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York University
Information Technology
ITEC 3010

Systems Analysis and Design I ITEC3010 – Fall 2010 – Luiz Cysneiros Lecture 6 – Data Flow Diagrams – Oct 26 Chapter 6 – Data Flow Diagrams (DFD) - Data Flow Diagram (DFD) o A graphical system model that shows all of the main requirements for an information system: inputs, outputs, processes and data storage o Everyone working on the project (and end users) can see all the aspects of the project in the diagram with minimal training (simple – only 5 symbols) - The square is an external agent o A person or organization, outside the boundary of a system that provides data inputs or accepts data outputs - The rectangle with rounded edges is a process o A symbol that represents an algorithm or procedure by which data inputs are transformed into data outputs - The lines are data flows o Represents movement of data - The flat three-sided rectangle are data stores (a file or part of a database) o A place where data is held Data Flow Diagrams and Levels of Abstraction - Levels of abstraction o Particular to any modeling technique that breaks the system into a hierarchical set of increasingly more detailed models o Example above – a DFD fragment – showing one process in response to one event o Other diagrams show the processing at a higher level (more general) or lower level (a more detailed view of one process) o Higher level processes in a DFD can be decomposed into separate lower level DFD (or some other diagram) Context Diagrams - Context Diagram: A DFD that summarizes all processing activity within the system in single process symbol o Describes highest level view of a system o All external agents and all data flows into and out of a system are shown in the diagram o The whole system is represented as one process - Example – fig. 6-5 shows a context diagram for a university course registration system that interacts with 3 agents: academic dept., student, and faculty member Notes on Context Diagram - Useful for showing system boundaries - External agents are outside the software scope (which is represented by the single process). But not from System Analysis - Data stores are not shown in the context diagram since they are considered to be in the software scope (i.e. the single process) - It is the highest level DFD - Context diagram does not show any details of what takes place within the system (i.e. that single process) DFD Fragments - DFD fragment: A DFD that represents the system response to one event within a single process symbol o A fragment is created for each event in the event list – it is a self- contained model showing how the system responds to a single event o Created one at a time o Fig. 6-7 shows 3 DFD fragments for a course registration system o Each fragment represents all processing for an event within a single process symbols o The data stores in the DFD fragment represent entities in the ERD (Entity Relationship Diagram – see last lecture) – Not Necessarily! Event-Partitioned System Model - Event-partitioned system model: a DFD that models requirements using a single process for each event o The entire set of DFD fragments can be combined into this single DFD called the event-partitioned system model or diagram 0 o Diagram 0 shows the entire system on a single DFD - Figure 6-10 (next slide) shows a set of four related DFDs o The top diagram shows the Context diagram for course registration (same as fig. 6-5 above) o The diagram below that (Diagram 0) is the decomposition of the one process in the context diagram AND consists of the a combined version of the 3 DFD fragments in fig. 6-7 above (in fig. 6-10, DFD fragment 1 is shown below diagram 0) o Finally, Diagram 1 is a decomposition of the process in DFD fragment 1 Dividing the System into Subsystems - The RMO customer support system involves 20 events, therefore the event- partitioned system model (diagram 0) would contain 20 processes o This can get to be a crowded diagram! o Solution is to divide the system into subsystems o Events are grouped into related subsystems based on similar:  Interactions with external agents  Interactions with database stores  Required processing - In the RMO example, we can break up the support system into 4 subsystems - Next Step: After the subsystems are defined: o A DFD is created to represent the division of the system into subsystems – the subsystem DFD o This subsystem DFD shows how the four RMO subsystems are connected (i.e. how they are related to all the outside sources and destinations of data)  Note that there is a process in the diagram for each of the four subsystems that were defined for RMO o See figure 6-13 for an example based on the 4 subsystems in the RMO example o Note - even with only 4 subsystems (rather than one process for each of the 20 events) the diagram gets cluttered - Next Step: can decompose a subsystem DFD into event-partitioned models - one for each subsystem: o In the RMO example can expand the subsystem in fig. 6-13 called “Order entry subsystem” into an event-partitioned model  This model has 5 processes within it – see next slide o The analyst would also create an event-partitioned model for the other 3 subsystems in the RMO example Summary – Relationship of All These Diagrams - Figure 6-12 shows the relationship among DFD abstraction levels when subsystems are defined - The figure starts off with the context diagram, which breaks down to the subsystem diagram (one for each subsystem) - The subsystem diagram is turn broken down into the event-partitioned subsystem diagrams - ETC. Decomposing Process to see Details of One Activity - Using this same principle of breaking down the model to more detailed level, we can take a DFD fragment (e.g. create new order fragment from the RMO example) and decompose it into sub-processes - In figure 6-15 this is shown o Since fragment “create new order” was the second DFD fragment defined for the RMO example (see fig. 6-8) we will label processes inside of it as processes 2.1, 2.2, 2.3 etc. o The diagram decomposes “create new order” into 4 sub-processes (see fig. 6-15) – labeled sub-processes 2.1-2.4 Evaluating DFD Quality - A quality set of DFDs is o Readable o Internally consistent o Accurately represents system requirements - Minimizing complexity o Want to avoid information overload  Occurs when too much informa
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