users & system developers 4 2

3 Types of modeling 2 types (a) Logical Data Modeling (LDM) Entity relationship Modeling (ERM) Entity Behavior Modeling (EBM) (b) Data Flow (process) Modeling Data Flow Diagrams (DFD) Logic modeling 5 (a) Logical (data) modeling It is a technique of identifying & modeling the data requirements of an IS. LDM consists of logical data structures (LDS) which represent entities and their relationships through ERD/ERM diagrams LDM is used in creation of databases and is also called database modeling 6 3

4 ERM uses 3 major abstraction to describe things to which data is related to: Entity, attribute, instance & relationship Entity = something for which data is recorded about e.g., persons, events etc Attributes:- characteristic/property of an entity that has a unique value e.g., student (entity) is described by attributes such as ID No., Name, Age etc. They are stored as fields in a Table 7 Table: = A grouping of records = basic data structure E.g., Entity = Student Primary Key Attributes/ Fields Entity instance/ Record Student ID. Name Age Course I39/3458/12 Hilda 20 BTech Value of Attribute 8 4

5 Entity Instance (record) :- A single occurrence of an entity e.g., one instance of entity student is Name and value is Hilda Primary) Key:- Unique identifier for the records e.g., ID-No Relationships = Associations between entities. ERM uses these abstractions together with relationship between entities to create Databases 9 Creating a LDM Collect and divide data about entities into separate subjects (Fields) Group related fields into (ONE) table Create Primary Keys to link data in Tables Determine nature of relationship between entities E.g., Information relating to books in library and users (borrowers) can be grouped as follows: 10 5

6 Table: BOOK ISBN Author Date-Pub Publisher Title Table: BOOKINGS Date-Out Due Date User-Id Title Status Relationships Table: USER User-ID User_Name Tel Address Occupation NB Fields in a Table should not be repeated in other tables 11 Entity Relationship Modeling ERM shows the associations between entities. ERM Symbols Entity E.g., Student Student Courses Relationship between Entities 12 6

7 Degree of relationships (a) One : One For any entity A, only one member of entity B associated with it and vice versa. E.g., MD Works for Employs Company A kenyan has 1 ID Kenyan ID Card 1 Card is issued to 1 Kenyan 13 (a) One : Many A Mother has many children Mother Children Many Children belong to one Mother 14 7

8 1:M A dept has Many Employees Department Employee Many employees Belong to 1 Dept Cargo aircraft Will serve Distribution centres Is served by 15 (c) Many : Many Product Made of Raw material Part of Sales Person is assigned to Customer Is called on by 16 8

9 Activity A company has one CEO who is not employed by another company. The company has several divisions each of which employs several people, No employee works for more than one division. Each division produces several products. No product is made by more than one division. Each product may be made out of one or more raw materials. Any raw materials may go into one or more products. Construct ERD from the narrative 17 Soln Entities: Company, CEO, Division, Employee, pdts, raw materials Company CEO Division Employee Product Raw Materials 18 9

10 Once the relationships are developed, we then create Databases Database = An integrated collection of logical data relating to an entity or collection of related fields 19 (b) Process & Logic modeling Process Modeling = technique of identifying & modeling how data moves around an IS by examining processes that transform data, data stores and external entities using DFDs. Logic modeling shows the logical, policies & procedures to be implemented by a system s process 20 10

11 (i) DFDs DFDs shows processes (activities) that transform data from one form to another, data sores (holding areas of data), External entities (things that sent data into or receive data from a system) and data flows (routes by which data flows types of DFDs Logical DFD Physical DFD Logical DFD Shows actual data flows and processes that transform data NOT concerned about HOW the system works or will be constructed 22 11

12 DFD symbols External entities = Source or destination of data e.g., Student, Customer. Processes = Actions that transform data Data stores = Temporary or permanent stores of data that are inputs to or outputs of processes D3 Student 2 Calculate fees Student file Data flows = transfer of data Student Data 23 Developing DFDs DFDs are developed using top-down approach (a process called Leveling ) PROCEDURE 1. Define entities, data flows, processes & data stores 2. Draw content diag (level 0) 3. Draw Parent diag (Level 1) 4. Processes on level 1 may in turn be exploded to create more detailed child diags (levels 2 n) 24 12

13 Narrative Context Diag (LEVEL 0) Parent Diag (LEVEL 1) Start with Context diag that shows ONLY a single process ( the system ) External entities and data flows to and from the system NO DATA STORES Create Level 1 DFDs that shows More Processes and data stores Child Diags (LEVEL 2) Explosions of Child Diags (LEVEL N) Create CHILD DIAGS (Levels 2,3, n) that are explosions of Level 1, 2 n-1 DFDs until the diag shows only primitive processes. SPH_201_FWN, Nyongesa UoN F. W. 25 Example 1 Level 0 (Context Diag) Student 0 System Student Level 1 DFD Student 1 Process 2 Process Student D1 Data store 26 13

14 Level 2 DFD Shows child processes Student 1 Process 2 Process Student Process Process D1 Data store D2 Data store 27 Rules on creating Child diagrams 1. A Child diag is created from a process of the previous level diag 2. 1 ST sub-level (Child diags) are numbered to correspond to the respective parent e.g., 2.1, 2.2 etc 3. Each Child diag should have the same number of input and output data flow as the parent process i.e., data flow from parent process to child diag must match 4. Child diags may include additional data stores not shown in the parent process e.g., D

15 Example 2 Level 0 (Context Diag) Only 1 process (numbered 0) & NO Data stores Entity 1 Input A 0 System Output B Entity 2 29 Level 1 DFD (Parent Diag) Shows more (unexploded) processes & datastores Entity 1 Input A 1 Process A Data Flow B 2 Process B Output B Entity 2 Record A D1 Data Store 1 D2 Record B Data Store

16 Level 2 DFD (Child Diags) Shows 1 ST sub-levels of a process (i.e., explosion of a process e.g., process 2) Output flow must match Entity 1 Input A Record A 1 Process A Data Flow B Record B 2 Process B Entity 2 Output B D1 Data Store Process B1 2.2 Detailed Process B2 2.3 Detailed Process B3 Additional data store D3 Data Store 3 D2 Data store 2 31 Example 3 Content diag of Library System NB 3 outputs to user & 3 inputs from user user Register as member confirmation 0 Lib services Books to borrow Borrowed books user Clearance user 32 16

17 Level 1 DFD of Lib system NB 3 outputs to user & 3 inputs from user user Registration request Confirmation 1 User registration User D1 Borrow books Books User details Registered users User s file 2 Loans desk Copy of book card D2 Book card Borrowed books User Due Books Clearance 3 Book Returns Desk Books Shelving Books SPH_201_FWN, Nyongesa UoN F. W. 33 D3 Cleared cards D4 Dustbin Example 4 Airline Reservation system Level 0 DFD Passenger Travel request Passenger reservations Preferences & available flights 0 Airline reservation system Travel agent Ticketing information Airline 34 17

18 Rules in Constructing DFDs No data flow is permitted between Two Data Sources/External Entity Two Data Stores Data Stores and data source. D1 student Student D2 Student D1 35 Worked Example ABC is a book publishing company in Westlands. It is organized in sections e.g., accounting and sales When the clerk receives book orders from bookstores, he verifies the orders. He then checks the book information as well as the customer credit from the customer file and authorizes shipment through the warehouse. The shipment is verified before shipping and it is accompanied by an invoice. The clerk keeps a copy of the details of the order in a file called "bookstore orders". Accounts receivable are processed through the accounts department from forms filled out by an accounting clerk. Draw a logical model for the system 36 18

19 Soln Entities: Data Stores: Processes Data Flows 37 Soln D1 BOOK INFORMATION CUSTOMER ACCOUNTS 3. Credit Check 1. Orders D2 1.0 Verify orders CUSTOMER INFORMATION FILE 2. Check book inform 4. Shipping orders 8. forms D3 BOOKSTORE ORDERS 6. Copy of order 2.0 Verify Shipment 7. Shipment + Notice + Invoice CUSTOMER WAREHOUSE 5. Shipping Information SPH_201_FWN, Nyongesa UoN F. W

20 Physical DFDs Are created from Logical DFDs It depicts the system i.e., it shows HOW the system is physically implemented (designed or works) including personnel, hardware, software, databases, etc Shows certain details not shown in logical DFD such as type of activities that take place in a process 39 Example of physical DFD 40 20

21 Logical DFD of Doctors billing system D1 Physician a Verify Exam record & compute fees Examination Record Valid Fees Patient Applicable Patient fees Address FEE SCHEDULE Prepare Invoice Completed Insurance form Insurance Invoice Patient Invoice Insurance Schedule & Address PATIENT Patient Payment INSURANCE CARRIERS Carrier Payment D2 PATIENT RECORD D3 INSURANCE Co. Correct patient Payment Verify payment from patients Verify payment from insurance carriers SPH_201_FWN, Nyongesa UoN F. W. 41 D4 Correct carrier payment ACCOUNTS RECEIVABLE Advantages of DFDs Provides better understanding of the business to the analyst Provides better communication with users compared to narrative text Identification of error and redundancies is easy (in structured walkthroughs) Physical DFDs Show sequence of process, order and in detail how the system works from top to minute details using the levelling technique 42 21

22 Activity Distinguish between logical and physical DFD List advantages of physical DFD over logical DFD 43 Structured logic & Decisions Logic modeling shows the logical, policies & procedures to be implemented by a system s process Tools of Logic modeling: Decision tables :- Used to make a decision between two or more choices depending on the value of the condition Decision Trees :- Useful for sketching logical decisions where complex branching routines are involved e.g, calculating discounts or sales commissions etc. Structured English:- suited to model logical processes where sequencies of actions and decisions are required 44 22

23 (a) Decision Tables It s a 2-D matrix that shows the decision rules that apply when certain CONDITIONS occur and ACTIONS to be taken CONDITIONS (IF) Condition Entry Y/N) ACTIONS (THEN) Action Entry (X) No of columns required in the condition entry = 2 N where N =Number of conditions 45 Example The delivery charge to be added to customer s invoices is determined as follows: If the order value is at least 100,000/= and the delivery distance is within 1 Km, the delivery charge is 1,000/=. If the distance > 1 Km, the delivery charge is 1,500/=. If the order value < 100,000/= and the delivery distance is within 1 Km, the delivery charge is 2,000/=, else 3,000/=. Draw a decision table from the above

24 Soln 2 conditions: Order value => 1000,000 Delivery dist within 1 Km And 4 Actions (Charge 1,000, 1,500, 2,000 & 3,000) Entries = 2 2 =4 Order Value >= 100,000? Delivery Distance within 1 Km? Y Y Y N N Y Y N Charge 1,000 X Charge 1,500 X Charge 2,000 X Charge 3,000 X 47 Activity ABC bookshop gives customers discount based on the following policy: Bookstores get 25% discount on orders of 6 or more copies per book title and Nil for orders less than 6 copies. For orders from libraries and individuals, a 5% discount is allowed on orders of 6-19 copies per book title; 10% on orders for 20-49% copies per book tittle; 15% for 50 or more copies. Document this policy using Decision Tables Decision Trees Structured English 48 24

25 Soln 3 conditions: Bookstore; Lib & Order size 5 Actions (25, 15, 5 & 0%) Entries = 2 3 = 6 49 Soln 888 RULES IF Customer is bookstore Y Y Library or Individual Y Y Y Y Order size => 6 copies Y N N Order size 6-19 copies Order size copies Order size => 50 THEN Allow 25% discount Allow 15% discount Allow 10% discount Allow 5% discount Allow Nil discount X X X Y X Y X Y X 50 25

26 (b) Decision Trees Useful for sketching logical decisions where complex branching routines are involved in the decision process e.g., Customer Order size Discount Bookstore = > 6 < Discount policy Libraries & Individuals > < (c) Structure English suited to model logical processes where sequencies of actions and decisions are required borrows heavily from control structures in programming especially If..Then Else

27 Soln IF (order is from bookstore & = > 6 copies) THEN Discount = 25% ELSE: Nil END IF ELSE IF (Order from lib & Order > 50 copies) THEN Discount = 15% ELSE IF Order = copies Discount is 10% ELSE IF Order = copies Discount is 5% END IF 53 END 54 27

information process modelling DFDs Process description

Process modelling IMS9300 IS/IM FUNDAMENTALS information process modelling DFDs Process description processes are the action part of businesses process modelling graphically represents the processes which