DFD Reminder…

Developing a DFD

• There are no FIXED rules about how a DFD should be developed…

• There is no such a DFD call “CORRECT DFD”…

• Expert SAs may not need much…..but definitely need for YOU!!

Where to Begin Creating DFDs

• Start with the data flow from an external entity and work forwards

• Start with the data flow to an external entity and work backwards

• Examine the data flows into or out of a data store

• Examine data flows, entity connections and data stores associated with a particular process

• Note fuzzy, ill-defined areas of the system for further clarification

Description of Each Level

• Context Diagram: This is the highest level and represents the overall system and its interaction with its environment

• Level 0 Diagram: This shows the major subsystems and their interactions

• Level x Diagram: Shows the processes that make up each of the major subsystems

Level 0 Tips

• Generally move from top to bottom, left to right• Minimize crossed lines• Iterate as needed

– The DFD is often drawn many times before it is finished, even with very experienced systems analysts

Level 0 DFD Building Procedure

• Level-0 DFD – Identify what is being done between each input and

its corresponding output– Identify the processes– Identify external data flows between external entities

and processes– Identify internal data flows between processes

and data stores

When to Stop

• Each process is a single decision or calculation or a single database operation, such as retrieve, update, create, delete or read.

• Each data store represents data about a single entity such as a customer, employee, product or order.

• The system user does not care to see any more detail or when you and other analysts have dcoumented sufficient detail to do subsequesnt systems development tasks.

• Every data flow does not need to be split further to show that different data are handled in different ways.

• You believe that you have shown each business form or transaction, computer screen and report as a single data flow.

• You believe there is a separate process for each choice on all lowest-level menu options for the system.