An Introduction to Systems

Transcription

1 An Introduction to Systems Second edition Mark Lejk and David Deeks Addison-Wesley An imprint of Pearson Education Harlow, England London New York Reading, Massachusetts San Francisco Toronto Don Mills, Ontario Sydney Tokyo Singapore Hong Kong Seoul Taipei Cape Town Madrid Mexico City Amsterdam Munich Paris Milan

2 Contents Foreword xv Acknowledgements xvii Introduction 1 Part 1 The systems development environment 3 1 Setting the scene Introduction Techniques, tools, methods, strategies A hierarchy Techniques Tools Methods Strategies Approaches to systems development Explaining the term 'approaches' Approaches covered by this book Approaches not covered by this book More fundamentals Prototyping CASE (computer-aided software engineering) tools Data and information Making complex choices Introducing the weighted matrix A simple weighted matrix example Commercial use of weighted matrices Communication is the key Why communication is so important Communication skills ' Ongoing involvement with users Systems analysis principles 30 This chapter Further reading 30 Web addresses worth a visit 31

3 vi CONTENTS 2 Managing a systems development project The role of the systems analyst Background Personal qualities Planning, control, progress Working to a deadline Balancing thoroughness and time Methods for project control The need for clear objectives Establishing what you are being asked to achieve The theory of 'Management By Objectives' (MBO) The role of objectives The management of objectives Tasks and roles Getting organised The difference between a group and a team How to turn a group into a team Controlling it all Introducing project scheduling How to go about it Scheduling project teams 45 This chapter A useful exercise 45 Further reading 46 Part 2 Structured systems analysis techniques 47 3 Spray and tree diagrams Why spray and tree diagrams? Spray diagrams (the 'controlled brainstorm') Tree diagrams Drawing one More tree diagram applications 53 This chapter Useful exercises 55 Further reading 55 Web address worth a visit 55 4 Introducing data flow diagrams (DFDs) Physical and logical views An innate ability Applying relevant information Document flow diagrams A predecessor of DFDs Document flows and agencies 58

4 CONTENTS vii Using a document flow to improve a system Introducing data flow diagrams (DFDs) A more powerful technique DFDs and the two views DFDs and the four stages Some first DFD examples What you are doing at the moment Three ways of communicating One logical happening DFD notation Many conventions, four common elements Not to be confused with flowcharts The four elements explained Some other points to note DFD levelling A worked example - the stock control system Where to begin The document flow diagram The context diagram The DFDs Two more examples of DFDs The hairdressing salon The integrated accounting system 76 This chapter Useful exercises 78 Further reading 78 Web addresses worth a visit 78 5 Creating DFDs From theory to practice Unreal and artificial Context diagrams in context Functional areas and processes Example 1 - the boat building company The scenario Initial attempt at current level 1 physical DFD Second version of DFD - 'one process per functional area' Level 2 DFDs Example 2 - the student assessment system The scenario Initial discussion Initial attempt at current physical DFDs Further decomposition? When is a level 2 a level 1, etc.? 92

5 viii CONTENTS 5.6 A reminder about 'time' 94 This chapter A useful exercise 94 Further reading 95 Web addresses worth a visit 95 6 Specifying processes Introduction Reaching the bottom level Precise, accurate, unambiguous and complete The problem with ordinary English Structured English What it is Sequence Selection Iteration Variations on structured English Decision tables What they are Limited entry decision tables Extended entry decision tables Mixed entry decision tables Decision trees A worked example - the end-of-year assessment system Background Representing the narrative description 107 This chapter... Ill A useful exercise 111 Further reading 111 Web address worth a visit Entity modelling Processes and data Entities and attributes Entities Attributes Selecting entities Entity relationships Identifying relationships Degrees of relationship Optionality Entities and entity occurrences Further examples Many-to-many relationships Multiple relationships 124

6 CONTENTS ix Exclusive relationships Supertypes and subtypes Building an entity-model The example Selecting entities Identifying relationships between entities Drawing the relationships Completing the entity model Further examples 128 This chapter Useful exercises 131 Further reading 132 Web addresses worth a visit Normalisation Introduction What is a 'relationship'? The entity model Computer system relationships Normalisation of documents Introduction The student transcript The module class list Combining the student transcript and class list Normalisation and relational data analysis Normalisation of tables of data Composite keys Like a compound key - but not quite The tutor group class list Normalising the tutor group class list Combining the 3NF entity models Comparing the 3NF entity model with the logical entity model Aiming for the definitive entity model The entity model before normalisation Combining the two views 157 This chapter A useful exercise 158 Further reading 158 Web addresses worth a visit Logicalisation of DFDs Introduction Returning to DFDs Why logicalise? Business Process Re-engineering (BPR) 160

7 x CONTENTS 9.2 Logicalisation step by step Guidelines Example 1 - the student assessment system Example 2 - Marine Construction Conclusion 174 This chapter A useful exercise 175 Further reading 175 Web addresses worth a visit Developing the required system Introduction Learning techniques Putting techniques into practice The existing student assessment system The scenario Current physical DFDs Current entity model Current logical DFD The requirements of the new student assessment system The required system The required entity model The required DFD What is missing? No maintenance No alternatives 187 This chapter A useful exercise 188 Further reading 188 Web address worth a visit The effect of time Introduction Time and the data model The possible effects The vehicle breakdown and rescue system Time and the DFD Events What is an event? Finding events in DFDs Types of event Entity life histories (ELHs) What an ELH is Drawing an ELH The entity/event matrix

8 CONTENTS xi Creating an ELH by further analysis of the entity/event matrix The process/event matrix State indicators and the control of processes 11.6 State transition diagrams 11.7 Conclusion This chapter... A useful exercise Further reading Web address worth a visit Part 3 Process Improvement for Strategic Objectives (PISO) Introducing PISO A new development Origins Like BPR, but Views, objectives and stakeholders A new rationale for DFD logicalisation 'Before and after' PISO examples The effect of the strategic objective The Pontefract General Infirmary example The Marine Construction example The PISO stages and steps The PISO steps that use DFDs PISO and 'green field' systems development 225 This chapter A useful exercise 225 Further reading 226 Web address worth a visit PISO and DFDs Reminders The PISO steps that use DFDs People doing it for themselves The Marine Construction PISO analysis Marine Construction step Preparing current system physical DFDs Marine Construction step Logicalising for systems efficiency < Marine Construction step Logicalising for strategic objectives Marine Construction step Physical DFD of recommended system Marine Construction - what has been achieved? 239

9 xii CONTENTS 13.3 Rontefract General Infirmary PISO analysis Pontefract step Preparing current system physical DFDs Pontefract step Logicalising for systems efficiency Pontefract step Logicalising for strategic objectives Pontefract step Physical DFDs of recommended system Pontefract - what has been achieved? 247 This chapter A useful exercise 248 Further reading 248 Web address worth a visit 248 Part 4 Object-oriented analysis techniques Introducing objects Background Structured analysis Software re-use Software maintenance and testing Dealing with today's complex systems 14.2 Objects and object classes 14.3 Objects, entities and class diagrams The 'object' concept Association Inheritance Aggregation and composition 14.4 Operations and class diagrams This chapter... A useful exercise Further reading Web address worth a visit Modelling object behaviour 15.1 Use cases 15.2 Scenarios 15.3 CRC cards 15.4 Sequence diagrams 15.5 Collaboration diagrams 15.6 State diagrams This chapter... A useful exercise Further reading Web address worth a visit

10 CONTENTS xiii Part 5 And finally Drawing it all together Some principles of systems analysis Maintenance and use - information systems and motor cars Maintaining Using Which approach to use? New and evolving Object-orientation and persistent data Structured analysis... and design Structured and object-oriented analysis Soft Systems A final finally 285 A useful exercise 285 Further reading 285 Web addresses worth a visit 285 Bibliography 286 Index 288