Inst_03_04
(073014)
From the CONTEXT DFD we move down to begin the decomposition at the system level. First,
we will use the data stores created in the CONTEXT DFD. We will cover nesting later.
This is a great time to review the basic modelling rules.
Sample Rules_DFD
The system level DFD is the first decomposition. Consider the bookstore CONTEXT DFD and
look at the (squared box) process BSD 00 as the roof of your house. Now consider removing
the roof from the process box and looking inside to find different “rooms” inside. A kitchen, bath,
den, and so on. This is the first decomposition of the project moving towards the many atomic
levels that will be built. Instead of rooms we will find major level processes such as marketing,
human resources, sales, and facilities.
Traditionally there are between five and seven system level processes. Think of what
department or processes areas are found within the organization. Remember we are only going
to model what happens inside the theoretical building.
The system DFD is also labelled NAME 00 as the CONTEXT. Note that the name of the DFD is
from the parent and the children processes are NAME 00 P00, P 01, etc. The next
decomposition for the DFD labelled NAME 00 P 00 will have children named NAME 00 00 P 00,
P 01, etc. The decomposition for the DFD labelled NAME 00 P 03 will have children named
NAME 00 03 P 00, P 01, etc.
Defined - Atomic level process The last (bottom) process of the firm decomposition. This is the
point where the actual transaction occurs. Consider a checkout lane at a grocery store. You
give the item to the clerk to scan, it reads the UPC, finds the inventory data, identifies the price
and reduces the inventory level, transfers the charge to your receipt, and moved to the atomic
level of payment. We will arrive back to the atomic level at the end of the project.
Consider the atomic level processes in the firm you selected.