Intro to DFD 2

In a DFD external entities are represented by a

• Circle

• Diamond shaped box

• Ellipse

• Rectangle

External Entities may be a

• Source of input data only

• Source of input data or destination of results

• Destination of results only

• Repository of data

A data store in a DFD represents

• A disk store

• A repository of data

• A random access memory

• A sequential file

By an external entity we mean a

• A unit which is not part of DFD

• A unit external to the system being designed

• Unit outside the system whose behaviour is independent of the system being designed

• Unit outside the system being designed which can be controlled by an analyst

A data flow can

• Only enter a data store

• Either enter or leave a data store but not both

• Only leave a data store

• Enter or leave a data store

A circle in a DFD represents

• An input unit

• A data store

• A process

• A an external entity

External entity to process

• Process to external entity

• Process to store and back

• Data can flow from

• Process to process

When to stop decomposing DFDs?

• Ideally, a DFD has at least three levels.

• Common errors in DFD

• Validating DFD

• When the system becomes primitive i.e. lowest level is reached and further decomposition is useless.

Logical and Physical DFD

• Physical DFD may depict physical movements of the goods

• DFDs considered so far are called logical DFDs

• A physical DFD is similar to a document flow diagram

• Physical DFDs can be drawn during fact gathering phase of a life cycle

Physical DFD for Cheque Encashment

• Bad Cheque Store

• Update Balance

• Verify A/C Signature

• Cash Clerk