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Data Flow Diagrams. Introduction. SSADM What is a Data Flow Diagram? Why do we use DFDs? Levelling Conventions Decomposition and Abstraction The Elements Process and Data Stores Outside Entity Data Flow The Levels Rules The Procedure for Constructing DFD’s The Document Flow Diagram
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Introduction SSADM What is a Data Flow Diagram? Why do we use DFDs? Levelling Conventions Decomposition and Abstraction The Elements Process and Data Stores Outside Entity DataFlow The Levels Rules The Procedure for Constructing DFD’s The Document Flow Diagram The Context Diagram Draw the external entities and data stores Level 1 Physical DFD - Complete
S.S.A.D.M. • S.S.A.D.M. - Structured Systems Analysis and Design Method • Uses different techniques to model a system • Data Flow Diagrams • Entity Relational Model (Logical Data Stores) • Normalisation
What is a Data Flow Diagram? • Known as DFDs • A way to model a real world situation • They are the interface between the real world activities and an understanding of how this can be converted into a computer system.
Why do we use DFDs? • It is a way of taking the physical view and converting it into a logical view. • The physical view - all documents involved • The logical view - the data they contain • Their main purpose is to communicate with the user, the analyst’s understanding of the scope of the required system
Levelling • DFDs are expanded or decomposed into levels. • Separating each process into sub processes • Uncovers more and more detail
Conventions Balancing Process at lower level should have identical data flows if they flow out of a process Modelling Data Stores Only use DATA STORES used within this process on the diagram Numbering 1 - 1.1 - 1.1.1 1.2 - 1.2.1 Labels Should carry as much meaning as possible
Decomposition and Abstraction • Decomposition - Divide and subdivide into manageable size problems • Abstraction - Concentrate on the important issues and ignore the irrelevant
The Elements The four main elements of DFDs notation • Data Flows, with a label to indicate what data is flowing • Processes, that handle the data • Data stores, within the system (diary, filing cabinet or computer file) • Outside entities, outside sources of data
Process and Data Stores A process is made up of Data Stores Destination (Place or Name) Process Number Process description Should be descriptive, starting with a verb. Can be M for manual or D for computer base data stores. Name of Store M1
Outside Entity Is anything outside the system that is of interest to the system. Can be a person, a company or another system. Customer a Outside entity shows the Name and a lowercase alpha character is used to uniquely identify it. Customer a If an outside entity is repeated for the purpose of neat layout a line is added across the top.
DataFlow Is shown by a line with an arrowhead, indicating the direction of the flow of data. Each data flow should be named to indicate what data is being passed. Nouns or adjectives only no verbs are permitted.
The Levels • Context - Overview - contains only one process • Level 1 - Utilises all four elements • Level 2 - A breakdown of a level 1 process • Level 3 - A breakdown of a level 2 process • There is no rule as to how many levels of DFD that can be used.
Rules Sequence not important - getting the Process correct is • Context or Level 0 - Identifies the system/ boundary/External Links • Level 1 - Overview of function • Level 2 - Breakdown to Understand Hard to know where to stop Rule of Thumb If there are more than 8 data flows break it Process of Identifying major Processes
Process 3 Level 2 3 Hair/Reception Client a Clie a 3.1 Hairdresser Appointment Details Conduct Appointment Hair Details M2 Diary 3.2 Hairdresser Inform Reception Change of Hair Details 3.3 Receptionist Complete Appointment M3 Client Card
Summary SSADM What a DFD is & Why we use them The different conventions What the elements are Example Next Week:- Entity Relational Model
