Data Flow Diagram
A process model is a picture of the flow of data through a system and the processing performed on that data.
These pictures are often easier to read than any other form. Process modeling helps us grasp inputs, outputs, processing and the relationship between processes.
The term process modeling comes from the depiction of processes and how they interact of interface with one another. These interactions take place in the form of data flows between the processes which are called data flow models or diagrams.
Data flow diagram conventions and guidelines:
A data flow diagram illustrates the flow of data through a system and the work or processing performed by that system.
Language and symbols:
DFD's are quite easy to read and understand, but to draw a DFD we must be familier with the symbols to be used. Let us now see the symbol set, a technique developed by Gane-Sarson and Yourdon.
1. Internal or external entity:
Entities are called as sources of data (input to the system) or designations (output from the system).
Entities are people, organizations and other systems with which the system interacts. They are always one of the following:
(i) An office, department or person who provides input to the system or receives the output from the system is called an internal entity.
(ii) Organizations, agencies or individuals that are outside your company, who are providing input or receiving output are called as external entity.
(iii) Systems which are outside the boundary of the system, but which receives data from system are called as an external entity.
Labels [Naming of entity]:
Labels to the entity should be descriptive. Always use titles instead of people's names, as it is likely that persons will change, but titles/designations will not change.
Duplication of entity in a diagram:
To avoid crossing data flow lines on the DFD, it is permissible to duplicate internal or external entities ont eh DFD. All duplicated entities should be marked.
2. The process
The processes are work or actions that are performed by people, machine or computer on the incoming data flow to produce an outgoing dataflow. The process name depends upon the level at which the DFD is drawn.
The process accepts data to produce an output data flow. For some this may occur frequently and the process may be executed, while for others, it may occur optionally under certain conditions. The DFD does not show which data is mandatory or when it will occur.
3. The data store
It is used to store the data which can be used at later stages. The names given to them should be appropriate adjectives. To avoid a crossing of dataflow, duplication of data stores is allowed, which can be marked accordingly.
(a) A data flow from the process means that the process uses the data.
(b) A data flow to a data store from the process means that the process updates the data store.
Updates may include any or all of the following:
(i) Adding a new record or form.
(ii) Deleting or removing old records.
(iii) Changing an existing record or form.
(c) As a programmer, you know that read and write occurs in pairs, but don't try to show the pair for each flow unless it is really necessary.
4. The data flow
It describes the data flowing from the entity to the process or vice-versa. It may also flow from the data store to the process or vice-versa. Each dataflow is a known composition, which must be labeled as a noun clause but not a verb. Sometimes data flows in packets and they should not be separated.
Rules which must be followed at the time of drawing DFDs:
1. An entity to entity data flow is not allowed.
2. An entity to data store data flow is not allowed.
3. A data store to entity data flow is not allowed.
4. A data store to data store data flow is not allowed.
5. A process has inputs (data flows) but no outputs (data flows). We call such a process a BLACK WHOLE.
6. A process has outputs (data flows) but no inputs (data flows). We call such a process a GRAY WHOLE.
7. A data flow shout not be hanging.
Software Engineering Assessments and Assessments Report Writing Help
Expertsmind.com offers help with software engineering homework and assignments, software engineering assessments and problems solutions, review software products and assessments, report writing help, help with software engineering projects and questions from online software engineering tutor's help. We offer email based homework help and assignment help for software engineering subject and topics. Get solved software engineering assessments and problems from qualified and experienced computer science experts at Expertsmind.com.
Tags: Computer Science Engineering Help, Computer Science Assignment Help, Software Engineering Homework Help, Software Engineering Assignment Help, Data Flow Diagram Homework , Data Flow Diagram Homework Help , Data Flow Diagram Homework Help Services , Live Data Flow Diagram Homework Help , Data Flow Diagram Homework Tutors , Online Data Flow Diagram Homework Help , Data Flow Diagram Tutors , Online Data Flow Diagram Tutors , Data Flow Diagram Homework Services , Data Flow Diagram