data flow diagram

This content is an extract from GetAhead in Business Analysis - the FULL course is in eBook and multimedia CD-Rom format.

Processes.
A process on a data flow diagram shows a transformation or manipulation of data flows (DFs) within the system. The symbol used is a rectangular box which contains 3 descriptive elements: Firstly an identification number (i.e. 6) appears in the upper left hand corner. This is allocated arbitrarily at the top level of the data flow diagram and serves as a unique reference. Secondly, a location appears to the right of the identifier and describes where in the system the process takes place. (i.e. Accounts). This may, for example, be a department or a piece of hardware. Finally, a descriptive title is placed in the centre of the box. This should be a simple imperative sentence with a specific verb, for example 'maintain customer records' or 'find driver'.

Data Flow.
A data flow (DF) on a data flow diagram (DFD) shows the flow of information from its source to its destination. A DF is represented by a line, with arrowheads showing the direction of flow. Information always flows to or from a process and may be written, verbal or electronic. Each DF may be referenced by the processes or data stores at its head and tail, or by a description of its contents. A data store (DS) is a holding place for information within the system. It is represented by an open ended narrow rectangle. Data stores may be long-term files such as sales ledgers, or may be short-term accumulations: for example batches of documents that are waiting to be processed. Each DS should be given a reference followed by an arbitrary number.

Resource Flow.
A resource flow shows the flow of any physical material from its source to its destination. For this reason they are sometimes referred to as physical flows.
The physical material in question should be given a meaningful name. Resource flows are usually restricted to early, high-level diagrams and are used when a description of the physical flow of materials is considered to be important to help the analysis.

External Entities.
It is normal for all the information represented within a system to have been obtained from, and/or to be passed onto, an external source or recipient. These external entities may be duplicated on a diagram, to avoid crossing DF lines. Where they are duplicated a stripe is drawn across the left hand corner, like this. The addition of a lowercase letter to each entity on the diagram is a good way to uniquely identify them. When naming processes try to avoid glossing over them; without really understanding their role. Indications that this has been done are the use of vague terms in the descriptive title area - like 'process' or 'update'. The most important thing to remember is that the description must be meaningful to whoever will be using the diagram.

Double headed arrows can be used (to show two-way flows) on all but bottom level diagrams. Furthermore, in common with most of the other symbols used, a DF at a particular level of a diagram may be decomposed to multiple DFs at lower levels.
Each DS should be given a reference letter, followed by an arbitrary number. These reference letters are allocated as follows:
'D' - indicates a permanent computer file
'M' - indicates a manual file
'T' - indicates a transient store, one that is deleted after
processing.
In order to avoid complex flows, the same DS may be drawn several times on a diagram. Multiple instances of the same DS are indicated by a double vertical bar on their left hand edge (as shown on the ‘Withdrawals’ store).

External Entities & Data Flows.
There are rules governing which DFs are permitted between the various components of a business process diagram. DFs and resource flows are allowed between external entities and processes. DFs are also allowed between different external entities. However, DFs and resource flows are not allowed between external entities and DS's.

Processes & Data Flows.
For processes the DF rules are as follows:
DFs and resource flows are allowed between processes and external entities and between processes and DS's.
They are also allowed between different processes. In other words processes can communicate with all other areas of the business process diagram.

Data Stores & Data Flows.
For DS's the DF rules are as follows:
DFs and resource flows are allowed between DS's and processes.
However, these flows are not allowed between data stores and external entities or between one DS and another. In practice this means that DS's cannot initiate a communication of information, they require a process to do this.

Stepwise Diagram Evolution.
Firstly, draw the process symbol for the system under investigation. Remember, this should consist of a box containing an identification number in the upper left hand corner. It should also have a location to the right of the identifier; which describes where in the system the process takes place and a descriptive title in the centre of the box.
Only when you have drawn your version should you go to the next page, to see a suggested solution.
The next step is to identify the external entities and add them to the diagram. In this case there is only one possible external entity. This should be represented by an oval, containing a meaningful and unique identifier.

Only when you have drawn your version should you go to the next page, to see a suggested solution.
Now identify the DFs between the Customer and the ATM process and add them to the diagram. A Customer may request information, for example a balance enquiry, or they may receive cash or information from the ATM - this represents a data or resource flow from the process to the Customer.
Only when you have drawn your version should you go to the next page, to see a suggested solution.
Finally, think about the DFs and the DS's required to support the transactions already described.
Only when you have drawn your version should you go to the next page, to see a suggested solution.

Customer transactions will usually depend on their account details being checked; and this is shown on the diagram. However, whilst information updates are always shown, the 'reading' of DS's is not normally shown on business process diagrams. The reason for this is to avoid crowding the diagram with a multitude of low level DFs.

This content is an extract from GetAhead in Business Analysis - the FULL course is in eBook and multimedia CD-Rom format. TOP