SE-381Software Engineering

BEIT-V

Lecture no. 21

(Data Dictionary)

Logical Data Dictionary• Data Dictionary

• Is a mean of recording metadata of a system• The Logical Data Model is to record and

analyze data requirements independently of how these requirements are going to be met

• The Physical Data Model is to record design decisions in terms of its implementation.

Hence Data Dictionary • Is a mechanism for recording the data require-

ments and data resources of an organization.• Is a tool for Analysis and Design phases of SD

Data Dictionary (DD)– Data Dictionary

– Is simply a record of data about data, or metadata– Can be compiled manually or by a fully automated

package– Links different techniques and components of system

together– Is not a static mechanism, but the information stored in

will improve/increase and will be updated with time– Provides a logical bridge between Analysis & Design– Serves as back-bone for CASE toolsIn Structured System Analysis and Design, DD holds data

about 3 of the 4 components of DFDs i.e. Data Stores, Processes and Data Flows. Further, the Data Elements and Data Structures need be defined to elaborate composition of Data Flows and Data Stores

Templates - DD Components

– Data Element:Name: A meaningful unique nameDescription: Short description of meaning of DEAliases: Several dept may refer same DE by different

names or termsType: Character, Numeric or AlphanumericFormat: Used to prepare format checks in subsequent

system designValues: To embody different codes to represent different

categoriesSecurity: Who can modify, add or delete the given DEEditing: How +ve or –ve numbers be differentiatedComments: To record some special information about the

DE

Templates – DD Components.

– Data StructureA Data Structure is made up of data elements and other

data structures. Thus Data Dictionary should contain its complete info, explicitly mentioning which elements are Optional, Repeated or mutually exclusive.

Optional Structure: Placed in square brackets eg [PREV-SURNAME]

Alternate Structure: Place in braces eg {FATHER-NAME, HUSBAND-NAME}

Iterations of Structure: Marked with an asterisk eg COURSE-REGISTERED * (1-5) with number of iterations placed, if known, in parentheses, here applicant can register in 1,2,3..5 courses

Volume Information: Collected at the end of the form and used for resource estimation

Backus Naur FormA Notation primarily used to define the syntax of programming languages, can also be used to define Data Elements and Data Structures

= Left of the sign consists of whatever is on right

+ Equivalent to ‘and’

{…;…;…} Only one of the item is to be chosen – Selection

[ … ] Optional i.e. zero or one occurrence

(…) Item contains from zero to an infinite number of occurrences of whatever inside braces – Iteration

*…* Comment i.e. it does not constitute the part of def

Some examples of terms of Data Dictionary

AGREED-PURCHASE-PRICE = *Price provisionally agreed between the purchaser and vendor *

CENTRAL-HEATING = CENTRAL-HEATING-TYPE + CENTRAL-HEATING-DEGREE

CENTRAL-HEATING-TYPE = {GAS ; ELECTRIC ; SOLID-FUEL }

CENTRAL-HEATING-DEGREE = { MAJOR ; AVERAGE ; MINOR }

Writing Data Dictionary Items in BNF

Templates – DD Comps ..

– Data Store– Contents of Data Store can be written more clearly and

with less chance of error. Further, interrelationships of parts of systems are also represented by the occurrences of the structures. All data flows coming-in and going-out are recorded

– Data Flow– Its Source, Sink and composition be recorded.

– Process– Along with inputs and outputs to the process, its logic or

working can also be recorded into the template

Data Dictionary (DD)– Data Dictionary

– Is simply a record of data about data– Can be compiled manually or by a fully automated

package– Links different techniques and components of system

together– Is not a static mechanism, but the information stored in

will improve/increase and will be updated with time– Provides a logical bridge between Analysis & Design– Serves as back-bone for CASE toolsIn Structured System Analysis and Design, DD holds data

about 3 of the 4 components of DFDs i.e. Stores, Processes and Data Flows. Further, the data Elements and Data Structures need be defined to elaborate composition of Data Flows and Data Stores

References

• Tom DeMarco (1978); Structured Analysis and System Specifications

• Chris Gane and Trish Sarson (1979) Structured Analysis : Tools and Techniques

• Paul Beynon-Davies (1989); Information Systems Development, Macmilon, London, UK

• Steve Skidmore and Brenda Wroe (192); Introducing System Analysis, NCC Publications, BPB Plublications, India

• Wayne P Stevens (1991); Software Design: Concepts and Methods, Printice Hall, London, UK

DFDs for ATM System

0-Level or Context Diagram

1st-Level Diagram

2nd-Level Diagram (Process 3.0 Draw Cash)

Guidelines for Constructing DFDs– Read the Problem Specification or listen carefully to the

verbal specification of Problem– Analyze and identify the Externals– List down the Activities and Sub-activities, preferably in

indented format, depicting hierarchy– In reply to question ‘How you managed to publish so

much?’ Prof C A Hoar told:‘I take a ream of white paper, handful of sharpened lead pencils, a thick good quality eraser, sit at a lonely place and start writing’

[Prof C A Hoar]‘

– Follow Prof Hoar’s advice

– On a A4 sheet draw Externals at the periphery, mark the Data Flows originating from or destined to these Externals, and then draw a Process in the Middle of the page, with a name most appropriate for the system.

– Link Externals to this central Process by extending Data Flows. Name the Data Flows

– This is the 0-Level or Context Diagram

Be Reminded - first few DFDs would be (probably) wrong, so Be patient and persistent. Failures are to learn and key to SUCCESS

– To Identify Externals, read problem specs, mark or underline Nouns; these can be Externals, data flows or Data Stores, so look into the context how these are used, if these are acting as sources or destinations for data, then these are Externals

– Mark Verbs representing actions these indicate activities or sub-activities, you have to decide their correlation-ship, i.e. which is sub-activity of which activity

– To draw, 1st level or Overview Diagram, take an A4 sheet, draw a dashed line boundary, a larger rectangle and transfer all data flows from the Context Diagram to this

– Now look for each of the activities and associate a Process to it, and so draw Processes for them.

– Concentrate on each of the processes, and workout the possible data flows coming-in and going-out of these Processes, to demonstrate the requisite functionality

– Introduce any of the Data Stores, if needed, and Data Flows thru these Data Stores is preferred, it helps in de-linking the Processes later

– Now extend the inherited Data Flows, to the respective Processes, instead of their termination or origination from Boundary

– Label or name and number all Processes, Data Flows and Data Stores as per Guidelines

– Explode the Processes, where needed i.e. the ones having more functionality and activity

Courses Registration at KICSIT

Read the Case Study and draw logical Data Flow Diagrams, up to three levels, i.e

Context or 0-Level,

1st Level or Overview Diagram and

2nd Level diagrams for the processes having sufficient functionality

Courses Registration @ KICSIT Home Assignment

– I hope every body tried to • Understand the Problem and • make the Data Flow Diagrams up to 2nd or 3rd

level

– Results of an attempt to make these DFDs are presented:

• These are correct, but still open for discussion and correction, if any.

0-Level or Context Diagram

The DFDs are to be revised and updated accordingly, and top-down and bottom-up iterations among different levels of DFDs are to be carried out to ensure consistency and balancing.