Question

What are the primary steps in a Data Flow Diagram for a Payroll Account with External Controls in the process?

Answer 1

Steps: - Break the system into suitably tractable units by means of transaction analysis - Convert each unit into into a good structure chart by means of transform analysis - Link back the separate units into overall system implementation.

Transaction Analysis

The transaction is identified by studying the discrete event types that drive the system. For example, with respect to railway reservation, a customer may give the following

The three transaction types here are: Check Availability (an enquiry), Reserve Ticket (booking) and Cancel Ticket (cancellation). In a typical situation, any one stimulus may be entered through a particular terminal. The human user would inform the system her preference by selecting a transaction type from a menu. The first step in our strategy is to identify such transaction types and draw the first level breakup of modules in the structure chart, by creating separate module to co-ordinate various transaction types. This is shown as fol

The Main ( ) which is a over-all coordinating module, gets the information about what transaction the user prefers to do through TransChoice. The TransChoice is returned as a parameter to Main(). The actual details of how GetTransactionType ( ) is not relevant for Main ( ). It may for example, refresh and print a text menu and prompt the user to select a choice and return this choice to Main ( ). It will not affect any other components in our breakup, even when this module is changed later to return the same input through graphical interface instead of textual menu. The modules Transaction1 ( ), Transaction2 ( ) and Transaction3 ( ) are the coordinators of transactions one, two and three respectively. The details of these transactions are to be exploded in the next levels of abstraction.

We will continue to identify more transaction centers by drawing a navigation chart of all input screens that are needed to get various transaction stimuli from the user. These are to be factored out in the next levels of the structure chart (in exactly the same way as seen before), for all identified transaction centers.

Transform Analysis

Transform analysis is strategy of converting each piece of DFD (may be from level 2 or level 3, etc.) for all the identified transaction centers. In case, the given system has only one transaction (like a payroll system), then we can start transformation from level 1 DFD itself. Transform analysis is composed of the following five steps:

Draw a DFD of a transaction type (usually done during analysis phase)

Find the central functions of the DFD

Convert the DFD into a first-cut structure chart

Refine the structure chart

Verify that the final structure chart meets the requirements of the original DFD

Let us understand these steps through a payroll system example:

The central transform is the portion of DFD that contains the essential functions of the system and is independent of the particular implementation of the input and output. One way of identifying central transform is to identify the centre of the DFD by pruning off its afferent and efferent branches. Afferent stream is traced from outside of the DFD to a flow point inside, just before the input is being transformed into some form of output (For example, a format or validation process only refines the input – does not transform it). Similarly an efferent stream is a flow point from where output is formatted for better presentation. The processes between afferent and efferent stream represent the central transform (marked within dotted lines above). In the above example, P1 is an input process, and P6 & P7 are output processes. Central transform processes are P2, P3, P4 & P5 - which transform the given input into some form of output.

First-cut Structure Chart

To produce first-cut (first draft) structure chart, first we have to establish a boss module. A boss module can be one of the central transform processes. Ideally, such process has to be more of a coordinating process (encompassing the essence of transformation). In case we fail to find a boss module within, a dummy coordinating module is created