IMAT1906 Systems Development

1. IMAT1906 Systems Development Lecture week 8: systems analysis (3) : logical system

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

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22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

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29. 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

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31. 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

32. 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

33. 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

34. 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

35. 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