TheRAGE
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Hey there, im new to these here parts, Im currently doing a foundation degree in Computer Science and the first module/part of the first module is to create a requirements specification document for tesco.com, i havn't really done much yet as i keep getting confused over diagrams so i was wondering if someone could explain the following (using examples) use case diagram, data flow diagram, context diagram. I know its a lot to ask for but its my only hope.

thanks in advance-

-TheRAGE-
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iisjman07
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A DFD looks something like this: http://databaseservice.co.uk/dataflow_diagram.gif. It maps entities (people, basically), processes (things that perform a process that manipulates data), and data stores (anything from a filing cabinet to a SQL database). As well as these you show what data flows and where between which parts of the DFD. This website seems helpful from what I can see': http://www.visual-paradigm.com/produ...orials/dfd.jsp, but your first steps are to identify those things I've mentioned, so identify the entities, processes and data stores (there may be an element of assumption depending on how thorough the brief is).

Here is a basic (very basic) example of a use case demonstration how an ATM works: http://argouml-stats.tigris.org/docu...ltiplicity.gif. The first step to doing one is drawing a square; this is the system boundary and it's important not to forget it. Then you need to identify the actors; the people who interact with the system. These are then split into two group, primary and secondary, primary being the people the system was designed to be used by (eg: bank customers) and secondary being people who use the system but weren't the intended main users (eg: maintenance people). Draw these actors as stick people outside the square you've drawn. You then need to identify the actions (probably the best word I can think if this early) that the actors do from a very high-level view, eg: http://www.agilemodeling.com/artifac...aseDiagram.htm. There are a couple of reasonable examples in there that might help you. Draw ovals round these and put them in the system boundary. Once you've done those, you can add extend or include functionality (you haven't mentioned it but it's pretty standard), this would be where you might be modelling an ATM, you might have a process in the system boundary called 'withdraw' which may extend or include the 'print' process (include and extend are essentially when a process uses another process or has the option of using another process, but I can't remember which one is which right now). That's pretty much it I believe, starting is quite daunting but once you're going it's not that bad, and expect that you probably won't get it right first time.

I've not done a context diagram before but looking at stuff online it looks like you draw the name of the system in a bubble with external entity coming off with arrows indicating the direction of flow, and then write what each entity does underneath.

I hope this makes sense and that it helps.


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