Imat1906 systems development
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IMAT1906 Systems Development. Lecture week 8: systems analysis (3) : logical system. Today’s Agenda. Modelling logical system to meet requirements Use case model Data flow diagrams Blackboard survey. Purpose. By now we have found the requirements, from our fact-finding activities

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Imat1906 systems development

IMAT1906 Systems Development

Lecture week 8: systems analysis (3) : logical system


Today s agenda

Today’s Agenda

  • Modelling logical system to meet requirements

  • Use case model

  • Data flow diagrams

  • Blackboard survey

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Purpose

Purpose

  • By now we have found the requirements, from our fact-finding activities

  • Now we need to model the logical system to meet the requirements

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Modelling tools

Modelling tools

  • There are several modelling tools we can use

    • Use case model

    • Data flow diagrams

    • Data model

    • Structured English

    • Decision tables

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case model

Use case model

  • Use case model consists of

    • Use case diagrams + use case descriptions

  • Use case diagram

    • Simple model that shows who requires which function in the system

    • Functions and requirements identified in overall fact finding

  • Use case description

    • Clear concise explanation of what the function does

    • Entries come from detailed fact finding

  • We have seen these for documentation

  • Can also be used for analysis and design of a new system

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case diagram 1

Use case diagram (1)

  • Actors

    • Users and other systems that interact with this system

    • Shown as matchstick figures

  • Use cases

    • Things the system does

    • Things the users need the system to do for them

    • Functions or processes

    • Shown as ovals

  • Connections

    • Link actors with use cases

    • Shown as lines

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case diagram 2

Use case diagram (2)

  • System boundary

    • Depicts system scope

    • Actors are outside the system

    • Use cases are inside the system

    • Shown as a box

  • Dependencies

    • Use cases may relate to each other without being the same

    • One may always include the same steps: use <<include>>

    • One may sometimes lead to another: use <<extends>>

  • Can be drawn by hand or on a CASE tool

  • We saw examples of bookshop and Monte Cerino

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case description 1

Use case description (1)

  • Gives the details behind a use case

    • One description per use case

    • Several entries

      • More than one possible title for some entries

      • Doesn't matter which you choose

      • Be consistent across the use case model

      • Use same entries on all descriptions, even if blank or not applicable

    • Needs to be clear but not over-wordy

  • Name

    • The name of the use case on the diagram

    • Reflects the function or process being described

    • Use whatever term the business people talked about

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case description 2

Use case description (2)

  • Actor(s)

    • Anyone or anything that will interact directly with the system

    • Can have more than one actor

  • Goal or description

    • What process is being described

    • Brief summary of what the use case does

    • If business people have used more than one phrase to describe a function’s purpose, include their phrases so they can relate to the use case

  • Scope

    • Which system the use case is part of eg bookshop

    • What unit of work the use case covers eg single book

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case description 3

Use case description (3)

  • Primary actor

    • Sometimes there is one particular actor who instigates a use case or starts it running

  • Stakeholder(s)

    • Anyone with a work-related interest in the function

  • Preconditions

    • What needs to have been done or what needs to be true before the use case can start

  • Successful completion

    • Steps taken by both actor and system to carry out the process

    • Process flow when nothing goes wrong

    • Reads like a conversation between actor and system

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case description 4

Use case description (4)

  • Alternatives

    • Steps taken by both actor and system to deal with error situations

    • Can also describe non-standard or unusual situations

    • Sometimes called Extensions

  • Postconditions

    • What has been done or what is true after the use case has completed

    • What has changed as a result of the process

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case model examples

Use case model examples

  • We have seen use case diagrams and descriptions for

    • Restaurant – Monte Cerino model, in the first Qsee trainer

    • Bookshop - Student 2 Student, in the lab sessions

  • Bookshop is a simple system

    • And it has a simple use case model

  • Restaurant is a little more complex

    • And it has more use cases in its diagram

    • Also more connections between use cases

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Use case model summary

Use case model summary

  • Use case model shows what happens in the system

  • Can be used to depict overall or outline requirements

  • Can be used to design processes needed in a new system

  • Can be used to indicate which processes link together

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Where we are on agenda

Where we are on agenda

  • Modelling logical system to meet requirements

  • Use case model

  • Data flow diagrams

  • Blackboard survey

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Data flow diagrams dfds

Data flow diagrams (DFDs)

  • Show what happens in a system and the logic of how it happens, along with the data needed

  • Can be used at several stages in system development process depending on needs:

    • If replacing existing system, DFDs can show current physical system ie what is done and how

      • this DFD gives the current physical model

    • DFD can show the logic behind the current physical system, concentrating on what is done without reference to how

      • this DFD gives the current logical model

    • DFD can show the logic and data needed in the new system

      • this DFD gives the required logical model

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Levels of complexity

Levels of complexity

  • Levels of diagram and complexity

    • Context diagram – sometimes called Level 0 diagram

    • Level 1 diagram – shows main processes

    • Level 2 diagram – breaks a complex process down into smaller processes

  • Levels are in a hierarchy

  • Lower-level diagrams are said to further explain or decompose higher-level diagram it came from

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Elements of dfd

Elements of DFD

  • Elements of DFD are the same at all levels of complexity

  • Process

    • shown as box

  • External entity

    • shown as oval

  • Data store

    • shown as open-ended box

  • Flow of information or data

    • shown as arrow

  • Every element is named to indicate what it does

  • We have seen these in the lab sessions weeks 7 and 8

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Duplicate elements

Duplicate elements

  • Duplicate elements

    • Included to make DFD more readable

    • Use diagonal line at top left corner to show duplicates

      • But note that QSee does not put the diagonal line on the diagram

    • Often used for external entities

    • Sometimes used for data stores

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Context diagram purpose

Context diagram - purpose

  • Shows where the system will fit in its surrounding context

  • Flow of information / data / requests / results between actors and system

  • Also shows scope of system

    • What is in the system

    • What is outside the system

  • Can be used to discuss the requirements with the business people

    • Good idea to confirm the scope of the system at the design stage before much development effort has been spent

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Context diagram contents

Context diagram - contents

  • Whole system shown as an empty box

  • Each actor or external entity shown as an oval

  • Each way in which actors interact with system is shown as a data flow

    • Arrow from actor to system for inputs or requests for information

    • Arrow from system to actor for outputs, results of requests, and reminders

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Imat1906 systems development

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Level 1 diagram overview

Level 1 diagram - overview

  • Shows what goes on inside the system

  • Links to context diagram for the system

    • Level 1 diagram decomposes the context diagram

  • CASE tool often uses the context diagram as a skeleton to start off the level 1 diagram

    • Tool knows from the context diagram what is outside the system and what interactions are planned

    • Puts those things on the level 1 diagram ready for connection to processes

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Level 1 diagram contents

Level 1 diagram - contents

  • Shows more detail about what’s in the system

    • Processes

    • Data stores

    • Data flows

    • External entities from the context diagram

  • Usually starts at the beginning of major processing

    • Works through main flow of data in system eg one order

    • Follows it through the system

    • Describes various processes that happen to the data (eg the order) along the way

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Level 1 diagram notation

Level 1 diagram – notation

  • Elements

    • Same as for context diagram

    • Processes in boxes

    • Data stores in open boxes

    • Data flows are arrows

    • External entities in ovals outside the system

    • All with names

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Level 1 diagram how to do 1

Level 1 diagram – how to do (1)

  • Data flows

    • Show information passed between components of the system

    • Flow pointing into a data store means some form of update - could be add, amend, delete

    • Flow pointing out of a data store means read without update

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Level 1 diagram how to do 2

Level 1 diagram – how to do (2)

26

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11

  • Data flow rules

    • Every process must have at least one input data flow and at least one output data flow

      • Data doesn’t come from nowhere

      • Process cannot swallow data

      • All data output from a process must be related to inputs

    • No data flow between external entity and data store

      • Some process needs to transfer the data

    • No data flow from data store to data store

      • Some processing needs to happen to get the data from place to place


Level 1 diagram how to do 3

Level 1 diagram – how to do (3)

  • Method

    • Start with first step in major flow through system

    • Add processes for the steps

    • Attach known data flows from context diagram to processes

    • Add data stores the processes work with

    • Attach data flows between processes and data stores

    • Check for unconnected data flows from external entities

    • Put in processes needed to connect them

    • Check for exception processes or minor processes that are missing

    • Put them in

    • Check the model – QSee can do a lot of the checking

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Imat1906 systems development

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Level 2 diagram

Level 2 diagram

  • If any process on the level 1 diagram is quite large then split or decompose the complex process by creating a level 2 diagram for it

  • The level 2 diagram is not usually so big as a level 1 diagram

  • Notation and method are the same as for the level 1 diagram

    • Processes as boxes

    • Data stores as open boxes

    • Data flows as arrows

    • Same rules for data flows

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Imat1906 systems development

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Data flow diagram summary

Data flow diagram summary

  • Shows what happens in system

  • How processing transforms inputs into outputs

  • What data stores are needed

  • Levels of complexity describe processes within processes

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Further information

Further information

32

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11

  • Use case models

    • Skidmore & Eva pp 100-107

    • Cadle et al pp 205-211

    • Shelly & Rosenblatt pp 147-148, 257-260

    • Schneider & Winters chapters 1-4

  • Data flow diagrams

    • Skidmore & Eva pp 111-119

    • Shelly & Rosenblatt pp 198-205


Where we are on agenda1

Where we are on agenda

  • Modelling logical system to meet requirements

  • Use case model

  • Data flow diagrams

  • Blackboard survey

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Survey background

Survey background

  • One thing I am doing, with other tutors, is research into student feedback and making it more meaningful to you

  • Tutors give you feedback:

    • In labs and tutorials

    • In comments on assignments

    • In answers to your questions

  • Students give us feedback:

    • At end of module

    • At end of year

  • But I want to know what you think now in the middle of the year so I can solve any problems

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


Survey method

Survey method

  • Short questionnaire on Blackboard

    • In the Student Feedback area – this is a new option on the left hand side menu of the module home page

    • There are about 12 questions

    • One module mark is available to those who take part

    • There is likely to be a similar survey at the end of next term

IMAT 1906 Lecture Week 8 (c) De Montfort University 2010-11


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