Btec National - IT SYSTEMS ANALYSIS AND DESIGN 1 IT Systems Analysis and Design SSADM – Design

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 2 Aims Learn about Data Flow Diagrams (DFDs) and what they are used for. Be able to create context DFD and Level one DFD Gain an insight into further levels of detail (DFD)

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 3 Data Flow Diagrams DFD –Data flows are used to show how a system will/does operate. –They show how the data flows around the system.

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 4 Data Flow Diagrams DFD –Data flow diagrams show the system boundary, the external entities, the processes and the data stores.

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 5 Data Flow Diagrams External entities – things outside the system, such as people, organisations and other systems Customer

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 6 Data Flow Diagrams Process, should always start with an active verb. The number in the corner is used to identify the parent process and to uniquely identify this process. Calculate Benefit 1.3 Unique ID Process description

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 7 Data Flow Diagrams Data Store –This is used to store and retrieve data from, these are not shown at level 0 (context diagram) Unique ID Data store name Customer Details D8

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 8 Data Flow Diagrams As in Yourdon diagrams, a flow is shown by a line with a flow name and an arrow at the destination end, e.g. The data flow name must be a noun (i.e. a thing not an activity) Data flows may only go:  between two processes;  between an entity and a process;  between a data store and a process. Data flow lines don’t merge or split (but more than one line can go to/from the same entity, process or data store) Data flows must not cross (use duplicate entities and/or data stores to avoid crossing lines) Customer address

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 9 Data Flow Diagrams Context Diagram –A context diagram is the highest level DFD (general overview), it shows the data flows, external entities, the process (system in focus, don’t be tempted to divide the process into too many steps or the diagram will become too complicated)

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 10 Data Flow Diagrams Context Diagram (high level) – Example (Notice no data stores) Booking confirmation Booking details Customer 1 Create Booking

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 11 Data Flow Diagrams Moving from high-level to low-level – –The high level DFD needs to be broken down into more detail (decomposed) to produce a low-level DFD. – –The first step is to divide the identified process in the context DFD into several, more detailed processes. – –This is generally performed by using the information gathered so far to determine what the divided processes should be. – –Some analysts say as a rule of thumb there should be a single process dealing with each data flow attached to an external entity.

Btec National - IT SYSTEMS ANALYSIS AND DESIGN Check Availability 1.2 Record new booking 1.3 Confirm booking Customer D1 Room Booking Details D2 Customer Details Booking details Customer Customer details Room booking details Customer details Booking details Booking confirmation Availability details Notice the system boundary

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 13 Data Flow Diagrams Moving from Level 1 to Level 2 – –A level 1 may need breaking down again into further processes i.e. if the confirm booking process (1.3) was complicated then that process could be broken down further to detail the complexities. The process IDs would start with 1.3.1, etc. – –This would continue until the analyst is satisfied that he/she has a clear understanding.

Btec National - IT SYSTEMS ANALYSIS AND DESIGN 14 Conclusion Learned about Data Flow Diagrams (DFDs) and what they are used for. We created context DFD and Level one DFD Gained an insight into further levels of detail (DFD)