Scoping A System

Subject: Scoping A System
From: Tony Markos <ajmarkos -at- yahoo -dot- com>
To: "TECHWR-L" <techwr-l -at- lists -dot- techwr-l -dot- com>
Date: Mon, 1 Nov 2004 10:02:16 -0800 (PST)

Jens Reineking asks:

How do you identify the sinks [on a Data Flow

Tony Markos responds:

This is part of the process of identifying the scope
(i.e., extent) of the system. The following are the
steps to scoping a system (including identification
the sinks):

1.) The system's sponsors and/or users are supposed to
define the scope of the system. In reality, their
understanding is incomplete and has inaccuracies.

Interview them and/or read their (most often written)
description of the system's scope to get whatever info
you can. Scope definition from these people, in its
best form, consists of high-level statements of the
format: System A inputs X from System B, and System A
outputs Y to System B. (Note: System B is a sink.)

2.) Draw a rough-cut Context diagram. A Context
diagram is a special kind of DFD in which the whole
system is represented as a single entity (i.e., single
box or circle). It also shows interfaces from/to the
system from/to the sinks.

(Note: Because a Context diagram displays the whole
system as a single entity, the focus is taken off of
the specifics of the system and is placed on the
inputs/outputs to/from the system to/from the sinks.)

3.) Walk throught the rough-cut context diagram with
sponsors and/or end-users to correct - as much as you
can. Even after a couple of walkthrough iterations,
there will still be mistakes and missing interfaces
and sinks. Don't worry; you will find them in the next

4.) "Explode" the context diagram into a high-level
regular DFD. Within this DFD, in the course of
following the data flows that you do have identified,
you will reach points where you see that some of the
data needed to complete a given task is missing. You
then backtrack out of the high-level DFD and, with the
sponsors and/or end-users help, you identify the
missing interface to the outside world and the
interface's sink.

Tony Markos

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