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Systems Analysis and Design

Systems Analysis and Design. Plan. Introduction Structured Methods Data Flow Modelling Data Modelling Relational Data Analysis Feasibility Maintenance. Plan. Introduction Structured Methods Data Flow Modelling Data Modelling Relational Data Analysis Feasibility Maintenance.

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Systems Analysis and Design

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  1. Systems Analysis and Design

  2. Plan • Introduction • Structured Methods • Data Flow Modelling • Data Modelling • Relational Data Analysis • Feasibility • Maintenance

  3. Plan • Introduction • Structured Methods • Data Flow Modelling • Data Modelling • Relational Data Analysis • Feasibility • Maintenance

  4. Data Flow Modelling

  5. What are Data Flow Models? • A model of the system • Software modelling is unlike other types of modelling • A communication tool • Should be clear and unambiguous • An analysis tool • Should help us to understand systems • A design tool • Should help us to develop and improve systems

  6. What are Data Flow Models? • They only represent a single view of the system • We cannot describe the system fully in a single diagram • Represents the Process View • What processes act on data as it flows through our system

  7. When do we use them? • Feasibility • High Level of abstraction • Enough Detail to estimate Data/Processing requirements • Analysis • Analysis of Current System and Requirements • Design • Used as source for design of new system

  8. Structured Analysis + Design

  9. What do they look like? • As usual there are a number of “standards” for drawing DFDs • They share a set of common elements • Data Flows • Processes • Entities • Data Stores

  10. Example 1 – Gane and Sarson

  11. Example 2 – DeMarco/Yourdon

  12. Example 3 - SSADM

  13. What do they look like? • We will use the SSADM style • Used in Lejk and Deeks, 2002 • You should be able to read diagrams using another style

  14. How do we build one? • Top down • More likely to be done in Tutorials • Good way of learning the technique • More detail added in successive iterations • Bottom up • More likely to be done in practice • Interviews provide localised detail not context

  15. Building a Data Flow Model • Understand the system • Establish the context • Identify major processes • Identify Data Flows: • What do processes produce as output • What Data is required to produce output • Identify other elements • Where does data come from/go to • Check against System Description

  16. Document Flow Diagram (DocFD) • A precursor to DFDs • A good way of establishing the context • Identify the entities in the system (departments) • Identify documents flowing between them • Establish system boundary • Bottom up and top down?

  17. DocFD Elements • Entity (also called ‘Agencies’) • Symbol • Duplicate symbol acustomer acustomer

  18. DocFD Elements • Document Flow Sales Order

  19. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers dbar/restaurant weekly statement (copy) caccounts dept Example Diagram (CD Hotel)

  20. Identifying the system boundary • This is rarely an easy task • There may be different opinions on this boundary • Unless it is finalised development will fail • Once identified, the boundary will be clearly stated in the terms of reference for the project

  21. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers dbar/restaurant weekly statement (copy) caccounts dept Example Diagram (CD Hotel)

  22. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers dbar/restaurant weekly statement (copy) caccounts dept Example Diagram (CD Hotel)

  23. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers weekly statement (copy) dbar/restaurant caccounts dept Example Diagram (CD Hotel)

  24. Document Definitions • It is important at this stage to identify the information in the documents that flow across the system boundary • These will inform us of the interface between the system and external entities • Data Interface and possibly GUI • These will form the basis of our data dictionary

  25. Data Dictionary Entry (Mason and Wilcocks, 1994)

  26. DFD Context Diagram • Now that the system boundary has been established we can show the context in which our system operates • All internal details are removed • System boundary is changed to a process box • All input and output data flows have been identified

  27. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers dbar/restaurant weekly statement (copy) caccounts dept Example Diagram (CD Hotel)

  28. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers dbar/restaurant weekly statement (copy) caccounts dept Example Diagram (CD Hotel)

  29. aguest booking+deposit payment confirmation weekly statement bbooking dept final statement deposit vouchers weekly statement (copy) dbar/restaurant caccounts dept Example Diagram (CD Hotel)

  30. aguest booking+deposit confirmation CD HotelBookings weekly statement bbooking dept final statement deposit vouchers weekly statement (copy) dbar/restaurant caccounts dept Example Diagram (CD Hotel)

  31. DFD Context Diagram We now know what Systemwe are going to develop

  32. Adding More Detail • Now we have established the system context we can start adding more detail • To do this we create a Data Flow Diagram (DFD) • Identify Processes • Identify Data Flows • Identify Data Stores • Connect them to External Entities

  33. 1 Sales Assistant 2 Accounts Dept Create Reservation Maintain Sales Account * Elements of a DFD • Process Box • Symbol • Active Component • What goes in must come out or be transformed • Physical or Logical? Indicates Process at lowest level of detail

  34. Sales Order Elements of a DFD • Data Flow • Does not change data from end to end • Changes imply there is a process – so add one • Used to create the data model – must be labelled

  35. M1 M1 Sales File Sales File Elements of a DFD • Data Store • Symbol • Duplicate • Passive Component • Does not process data • Manual or Digital (M/D)

  36. Elements of a DFD • External Entity • also called ‘Agencies’, source and sink • Symbol • Duplicate symbol acustomer acustomer

  37. Example - SSADM

  38. Example (CD Hotel) • When a guest sends us a booking, with deposit, we change the vacancy list to show one less and enter the customer details into the reservation file. The cash from the deposit is sent to the accounts department. A letter of confirmation is sent to the customer and we also open a guest account in our ledger

  39. Example (CD Hotel) • When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the reservation file • The cash from the deposit is sent to the accounts department • A letter of confirmation is sent to the customer • …and we also open a guest account in our ledger

  40. aguest booking+deposit confirmation CD HotelBookings weekly statement bbooking dept final statement deposit vouchers weekly statement (copy) dbar/restaurant caccounts dept Example (CD Hotel)

  41. High Level Processes can often be identified as business functions Accounts Sales Purchasing Lower level Processes will be implied by verbs such as Create Sort Compile Change Identifying Processes

  42. Example (CD Hotel) • When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the reservation file • The cash from the deposit is sent to the accounts department (splitting documents) • A letter of confirmation is sent to the customer • …and we also open a guest account in our ledger

  43. aguest booking+deposit confirmation CD HotelBookings weekly statement bbooking dept final statement deposit vouchers weekly statement (copy) dbar/restaurant caccounts dept Example (CD Hotel)

  44. aguest 1 Reception Create Booking Example (CD Hotel) aguest CD Hotel Bookings baccounts dept

  45. These may be obvious: File Catalogue Tray Folder Database ..or implied Check against Put into Read Identifying data Stores

  46. Example (CD Hotel) • When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the reservation file • The cash from the deposit is sent to the accounts department • A letter of confirmation is sent to the customer • …and we also open a guest account in our ledger

  47. aguest 1 Reception Create Booking Example (CD Hotel) aguest CD Hotel Bookings baccounts dept

  48. acustomer 1 Reception Create Booking M1 M2 M3 Vacancy List Reservation file Guest account Example (CD Hotel) aguest CD Hotel Bookings baccounts dept

  49. Identifying data flows • These will be present wherever there is movement of data between: • A data store and a process • An entity and a process • Two processes • But Never: • Between data Stores • From an entity to a data store

  50. Example (CD Hotel) • When a guest sends us a booking, with deposit, • …we change the vacancy list to show one less • …and enter the customer details into the reservation file • The cash from the deposit is sent to the accounts department • A letter of confirmation is sent to the customer • …and we also open a guest account in our ledger

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