Recap on last week

1. Recap on last week • Last week we learnt about: • Data Security • Encryption • Legislation • Hackers • IT Policies

2. Recap exercise • In your groups – arrange the words to form sentences used in last week’s lesson. • Then you will need to expand and explain the meaning of the sentences. • You have 5 minutes to complete this exercise.

3. Unit 2 Data Structures and Data Flow Diagrams

4. Data Flow Diagrams Objectives:-By the end of the session • All students will be able to • List the symbols used on a Data Flow Diagram • Recognise the terminology used • Most students should be able to • Describe how DFDs can be used at different levels • Describe the advantages of using DFDs • Some students may be able to • Apply the technique to a case study

5. Data Flow Diagrams • Pictorial representations of business activities are more effective than narrative ones. ‘A picture is worth a thousand words’ • There are conventions, as there are with other diagramming techniques such as flowcharts, but everything in a DFD has an exact equivalent in the activity being described. It is much easier for the user, the systems analyst and the designer to relate to • The technique of decomposition is used and this structured approach allows simple definitions at a high-level, and levelled definitions at lower levels as necessary

6. Case Study – Tim Horton’s • Read through the pre-release case study from June 2009. • Highlight areas you think identify data that might flow around the organisation • You have 10 minutes to complete this exercise.

7. Definition of a Data Flow Diagram • A DFD uses four main symbols: • A rectangle. This is called a process. • A line is a data pipeline. Data flows along the line between two processes, or between an external body and a process. • A file symbol means there is a temporary or permanent storage of data, referred to as a data store. This may be a 'receiver' of data for later use or for use by some other activity or it may be a reference source from which standard data can be retrieved • A terminator or external entity on the diagram indicates that data arises from outside the system, i.e.source, or it leaves the system, i.e. a sink.

8. Place/person 1 Place where the process takes place or person who does it Unique reference Process description Must contain a verb Definition of a Data Flow Diagram • In a DFD the rectangle implies that some activity takes place, transforming the input data in some way to produce the output data • Any decisions which are taken are contained within a process. There is no separate decision symbol

9. Definition of a Data Flow Diagram • There must be no increase or decrease in the data as a result of the process • The purpose of a DFD is to show what happens, not when, nor under what external influences, nor why it happens 6 Doctor’s receptionist Record appointment details

10. Definition of a Data Flow Diagram • The Dataflow patient’s name appointment date and time of appointment

11. M1 Patient card index Manual store D1 Patient file Computer (Digital) store Definition of a Data Flow Diagram • The Data Store Symbol • When a data store is indicated on a DFD, the flow to or from the data store can be of two types: • When the data store is used only for reference, then the flow is in one direction from the data store to process • When the data store is updated, two data flows are possible but it is normal to show only the net flow either to or from the data store

12. patient Definition of a Data Flow Diagram • The Sources and Sinks • The fundamental purpose of the external entity symbol used as a source or sink is to indicate that, whatever happens at the end of the data flow, it is not the concern of the activities being studied or designed in the DFD

13. Reception 1 System Boundary Date & time Make appointment Doctor Name Patient M1 Patient records Patient record Appointment details 2 Reception Appointments D1 Check in patient Appointment details Name Definition of a Data Flow Diagram • The way a DFD is bounded is shown by the data flowing into the area from the source(s) and the data flowing out to the sink(s).

14. Case Study – Tim Horton’s • Look back through the case study and the answers we gave earlier • Match up the answers we gave and decide if they’re a process, source, sink or data store. • You have 5 minutes to complete this exercise.

15. Name Patient Appointments system Patient record Doctor Patient Date & time Preparing a DFD for an Existing System • The 'Context Diagram' • There is a level of DFD called the 'Context Diagram', also called ‘Level 0 DFD’ • Drawn as a single process, with all the inputs from the sources and outputs to sinks identified

16. Preparing a DFD for an Existing System • Level 1 Physical DFD • The next level of DFD is called level 1 or the high-level DFD. It is used to represent the physical organisation of the current system • From answers obtained during interviews, from observation, and from any pre-knowledge it is possible to sketch out a level 1 DFD, though it is unlikely to be right first time

17. Preparing a DFD for an Existing System • Guidelines for Building a Level 1 DFD • Identify the Vital Input and Output Data Flows • These could be, for example, documents, telephone/email messages, electronic storage media, screens/monitors, data links, • Identify Sources and Sinks • Decide where the inputs and outputs identified come from or go to, such as departments, individuals, existing computer systems

18. Case Study – Tim Horton’s • Read through question paper from June 2009. • Compare the answers you gave earlier with the questions on DFD’s • You have 10 minutes to complete this exercise.

19. Preparing a DFD for an Existing System • Agree the System Boundary with the User • Determine the path of each data flow from its source to the first functional area which makes use of it, and through subsequent functional areas until it reaches its final sink. This makes it easier to clearly identify and agree with the user which functional areas are within the scope of further investigation. • Identify Processes and Data Stores • For each of the input and output data flows already identified, look for a receiving or generating process, together with its associated data stores. It is helpful to ask what process is triggered by the input data flows and what other processes are triggered by time alone.

20. Preparing a DFD for an Existing System • To identify data stores, look for: • standing files of reference data, for example customer names and addresses; • historical information; • transactions accumulated and then passed from one process to another; • files of information extracted from larger files for reporting;

21. Preparing a DFD for an Existing System • Checking the Level 1 DFD for Completeness • When the first draft of the level 1 DFD has been drawn, some checks for completeness can be made: • the data stores should be checked to ensure they have flows both entering and leaving • each flow should be followed to ensure its complete life is shown

22. T(M)1 In-tray Temporary Manual store Email inbox T1 TemporaryComputer store Labelling Data Flow Diagrams • Temporary data stores are data stores where data is read once and then removed or deleted. They are denoted by the letter ‘T’ in their reference:

23. Preparing a DFD for an Existing System • Naming Processes and Data Flows • On high-level DFDs, many different functions are grouped together in order to give the broad picture. Often processes on the top level diagrams are whole departments or IT sub-systems • At the lower levels, however, each process should correspond to a definite identifiable business operation, and each data flow to an existing document or file contents. e.g. an invoice

24. 1stlevel DFD 2ndlevel DFD Decomposes into 3rdlevel DFD Is supported by Mini-spec or EPD Preparing a DFD for an Existing System • Lower Level DFDs • The second level DFD illustrated has the extra data store shown. This file was omitted from the higher level

25. 1.1 1.1.1 Decomposes into 1.2 1.1.2 1 Labelling Data Flow Diagrams

26. Top level DFD Further detail Decomposes into Lowest level DFD Is supported by Mini-spec or EPD Definition of a Data Flow Diagram • Levels • Diagram below shows top level DFD decomposed into lower level DFDs

27. Definition of a Data Flow Diagram • How Many Levels? • There are no rules about the number of levels of DFD which can be used. In some cases, a single DFD will show all that is needed, supported by one mini-spec per process • In other cases, two, three or more levels of DFD may be necessary. Mini-specs should be mini, i.e. small, ideally about half a page

28. Doctor Patient record Patient record Appointments D1 Patient Patient records M1 Appointment details Name Reception 2 Check in patient Labelling Data Flow Diagrams

29. 2 Check in patient Doctor Name Reception 2.1 Check patient’s name against appointments Appointment details Patient Name Appointments D1 Patient record Reception 2.2 Patient record Send patient’s record to doctor M1 Patient records Labelling Data Flow Diagrams • Process 2 could be shown at a lower level:

30. Data Flow Diagrams • Summary • DFDs are flexible • can be discussed with, and understood by, users • can be modified and re-drawn as often as necessary • they also facilitate the process of moving from an existing system to a new required logical system