IMAT1906 Systems Development

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# IMAT1906 Systems Development - PowerPoint PPT Presentation

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

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

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

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
• 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 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)
• 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)
• 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)
• 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

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

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)
• 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)
• 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
• 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 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
• 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)
• 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 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 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
• 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
• 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
• 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
• 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
• 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
• 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)
• 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)

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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)
• Method
• 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
• 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
• 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

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 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
• 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
• 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
• 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