Chapter 3.8 Systems Development, Implementation, Management and            Applications3.8 (a)       Techniques for Develo...
Entity                Outside environment (e.g. Customer)                                      e.g.             2      Acc...
is happening now or what is required.   10. Fill in the details.                                          Table 3.8 (a)1  ...
Customer                                              Enquiry                                                             ...
store. Also, there can be no data flows between an external entity and a data store. Theflow of data from, or to, an exter...
It shows which can overlap, and by how much.Finally, it shows how long the project will take. (Your ideas of how a bungalo...
Complex computing projects require effective management or they may get out ofcontrol. Different personnel will be needed ...
User Request                    Initial Study                              Feasibility                                Stud...
3.8    Example Questions1.    Explain what is meant by the term entity model.                              (4)2.    A comp...
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  1. 1. Chapter 3.8 Systems Development, Implementation, Management and Applications3.8 (a) Techniques for Developing Computer SystemsThe figure below shows the stages involved when using Structured Systems Analysis andDesign Method (SSADM). Feasibility Study Requirements Analysis Requirements Specification Logical System Specification Physical DesignData Flow Diagrams DFDs provide a graphic representation of information flowing through a system. Thesystem may be manual, computerized or a mixture of both.The advantages of using DFDs are that it  is a simple technique;  is easy to understand by users, analysts and programmers;  gives an overview of the system;  is a good design aid;  can act as a checking device;  clearly specifies communications in a system;  ensures quality.DFDs use only four symbols. These are shown in Fig. 3.8 (a)2. 6.2 - 1
  2. 2. Entity Outside environment (e.g. Customer) e.g. 2 Accounts Dept Process Produce Invoices Data Flow Order e.g. D1 Data Store e.g. D1 Customer Orders Fig. 3.8 (a)2All names used should be meaningful to the users, whether they are computer literate ornot.The steps to be taken when developing DFDs are given in Table 3.8 (a)1.Step Notes1. Identify dataflows. e.g. documents, VDU screens, phone messages.2. Identify the external entities. e.g. Customer, Supplier3. Identify functional areas. e.g. Departments, individuals.4. Identify data paths. Identify the paths taken by the dataflows identified in step 1.5. Agree the system boundary. What is inside the system and what is not.6. Identify the processes. e.g. Production of invoices, delivery notes, payroll production.7. Identify data stores. Determine which data are to be stored and where.8. Identify interactions. Identify the interaction between the data stores and the processes.9. Validate the DFD. Check that meaningful names have been used. Check that all processes have data flows entering and leaving. Check with the user that the diagram represents what 6.2 - 2
  3. 3. is happening now or what is required. 10. Fill in the details. Table 3.8 (a)1 Fig. 3.8 (a)3 shows the different levels that can be used in DFDs. 0 Payroll System Level 1 (Top 1 Get hours worked 2 Calculate wages 3 Produce wage slips Level)Level 2(Lower 2.1 Validate 2.2 Calculate 2.3 Calculate 2.4 CalculateLevel) Data gross wage deductions net wageLevel 3 (Notalwaysneeded) Fig. 3.8 (a)3 Now consider the following scenario. A hotel reception receives a large number enquiries each day about the availability of accommodation. Most of these are by telephone. It also receives confirmation of bookings. These are entered onto a computer database. While a guest is resident in the hotel, any expenses incurred by the guest are entered into the database by the appropriate personnel. If guests purchase items from the bar or restaurant, they have to sign a bill which is passed to a receptionist who enters the details into the database. When guests leave the hotel they are given an invoice detailing all expenditure. When they pay, the database is updated and a receipt is issued. The flow of data in this system is shown below: 6.2 - 3
  4. 4. Customer Enquiry Drinks Bill Reply Drinks Order Confirmation 1 Reception 1 Bar of booking Process Process bookings and bookings and Final Bill Customers accounts accounts Customer Payment expenditure Receipt Customer Customer Drinks Bill Details Details D2 Customer Accounts D1 Customer Details Food Bill 1 Restaurant Food Order Process bookings and Customer accounts Food BillThe symbol for Customer, an external entity, has a diagonal line to indicate that it occursmore than once. This does not mean that these symbols represent different customers. Itis simply used to make the diagram clearer. Without this, there would be too many flowlines between this symbol and the internal processes.Data stores may also be duplicated. This is done by having a double vertical line on theleft hand side as shown in Fig. 3.8 (a)5. M1 Customer data Fig. 3.8 (a)5Notice this data store is numbered M1 whereas those in Fig. 3.8 (a)4 were numbered D1and D2. In data stores, M indicates a manual store and D indicates a computer based data 6.2 - 4
  5. 5. store. Also, there can be no data flows between an external entity and a data store. Theflow of data from, or to, an external entity must be between the external entity and aprocess.The example of the hotel system only shows one external entity, the customer. Usuallythere is more than one external entity. Suppose we are dealing with a mail ordercompany. Clearly, one external entity is the customer. However, another is the supplier.Note that although there are more than one customer and supplier, in the diagram they arewritten in the singular.When a major project is undertaken, there will be a number of different tasks to becarried out. (If the project consisted of a single task it could hardly be called ‘major’).With a number of tasks, some may be possible at the same time, while with others itbecomes important to do them in the correct order.It may also be important to work out how long the project should take to complete.Major projects like this can be represented graphically to show the different tasks andhow they join together, or relate to each other.Take, as an example, the major project of building a bungalow. It can be divided into anumber of tasks.A Concreting the foundations takes 4 daysB Building the walls takes 4 daysC Making the doors and windows takes 7 daysD Tiling the roof takes 2 daysE Installing the plumbing takes 3 daysF Doing the interior carpentry takes 4 daysG Installing the electrics takes 6 daysH Decorating takes 5 daysOne way of deciding how long the bungalow takes to build is to add up all the separatetimes, 35 days. On the other hand they are separate jobs so as long as enough people areworking, it will only take as long as the longest task, 7 days. This is silly, the decoratingcan’t be done before the roof is on!The real time for the project is somewhere between 7 and 35 days.HGFEDCBA 4 8 12 16 20 24 No of Days Fig. 3.8 (a)6Fig. 3.8 (a)6 shows when all the different tasks can start and when they must end. 6.2 - 5
  6. 6. It shows which can overlap, and by how much.Finally, it shows how long the project will take. (Your ideas of how a bungalow is builtmay be different, but this is the method used by B and L Construction.)Another type of graph might be similar to a flow diagram. The circles represent stagesand the arrows the tasks needed to reach that stage and the time, in days, needed to carrythem out. 4 1 A 2 4 B 2 3 7 4 D 5 6 C G 4 3 F 7 E 5 6 H1 Start2 Foundations finished3 Start making the windows and doors4 Walls finished5 Roof finished6 Interior finished7 Bungalow finishedThere are many routes through the diagram. The one that gives the time taken tocomplete the bungalow is1 2 4 5 6(G) 7A total of 21 days. This is the critical path. The bungalow cannot be built in a shortertime.The arrows show the order in which the tasks must be carried out, so E (the plumbing)cannot be done before B (building the walls), but can be done at the same time as tilingthe roof (D). Each node can have an optimum time worked out. Node 6 has an optimumtime of 4+4+2+6 = 16 days. This is the time to be sure of getting to node 6 whicheverroute you choose. Each node can also have a latest starting time before it holds upanother node. Node 1 must start immediately otherwise the walls (4) won’t be finishedby day 8. However, node 3 could start immediately or wait a day without affecting therest. 6.2 - 6
  7. 7. Complex computing projects require effective management or they may get out ofcontrol. Different personnel will be needed for different tasks. Their time must bebooked in advance so that they are free from working on other tasks or projects whenthey are needed. This means that an accurate prediction of the start times of the varioustasks must be made. It is also necessary to assess how long the different tasks will last sothat other projects can book personnel.There is now specialist project management software available that can take morecomplex projects than the one that we were considering and produce the type of analysisthat we have been talking about. If the software is compatible with the diary softwareused by a firm then it can automatically book the workers when they are needed.3.8 (b) The Purpose of DocumentationIt is important that the design of a solution is accurate BEFORE it is implemented. Forthe design to be accurate, the original specification must be accurate. This means that,when we are asked to produce a solution, we must make sure that we thoroughlyunderstand the problem. This can only be achieved by checking our analysis with thosewho are going to use our system. Users are not usually technical people and so werequire simple ways of showing our understanding of the problem. One of the best waysof doing this is to use diagrams.An E-R diagram shows the relationships between entities so that we can check theserelationships with the end user. These diagrams help us to ask questions like Can a customer only have one product? Can a customer own many products? Can a student borrow many books?Thus, we can ensure that the relationships are correct. We can also ensure that we haveincluded all the entities.Similarly, our Data Flow Diagrams (DFDs) show how data is moving through theorganization and we can ask questions like Are there any other data requirements? Are all data moving between the right departments? Are all the external entities present?This continual validation process is essential if we are to reduce the cost of maintenancedue to errors and omissions. The careful documentation also helps to maintain a piece ofsoftware when it is to be upgraded by adding extra facilities. 6.2 - 7
  8. 8. User Request Initial Study Feasibility Study Systems Analysis Systems Design Implementation Change Over Evaluation and Maintenance3.8 (d) Appropriate Response TimesA classic example is the file of goods that needs to be accessed directly if it is to be usedat the point of sale terminal, and sequentially if the details are to be used for ordering ofreplacements. The solution is to store the file in an indexed sequential format. At thepoint of sale the item required is identified by a laser scanner whereas at the head officethe item name will likely be typed in at a keyboard. If a single item needs to be foundthen a program can be written to find the details directly from the key field by using ahashing algorithm, alternatively the items can be held sequentially in an index and aprogram can be written to find the particular item by using a binary search of the index.Dependent upon the method of search the data needs to be stored differently.3.8 (e) Implementation Techniques  Direct Implementation  Parallel Implementation  Phased Implementation3.8 (f) Managing, Monitoring and Maintenance of Systems  Adaptive Maintenance  Corrective Maintenance  Perfective Maintenance 6.2 - 8
  9. 9. 3.8 Example Questions1. Explain what is meant by the term entity model. (4)2. A computer system has been designed and produced for a person who works from home for a publisher of fiction books as a proof reader of manuscripts. Another system is designed for a graphic designer who sends work to clients electronically. Describe how the hardware specifications for the two systems would differ. (10)3. Describe how the forms of implementation: (i) Parallel running (ii) Phased (iii) Direct are carried out. In each case describe one aspect of an application which would make that form of implementation appropriate in that case.4. Discuss the need for project management when a major project is being implemented. 6.2 - 9