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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

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

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

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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