Lecture 9 – PROCESS MODELLING PART 1

1. Lecture 9 – PROCESS MODELLING PART 1 Maria Petridou Data Flow Diagrams for Process Modelling Multi-level Data Flow Diagrams Logical Vs Physical DFDs Steps to Construct Data Flow Diagrams

2. Data Flow Diagrams for Process Modelling • Once the system requirements have been defined and refined: Process modelling and Data modelling • Process models can be of two types: • Logical process models – describe processes with no details about specific implementation. • Physical process models – produced in the design phase, provide further information necessary to build the system. Maria Petridou

3. Data Flow Diagrams for Process Modelling • Data Flow Diagram (DFD) • Structured analysis technique for constructing a graphical representation of processes. • One of the main methods available for analyzing data-oriented systems. • Simple diagram to model processes and represent flow of information. • DFDs emphasize the logic underlying the system. • Contains four types of symbols: process, data flow, data store, external entity. Maria Petridou

4. Data Flow Diagrams for Process Modelling • Advantages of DFDs: • Freedom from committing to the technical implementation too early. • Understanding of the interrelationships of systems and subsystems • Communicating current system knowledge to users. • Analysis of the proposed system. Maria Petridou

5. Maria Petridou

6. Data Flow Diagrams for Process Modelling Four Basic Symbols: • Process: • Manual or computerised activity or function that is performed for some specific business reason (e.g. A whole system, a subsystem and an activity). • Represented as a rectangle with rounded corners. • Always denotes changes in data. • Names should be in the form verb-adjective-noun • Complex processes may require the use of more formal process specification techniques such as structured English (pseudo-code), decision tables or decision trees. Maria Petridou

7. Data Flow Diagrams for Process Modelling • Data Flow: • Shows the data about a person, place, or thing that moves through the system. • Represented as an arrow labelled with the data name (a noun). • Data flows hold processes together and one end of the data flow will always come from or go to a process. • Direction of the arrow indicates destination of data. Maria Petridou

8. Data Flow Diagrams for Process Modelling • Data Store: • Collection of data stored in some way. Denotes long-term storage. • Represented as a rectangle with an open right side. • Has a descriptive name (noun) and a unique identification number such as D1, D2, D3. • Data stores are the starting point for the data model and are the link between the process model and the data model. • Must have at least one input data flow and at least one output data flow. Maria Petridou

9. Data Flow Diagrams for Process Modelling • External Entity: • Person or organisation that is external to the system but interacts with it. Also known as source or destination of data. • Represented as an square labelled with the entity name (a noun). • People that are part of the system are not considered as external entities because if they execute a process, then they are part of the process (not external to the system). • The same entity symbol can be used several times to avoid complex diagrams. Maria Petridou

10. Multi-level Data Flow Diagrams • Most business processes cannot be shown on a single DFD. Then, a hierarchy of data flow diagrams is required. • Lower level diagrams (child diagrams) show a portion of an upper level diagram (parent diagram) in more detail. • Balancing means that information presented at one level of a DFD is accurately represented in the next level DFD. Maria Petridou

12. Multi-level Data Flow Diagrams • Important Issues in Multi-level DFDs: • Context diagram shows the overall business process as a single process and shows data flows to and from external entities. • There is only one level 0 DFD and it shows the major high-level processes (typically up to 9 processes) and the data stores. • There is one level 1 DFD for each process in the level 0 DFD showing more details on how the high-level process operates. • If a given process at level n is decomposed into x processes at level n+1, then the x child processes should make up the parent process. • Correct numbering at the different levels helps to understand the structure of the business process. Maria Petridou

13. Logical Vs Physical DFDs • Logical DFDs • Shows how the business operates • Processes represent business activities • Data stores represent collections of data • Not important how the data is stored • Permanent collections • Controls are rules of the business Maria Petridou

14. Logical Vs Physical DFDs • Physical DFDs • Shows how the system will be implemented • Processes represent programs/functions • Data stores represent physical files and databases • Processes operating at different times must be connected via a data store • Controls are validation of user input, file formats and security measures Maria Petridou

15. Current Logical DFD New Logical DFD New Physical DFD Logical Vs Physical DFDs • Analyse the current system • Add features for the new system • Develop best methods for implementing the new system Maria Petridou

16. Logical Vs Physical DFDs • Logical DFDs – Advantages • Better communication with system users • Better stability for the system • Better business understanding for analysts • Better flexibility and maintenance • Physical DFDs – Advantages • Easier to categorise processes as manual or automatic • Better description of processes • Better for ordering processes into a sequence • Better for imposing controls Maria Petridou

17. Steps to Construct Data Flow Diagrams • Build the context diagram, including all external entities and the major data flow to or from them. • Create Diagram Level 0 by analyzing the major activities within the context process - Include the external entities and major data stores. • Decomposeto a child diagram (Level 1 DFD) for each complex process on Diagram 0. • Decompose level 1 processes into level 2 DFDs and decompose further if needed. • Balance and validate DFDs to ensure completeness and correctness. Maria Petridou

18. Steps to Construct Data Flow Diagrams • Context Level Data Flow Diagram • Contains only one process, representing the entire system • The process is given the number zero • All external entities are shown on the context diagram as well as major data flow to and from them. • The diagram does not contain any data stores Maria Petridou

19. Steps to Construct Data Flow Diagrams • Diagram Level 0 • Diagram Level 0 is the explosion of the context level diagram. • Should include up to 7 or 9 processes - Any more will result in a messy diagram. • Processes are numbered with an integer. • The major data stores and all external entities are included on Diagram 0. Maria Petridou

20. Steps to Construct Data Flow Diagrams • Child Diagrams • Each process on diagram Level 0 may be exploded (decomposed) to create a child diagram. • Each process on a lower-level diagram may be exploded to create another child diagram. • Each process is numbered with the parent diagram number, a period, and a unique child diagram number • 3.2 on Diagram 3, the child of process 3 • On Diagram 3, the processes would be numbered 3.1, 3.2, 3.3 and so on. • External entities are usually not shown on the child diagrams below Diagram 0. Reading: (Kendall&Kendall, chapter 7), (Dennis &Wixom, chapter 6). Maria Petridou

21. Steps to Construct Data Flow Diagrams Maria Petridou