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System Modelling (1) Specification of individual requirements allows the construction of various kinds of system models. Diagrams are used to represent.

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Presentation on theme: "System Modelling (1) Specification of individual requirements allows the construction of various kinds of system models. Diagrams are used to represent."— Presentation transcript:

1 System Modelling (1) Specification of individual requirements allows the construction of various kinds of system models. Diagrams are used to represent our picture of a system. Different approaches can focus on:-  processes and the data effected by processes  data entities and the relationships between them  states a system can exhibit/show and the events that shift that system from one state to another  the sequence of events in all or part of a system (a “scenario”) Dataflow analysis Entity-relationship (E-R) analysis State machine diagrams Sequence diagrams + other diagrammatic representations used in UML

2 System Modelling (2) Dataflow analysis 1 A dataflow diagram (DFD) represents all or part of a system in an abstract (non-physical) way using just 4 symbols Dataflow – data in motion. A “pipe” – unidrectional flow. The dataflow arrow does NOT mean “Go To” External Entity - external source or destination of system data Datastore – data of the same type (an entity) Stationary data or A Process – an actvity that effects data or

3 System Modelling (3) Dataflow analysis 2 Dataflow Dataflow contents e.g. amount_owing External entity Name of EE e.g. Supplier Process Process ID, e.g P1 or P2.1 Process name, e.g Verify_Pin Datastore Datastore ID, e.g D2 Datastore name e.g.Accounts

4 Dataflow analysis allows a system to be progressively “discovered” by “levelling” System Modelling (4) Dataflow analysis 3 warehouse Reorder System Purchasing System A context diagram puts the system in context in relation to related EE’s High-level context diagram

5 System Modelling (5) Dataflow analysis 4 warehouse Process_ stock_ transaction Purchasing System inventory P1 Produce_ ReOrder_ List P2 ReOrders D1 D2 ReOrder_detail Transaction_details ReOrder_detail exception_report Level 0 DFD

6 System Modelling (6) Dataflow analysis 5 warehouse Purchasing System Produce_ ReOrder_ List P2 inventoryD1 ReOrdersD2 ReOrder_detail trans_details exception_ report Level 1 DFD of P1 Get_trans P1.1 P1.2 Validate_trans trans_details transactionsD1 trans_details valid_trans stock_id, trans_qty valid_flag, trans_qty warehouse reject_ note P1.3 P1.4 Update_ inventory stock_id, trans_qty Create_reorder valid_trans ReOrder_detail Taking P1 to Level 1 has “exploded “ 1 process into 4 smaller ones

7 System Modelling (7) Dataflow analysis 6 Levelling should continue until an individual process need be decomposed no further – it is in a primitive form with probably no more than one dataflow output and whose logic could be described in a “short” piece of PDL Note that if the top-level DFD had 8 processes and each of these processes was taken through 2 levels of decomposition, each yielding another 8 sub-processes, then our system would end-up with 512 primitive Processes (8 x 8 x 8) There is no rule about how many levels are required but 2 to 3 levels is usually adequate and not all processes need to be taken down to the 3 rd level. (from Hawryszkiewycz, Systems Analysis & Design)

8 System Modelling (8) Entity-Relationship Modelling 1 Manager Project manages [n] 1 Entity set Relationship set Proj_Code Proj_Name Start_Date Tot_Budget Man_StartDate Proj_Locn Fin_Date... RK1 RK2 RK3 RK4 ABC XYZ IJK DEF Loc4 Loc9 Loc5 Loc /10/10 30/4/11 7/8/10 1/1/115/7/11 9/3/11 8/9/11 8/9/10 Proj_Code Man_No Man_Budget... RK RK2 RK3 RK /1/11 7/3/11 9/5/11 5/1/11 Man_No Man_Name Dept_ID Merkel,A Obama, B Fin Health... Attributes Separate instances (occurrences) of the entity Separate instances (occurrences) of the relationship Note that a relationship can have its own attributes

9 System Modelling (9) Entity-Relationship Modelling 2 Mapping a complete system using the E-R approach (from Hawryszkiewycz, Systems Analysis & Design)

10 System Modelling (10) State Machine Models 1  A state machine diagram shows how a system responds to internal or external events.  It does not show the flow of data within the system  Often used for modelling real-time systems because these systems are often driven by stimuli from the system’s environment Nodes represent system states Arcs represent events that shift the system from one state to another

11 System Modelling (11) State Machine Models 2 A state machine diagram for a microwave oven

12 System Modelling (12) Sequence Diagrams 1

13 System Modelling (13) Sequence Diagrams 2 Several different types of diagrams are available within UML for representing different aspects of OO. Sequence diagrams are dynamic models that show the sequence of object interactions that take place. A sequence diagram showing operations involved in collecting data from a weather station The objects are: :CommsController :WeatherStation :WeatherData Time is represented vertically Thin rectangle represents time when object is the controlling object in system summarise (report)

14 System Modelling (14) Sequence Diagrams 3 1 Object that is an instance of CommsController receives a remote request to send a weather report. It acknowledges receiving this request. 2. This object sends a message to an object that is an instance of WeatherStation to create a weather report. The instance of CommsController suspends itself. 3 WeatherStation sends a message to WeatherData to summarise the weather data 4 Summary is computed and control returns to the WeatherStation object 5. The WeatherStation object sends a message to CommsController requesting it to transfer the data to the remote system. The WeatherStation object suspends itself 6. The CommsController object sends the report to the remote station, receives an acknowledgement, and then suspends itself waiting for the next request.


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