dfd placement cell

8/18/2019 Dfd placement cell

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AIM: To study and draw data flow diagram for Online Training andPlacement System.

THEORY:

What is Data Flow Diagram?

A data flow diagram (DFD) is a graphical representation of the ”flow” of datathrough an information system, modeling its process aspects. A DFD is oftenused as a preliminary step to create an overview of the system, which can later

be elaborated. DFDs can also be used for the visualization of data processing(structured design). A DFD shows what kind of information will be input to andoutput from the system, where the data will come from and go to, and where thedata will be stored. It does not show information about the timing of process orinformation about whether processes will operate in sequence or in parallel (whichis shown on a flowchart).

Symbols in the data flow diagram

1. External Entity- External entities determine the system boundary. They are

external to the system being studied. They are often beyond the area of influence of the developer.

These can represent another system or subsystem. These go on margins/edgesof data flow diagram. External entities are named with appropriate name.

2. Process- Processes are work or actions performed on incoming data flows toproduce outgoing data flows. These show data transformation or change.Data coming into a process must be ”worked on” or transformed in someway. Thus, all processes must have inputs and outputs. In some (rare)cases, data inputs or outputs will only be shown at more detailed levels of

the diagrams. Each process in always ”running” and ready to accept data.3. Data flow-Data flow represents the input (or output) of data to (or from) a

process (”data in motion”). Data flows only data, not control. Representthe minimum essential data the process needs. Using only the minimumessential data reduces the dependence between processes. Data flows mustbegin and/or end at a process.

Data flows are always named. Name is not to include the word ”data”.Should be given unique names. Names should be some identifying noun. Forexample, order, payment, complaint.

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Figure 1: DFD notations

4. Data stores-Data Stores are repository for data that are temporarily or per-manently recorded within the system. It is an ”inventory” of data. These arecommon link between data and process models. Only processes may connectwith data stores.

There can be two or more systems that share a data store. This can occurin the case of one system updating the data store, while the other system

only accesses the data.Data stores are named with an appropriate name, not to include the word”file”, Names should consist of plural nouns describing the collection of data.Like customers, orders, and products. These may be duplicated. These aredetailed in the data dictionary or with data description diagrams.

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Rules in data flow diagram

Naming conventions:

• Processes: strong verbs

• dataflows: nouns

• datastores: nouns

• external entities: nouns

1. No more than 7 - 9 processes in each DFD.

2. Dataflows must begin, end, or both begin and end with a process.

3. Dataflows must not be split.

4. A process is not an analog of a decision in a systems or programmingflowchart. Hence, a dataflow should not be a control signal. Control sig-nals are modeled separately as controlflows.

5. Loops are not allowed.

6. A dataflow can not be an input signal. If such a signal is necessary, then itmust be a part of the description of the process, and such process must beso labeled. Input signals as well as their effect on the behavior of the systemare incorporated in the behavioral model (say, state transition graphs) of theinformation system.

7. Decisions and iterative controls are part of process description rather thandataflows.

8. If an external entity appears more than once on the same DFD, then adiagonal line is added to the north-west corner of the rectangle (representingsuch entity).

9. Updates to datastores are represented in the textbook as double-ended ar-rows. This is not, however, a universal convention. I would rather you didnot use this convention since it can be confusing. Writing to a datastore im-plies that you have read such datastore (you can not write without reading).Therefore, datastore updates should be denoted by a single-ended arrow fromthe updating process to the updated datastore.

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10. Dataflows that carry a whole record between a datastore and a process isnot labeled in the textbook since there is no ambiguity. This is also not auniversal convention. I would rather you labeled such dataflows explicitly.

• Conservation Principles: Datastores and Dataflows: Datastores can not cre-ate (or destroy) any data. What comes out of a datastore therefore mustfirst have got into a datastore through a process. Processes: Processes cannot create data out of thin air. Processes can only manipulate data theyhave received from dataflows. Data outflows from processes therefore mustbe derivable from the data inflows into such processes.

• Levelling Conventions: Numbering: The system under study in the contextdiagram is given number ‘0’. The processes in the top level DFD are la-belled consecutively by natural numbers beginning with 1. When a processis exploded in a lower level DFD, the processes in such lower level DFD areconsecutively numbered following the label of such parent process endingwith a period or full-stop (for example 1.2, 1.2.3, etc.). Balancing: The setof DFDs pertaining to a system must be balanced in the sense that corre-sponding to each dataflow beginning or ending at a process there should bean identical dataflow in the exploded DFD. Datastores: Datastores may be

local to a specific level in the set of DFDs. A datastore is used only if it isreferenced by more than one process. External entities: Lower level DFDscan not introduce new external entities. The context diagram must there-fore show all external entities with which the system under study interacts.In order not to clutter higher level DFDs, detailed interactions of processeswith external entities are often shown in lower level DFDs but not in thehigher level ones. In this case, there will be some dataflows at lower levelDFDs that do not appear in the higher level DFDs. In order to facilitateunambiguous balancing of DFDs, such dataflows are crossed out to indicatethat they are not to be considered in balancing. This convention of crossingis quite popular, but this text does not follow it. I would rather you followedthis convention.

CONCLUSION: Hence we have drawn and studied data flow diagram forOnline Placement Cell System system.

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Figure 2: Level 0 DFD

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Figure 3: Level 1 DFD

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Figure 4: Level 2 DFD for Student

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Figure 5: Level 2 DFD for Admin

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Figure 6: Level 2 DFD for Company

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dfd placement cell

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