Data flow diagram also called Data Flow Diagram (DFD). DFD is to describe the system modules in a smaller and less easy for the user to understand to understand the field of computer systems that will be done.
DFD also serves to describe the existing system or the new system will be developed logically without considering the physical environment where the data flows, or where data are stored. DFD is a tool that is used in the system development methodology is structured (structured analysis and design). DFD can describe the flow of data within the system with a structured and clear.
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
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.
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
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.
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.
Zero Diagram
Zero diagram is a chart that describes the process of DFD. This diagram provides a view of the overall system shows that the main function of the process or the flow of data and the external entity. At this level there is a data storage.
Detailed Diagram
Is a diagram that decipher what is the process in the diagram zero level or above.
Numbering level in the DFD:
Process Specification
Each process in DFD must have the process specification. in top level method used to describe process by using descriptive sentence. At more level detailed that is under process (functional primitive) required the specification structure.Process Specification will become guide to programmer in coding. Method used in the process specification : * description process in the story * decision table * decision tree
Data flow diagram levels
Level 1 (High Level Diagram)
A Level 1 Data flow diagram for the same system.
This level (level 1) shows all processes at the first level of numbering, data stores, external entities and the data flows between them. The purpose of this level is to show the major high-level processes of the system and their interrelation. A process model will have one, and only one, level-1 diagram. A level-1 diagram must be balanced with its parent context level diagram, i.e. there must be the same external entities and the same data flows, these can be broken down to more detail in the level 1, e.g. the "enquiry" data flow could be spilt into "enquiry request" and "enquiry results" and still be valid.
Level 2 (Low Level Diagram)
A Level 2 Data flow diagram showing the "Process Enquiry" process for the same system.
This level is a decomposition of a process shown in a level-1 diagram, as such there should be a level-2 diagram for each and every process shown in a level-1 diagram. In this example processes 1.1, 1.2 & 1.3 are all children of process 1, together they wholly and completely describe process 1, and combined must perform the full capacity of this parent process. As before, a level-2 diagram must be balanced with its parent level-1 diagram.
Data Dictionary
Data dictionary is a reserved space within a database which is used to store information about the database itself. Data dictionary is also called with a system data dictionary is a catalog of facts and data information needs of an information system. In function to help system agent to interpreting application in detail and organization all of data element that utilized by system exactly so user and system analyst have same understanding basic about entry, output, storage and process. In analysis phase, data dictionary used as communication between system analyst with user. in development system phase, data dictionary used to design input, and report database. Data flow in DFD have the character of globally, boldness more detailed can be seen in data dictionary.
Data dictionary load the followings :
- Name of data current: must note that readers who need further explanation about a flow of data can find it easily
- Alias: alias or other name of the data can be written when there is
- Forms of data: used to segment the data dictionary to use when designing the system
- Flow data: indicates from which data flows and where the data
- Description: to give an explanation of the meaning of the data flow
Balancing in DFD
Data flow into and out of a process must be the same as the flow of data into and out of the details of the process on the level / levels below. Name of the data flow into and out of the process must match the name of the flow of data into and out of the details of the process. Number and the name of an entity outside the process must be equal to the number of names and entities outside of the details of the process.
Things which must be gave attention to DFD owning than one level:
- There are must input balance and output between one level and level next
- Balance between level 0 and level 1 seen at input / output of data stream to or from terminal in level 0, while balance between level 1 and level 2 seen at input / output of data stream to/from pertinent process
- Data flow name, data of storage terminal and every level must be same if its same object
Prohibition in DFD
- Data flow may not from external entity directly to wend another external entity without passing a process.
- Data flow may not from direct data deposit go to external entity without passing a process
- Data flow may not from direct data deposit go to other data deposit without passing a process
- Data flow from one direct process go to other process without passing a data deposit better possible avoided.
References:
- Slide Part 4 - DATA FLOW DIAGRAM by ER Ngurah Agus Sanjaya
- http://www.getahead-direct.com/gwbadfd.htm
- http://en.wikipedia.org/wiki/Data_flow_diagram
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