 | MSc-IT Study MaterialJanuary 2011 Edition Computer Science Department, University of Cape Town| MIT Notes trang chủ | Edition Home | |
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As described previously, context diagrams (màn chơi 0 DFDs) arediagrams where the whole system is represented as a singleprocess. A level 1 DFD notates each of the main sub-processesthat together size the complete system. We can think of alevel 1 DFD as an “exploded view” of the contextdiagram.
A possible cấp độ 1 DFD for the Video-Rental LTD case study isas follows:
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Notice that the external entities have sầu been included on thisdiagram, but outside of the rectangle that represents theboundary of this diagram (i.e., the system boundary). It isnot necessary to lớn always show the external entities on màn chơi 1(or lower) DFDs, however you may wish to lớn do so to aid clarityand understanding.
We can see that on this level 1 DFD there are a number of datastores, và data-flows between processes & the data stores.
It is important to notice that the same data-flows to & fromthe external entities appear on this level 1 diagram và themàn chơi 0 context diagram. Each time a process is expanded to alower màn chơi, the lower màn chơi diagram must show all the samedata-flows inlớn, & out of the higher level process itexpands.
Constrsydneyowenson.coming màn chơi 1 DFDs
If no context diagram exists, first create one beforeattempting to constrsydneyowenson.com the cấp độ 1 DFD (or constrsydneyowenson.com thecontext diagram & level 1 DFD simultaneously).
The following steps are suggested khổng lồ aid the constrsydneyowenson.comion ofLevel 1 DFD:
Identify processes. Each data-flow into the system must be received by a process. For each data-flow into the system examine the documentation about the system and talk to lớn the users lớn establish a plausible process of the system that receives the data-flow. Each process must have sầu at least one output data-flow. Each output data-flow of the system must have been sent by a process; identify the processes that sends each system output.
Draw the data-flows between the external entities & processes.
Identify data stores by establishing where documents / data needs khổng lồ be held within the system. Add the data stores to the diagram, labelling them with their local name or mô tả tìm kiếm.
Add data-flows flowing between processes and data stores within the system. Each data store must have at least one input đầu vào data-flow & one output data-flow (otherwise data may be stored, và never used, or a store of data must have sầu come from nowhere). Ensure every data store has input đầu vào & output data-flows khổng lồ system processes. Most processes are normally associated with at least one data store.
Cheông xã diagram. Each process should have sầu an input đầu vào and an output. Each data store should have an input đầu vào & an output. Cheông chồng the system details so see if any process appears to be happening for no reason (i.e., some “trigger” data-flow is missing, that would make the process happen).