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Free Data Flow Diagrams Tutorial

Data Flow Diagrams - Introduction
Data flow diagrams can be used to provide a clear representation of any business function. The technique starts with an overall picture of the business and continues by analyzing each of the functional areas of interest. This analysis can be carried out to precisely the level of detail required. The technique exploits a method called top-down expansion to conduct the analysis in a targeted way.

The result is a series of diagrams that represent the business activities in a way that is clear and easy to communicate. A business model comprises one or more data flow diagrams (also known as business process diagrams). Initially a context diagram is drawn, which is a simple representation of the entire system under investigation. This is followed by a level 1 diagram; which provides an overview of the major functional areas of the business. Don't worry about the symbols at this stage, these are explained shortly. Using the context diagram together with additional information from the area of interest, the level 1 diagram can then be drawn.

The level 1 diagram identifies the major business processes at a high level and any of these processes can then be analyzed further - giving rise to a corresponding level 2 business process diagram. This process of more detailed analysis can then continue – through level 3, 4 and so on. However, most investigations will stop at level 2 and it is very unusual to go beyond a level 3 diagram.

Identifying the existing business processes, using a technique like data flow diagrams, is an essential precursor to business process re-engineering, migration to new technology, or refinement of an existing business process. However, the level of detail required will depend on the type of change being considered.

Data Flow Diagrams – Diagram Notation
There are only five symbols that are used in the drawing of business process diagrams (data flow diagrams). These are now explained, together with the rules that apply to them.
   
This diagram represents a banking process, which maintains customer accounts. In this example, customers can withdraw or deposit cash, request information about their account or update their account details. The five different symbols used in this example represent the full set of symbols required to draw any business process diagram.

External Entity
                   
An external entity is a source or destination of a data flow which is outside the area of study. Only those entities which originate or receive data are represented on a business process diagram. The symbol used is an oval containing a meaningful and unique identifier.

Process
                           

A process shows a transformation or manipulation of data flows within the system. The symbol used is a rectangular box which contains 3 descriptive elements:
Firstly an identification number appears in the upper left hand corner. This is allocated arbitrarily at the top level 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. 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 shows the flow of information from its source to its destination. A data flow 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 data flow may be referenced by the processes or data stores at its head and tail, or by a description of its contents.

Data Store
                            
A data store 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 data store should be given a reference followed by an arbitrary number.

Resource Flow
                              

Data Flow Diagrams – The Rules

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 data flow 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.

Processes
When naming processes, 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.

Data Flows
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 data flow at a particular level of a diagram may be decomposed to multiple data flows at lower levels.

Data Stores
Each store 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 data store may be drawn several times on a diagram. Multiple instances of the same data store are indicated by a double vertical bar on their left hand edge.

Data Flow Diagrams – Relationship Grid

There are rules governing various aspects of the diagram components and how they can relate to one another.

Data Flows
For data flows the rules are as follows:
Data flows and resource flows are allowed between external entities and processes. Data flows are also allowed between different external entities. However, data flows and resource flows are not allowed between external entities and data stores.

Processes
For processes the data flow rules are as follows:
Data flows and resource flows are allowed between processes and external entities and between processes and data stores. They are also allowed between different processes. In other words processes can communicate with all other areas of the business process diagram.

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

Data Flow Diagrams – Context Diagrams
      
The context diagram represents the entire system under investigation. This diagram should be drawn first, and used to clarify and agree the scope of the investigation.

The components of a context diagram are clearly shown on this screen. The system under investigation is represented as a single process, connected to external entities by data flows and resource flows.

The context diagram clearly shows the interfaces between the system under investigation and the external entities with which it communicates. Therefore, whilst it is often conceptually trivial, a context diagram serves to focus attention on the system boundary and can help in clarifying the precise scope of the analysis.

The context diagram shown on this screen represents a book lending library. The library receives details of books, and orders books from one or more book suppliers.

Books may be reserved and borrowed by members of the public, who are required to give a borrower number. The library will notify borrowers when a reserved book becomes available or when a borrowed book becomes overdue.

In addition to supplying books, a book supplier will furnish details of specific books in response to library enquiries.

Note, that communications involving external entities are only included where they involve the 'system' process. Whilst a book supplier would communicate with various agencies, for example, publishers and other suppliers - these data flow are remote from the 'system' process and so this is not included on the context diagram.

Data Flow Diagrams – Context Diagram Guidelines
Firstly, draw and name a single process box that represents the entire system.

Next, identify and add the external entities that communicate directly with the process box. Do this by considering origin and destination of the resource flows and data flows.

Finally, add the resource flows and data flows to the diagram.

In drawing the context diagram you should only be concerned with the most important information flows. These will be concerned with issues such as: how orders are received and checked, with providing good customer service and with the paying of invoices. Remember that no business process diagram is the definitive solution - there is no absolute right or wrong.

Data Flow Diagrams – Level 1 Diagrams
             
The level 1 diagram shows the main functional areas of the system under investigation. As with the context diagram, any system under investigation should be represented by only one level 1 diagram.

There is no formula that can be applied in deciding what is, and what is not, a level 1 process. Level 1 processes should describe only the main functional areas of the system, and you should avoid the temptation of including lower level processes on this diagram. As a general rule no business process diagram should contain more than 12 process boxes.

The level 1 diagram is surrounded by the outline of a process box that represents the boundaries of the system. Because the level 1 diagram depicts the whole of the system under investigation, it can be difficult to know where to start.

There are three different methods, which provide a practical way to start the analysis. These are explained in the following section and any one of them, or a combination, may prove to be the most helpful in any given investigation.

There are three different methods, which provide a practical way to start the analysis. These are introduced below and any one of them, or a combination, may prove to be the most helpful in any given investigation:

Data Flow Diagrams – Resource Flow Analysis
Resource flow analysis may be a useful method for starting the analysis if the current system consists largely of the flow of goods, as this approach concentrates on following the flow of physical objects.

Resource flow analysis may be a useful method for developing diagrams if the current system consists largely of the flow of goods. Physical resources are traced from when they arrive within the boundaries of the system, through the points at which some action occurs, to their exit from the system. The rationale behind this method is that information will normally flow around the same paths as the physical objects.

Data Flow Diagrams – Organizational Structure Analysis
The organizational structure approach starts from an analysis of the main roles that exist within the organization, rather than the goods or information that is flowing around the system.

Identification of the key processes results from looking at the organizational structure and deciding which functional areas are relevant to the current investigation. By looking at these areas in more detail, and analyzing what staff actually do, discrete processes can be identified.

Starting with these processes, the information flows between them and between these processes and external entities are then identified and added to the diagram.

Data Flow Diagrams – Document Flow Analysis
The document flow analysis approach is appropriate if the part of the business under investigation consists principally of flows of information in the form of documents or computer input and output.

Document flow analysis is particularly useful where information flows are of special interest. The first step is to list the major documents and their sources and recipients. This is followed by the identification of other major information flows such as telephone and computer transactions. Once the document flow diagram has been drawn the system boundary should be added.

Data Flow Diagrams – Top Down Expansion
               
The section explains the process of top down expansion, or leveling. Furthermore, it illustrates that whilst there can only be one context and one level 1 diagram for a given system, these normally give rise to numerous lower level diagrams.

Each process within a given business process diagram may be the subject of further analysis. This involves identifying the lower level processes that together constitute the process as it was originally identified. This procedure is known as top-down expansion or leveling.

As a business process diagram is decomposed, each process box becomes a boundary for the next, lower level, diagram.

Data Flow Diagrams – Top Down Expansion Illustrated
            

Data Flow Diagrams – Numbering Rules
     
The process boxes on the level 1 diagram should be numbered arbitrarily, so that no priority is implied. Even where data from one process flows directly into another process, this does not necessarily mean that the first one has to finish before the second one can begin.

Therefore the processes on a level 1 diagram could be re-numbered without affecting the meaning of the diagram. This is true within any business process diagram - as these diagrams do not imply time, sequence or repetition.

However, as the analysis continues beyond level 1 it is important that a strict numbering convention is followed. The processes on level 2 diagrams must indicate their parent process within the level 1 diagram. This convention should continue through level 3 diagrams, and beyond, should that level of analysis ever be required.

The diagram on this screen clearly illustrates how processes on lower level diagrams identify their ancestral path.

Data Flow Diagrams - When to Stop
It is important to know when to stop the process of top-down expansion. Usually this will be at level 2 or level 3.

There are 3 useful guidelines to help you to decide when to stop the analysis:

Firstly, if a process has a single input data flow or a single output data flow then it should be apparent that there is little point in analyzing it any further.

Secondly, when a process can be accurately described by a single active verb with a singular object, this also indicates that the analysis has been carried out to a sufficiently low level. For example, the process named validate enquiry contains a single discrete task.

Finally, ask yourself if anything useful will be gained by further analysis of a process. Would any more detail influence your decisions?

If the answer is no, then there is little point in taking the analysis further.

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