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¢ DATA FLOW DIAGRAMS


¢ Systems Analysis
¢ Focus is the logical view of the system, not the physical
¢ What the system is to accomplish, not how
¢ Tools:
“ data flow diagrams
“ data dictionary
“ process specification
“ entity-relationship diagrams



¢ Data Flow Diagram:
"a network representation of a system. The system may be automated, manual, or mixed. The DFD portrays the system in terms of its component pieces, with all interfaces among the components indicated."
- Tom DeMarco
hence DFDs:
focus on the movement of data between external entities and processes, and between processes and data stores
¢ Example Data Flow Diagram


¢ Data Flow Diagrams are:
¢ Used to perform structured analysis to determine logical requirements
¢ A graphical tool, useful for communicating with users, managers, and other IS personnel
¢ Useful for analyzing existing as well as proposed systems
¢ A relatively simple technique to learn and use



¢ Why Conduct Process Modeling?
¢ Understand components of current logical or physical system for purpose of rebuilding in a different physical form/technology, possibly with some changed functionality
¢ Find inefficiencies in current system
¢ Re-engineer current system



¢ Sources/Sinks
(external entities)
¢ Any class of people, an organization, or another system which exists outside the system you are studying.
¢ Form the boundaries of the system.
¢ The system and external entities exchange data in the form of data flows.
¢ Must be named, titles preferred to names of individuals - use a noun


¢ Data Flows
¢ data in motion
¢ marks movement of data through the system - a pipeline to carry data
¢ connects the processes, external entities and data stores
¢ Unidirectional
¢ originate OR end at a process (or both)
¢ name as specifically as possible - reflect the composition of the data - a noun
¢ do not show control flow! Control flow is easy to identify- a signal with only one byte - (on/off).
¢ HINT: if you can't name it: either it's control flow, doesn't exist or you need to get more information!
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¢
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¢ Processes
¢ transform incoming data flows into outgoing data flows
¢ represent with a bubble or rounded square
¢ name with a strong VERB/OBJECT combination; examples:
create_exception_report
validate_input_characters
calculate_discount



¢ Data Stores
¢ data at rest
¢ represents holding areas for collection of data, processes add or retrieve data from these stores
¢ name using a noun (do not use ˜file™)
¢ only processes are connected to data stores
¢ show net flow of data between data store and process. For instance, when access a DBMS, show only the result flow, not the request
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¢ Data Flow Diagram Don™ts
1. BLACK HOLES
2. MIRACLES
3. Let it get too COMPLEX: 7 ± 2 processes
4. Leave things UNLABELED
(corollary: labels should have meaning)
5. Data stores that are SOURCES or
SINKS
6. Data flows that are UNASSOCIATED with
a PROCESS
7. Expect your diagram to be perfect the
first time!
¢ Data Flow Diagram Don™ts
¢ Data Flow Diagram Don™ts
¢ Data Flow Diagram Don™ts
¢ Data Flow Diagram Don™ts
¢ Diagramming A System
¢ multiple DFDs are required to represent a system
¢ DFDs are created at increasing levels of detail
¢ Different Types of DFDs
¢ Context diagram
¢ Level-0 diagram (system diagram)
¢ Level-n diagram
¢ Primitive diagram
¢ Context Diagram
¢ defines the scope of the system by identifying the system boundary
¢ contains:
“ one process (which represents the entire system)
“ all sources/sinks (external entities)
“ data flows linking the process to the sources and sinks (external entities)
¢ Example Context Diagram
¢ Constructing a Context Diagram
¢ identify and list sources/sinks (external entities)
¢ identify and list inputs to and outputs from sources/sinks (external entities)
¢ create context diagram
¢ Level-0 Diagram
¢ describes the overall processing of the system
¢ show one process for each major processing step or functional requirement
¢ data flows from the context appear on system diagram also (level balancing)
¢ can show a single data store to represent all data in aggregate at this level
¢ can draw duplicate sources, sinks and data stores to increase legibility
¢ Drawing a Level-0 Diagram
¢ list the major data stores
¢ list major business steps
¢ draw a segment for each business step
¢ assemble into single DFD
¢ re-organize until satisfied
¢ number processes
¢ Functional Decomposition
¢ similar to a series of more detailed maps
¢ iterative process of breaking the description of a system into finer and finer detail to create a set of charts in which one process on a given chart is explained in greater detail on another chart
¢ referred to as exploding, partitioning, or leveling
¢ must use your judgment to decide what goes on each level
¢ show error and exception handling on lower levels (if at all)
¢ Lower Level Diagrams
¢ explode the processes shown on the level-0 diagram
¢ each process is represented by its own DFD


¢ balance data
“ data flows on upper level appear on lower level, or
“ data flows on upper level are broken into component pieces with components shown on lower level
¢ each lower level shows greater and greater detail
¢ follow numbering convention
¢ Balancing DFDs
¢ conserve data from level to level - inputs and outputs on the higher level must appears somewhere on the lower level


¢ Advanced Rules
¢ Composite data flow on one level can be split into its component data flows on the next level - but new data cannot be added and all data in the composite must be included in the sub-flows
¢ The inputs to a process must be sufficient to produce the outputs.
¢ Lowest level DFDs may add new data flows to represent exception handling, i.e., error messages
¢ May repeat data stores or sources/sink to avoid crossing lines


¢ Additional Guidelines
¢ the inputs to a process are different from the outputs of that process
¢ objects in a set of DFDs have unique names
¢ do not change data flow names on lower levels unless you are decomposing a data flow into component pieces.
¢ never explode a single process into another single process. If you cannot partition the process, then the lower level DFD is not needed.
¢ expect to iterate, put down the DFD and go back to it a few times to create something satisfactory.
¢ Other Questions about Lower level diagrams


1. How deep? (how many levels?)
“ if the process has only one input or one output, probably cannot partition further;
“ can you describe the process in English in about 1/2 page?


2. How broad? (how many processes on a level?)
“ 7 ± two is a reasonable heuristic
“ may temporarily place much of the system on a single diagram then re-draw into separate levels
¢ Quality Guidelines
¢ Completeness
“ all components included & in project dictionary


¢ Consistency
“ between levels: balancing, leveling
¢ Timing considerations
“ assume system never starts and never stops
¢ Iterative nature
“ revisions are common
¢ Drawing primitives (lowest level)
“ when to stop?

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Data flow diagram
From Wikipedia

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Data Flow Diagram example.[1]

A data-flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. DFDs can also be used for the visualization of data processing (structured design).

On a DFD, data items flow from an external data source or an internal data store to an internal data store or an external data sink, via an internal process.

A DFD provides no information about the timing of processes, or about whether processes will operate in sequence or in parallel. It is therefore quite different from a flowchart, which shows the flow of control through an algorithm, allowing a reader to determine what operations will be performed, in what order, and under what circumstances, but not what kinds of data will be input to and output from the system, nor where the data will come from and go to, nor where the data will be stored (all of which are shown on a DFD).

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DATA FLOWDIAGRAM(DFD)

Data Flow Diagram

A data-flow diagram (DFD) is a graphical representation of the "flow" of data through an information system. DFDs can also be used for the visualization of data processing (structured design).

On a DFD, data items flow from an external data source or an internal data store to an internal data store or an external data sink, via an internal process.

A DFD provides no information about the timing of processes, or about whether processes will operate in sequence or in parallel. It is therefore quite different from a flowchart, which shows the flow of control through an algorithm, allowing a reader to determine what operations will be performed, in what order, and under what circumstances, but not what kinds of data will be input to and output from the system, nor where the data will come from and go to, nor where the data will be stored (all of which are shown on a DFD).

Reference: http://studentbank.in/report-data-flow-d...z11e8IkabW

presented by:
Mr.Prasad Sawant

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Data Flow Diagrams (DFDs)
• Data flow diagram (DFD) is a picture of the movement of data between external entities and the processes and data stores within a system
DFD Symbols (Gane & Sarson)
Process
• Work or actions performed on data (inside the system)
• Labels should be verb phrases
• Receives input data and produces output
Rule 1: Process
• Can have more than one outgoing data flow or more than one incoming data flow
Rule 2: Process
• Can connect to any other symbol (including another process symbol)
Process: Correct/Incorrect?
Data Store
• Is used in a DFD to represent data that the system stores
• Labels should be noun phrases
Rule: Data Store
Must have at least one incoming and one outgoing data flow
Source/Sink (External Entity)
• External entity that is origin or destination of data (outside the system)
• Is the singular form of a department, outside organisation, other IS, or person
• Labels should be noun phrases
• Source – Entity that supplies data to the system
• Sink – Entity that receives data from the system
Context Diagram
• Top-level view of IS
• Shows the system boundaries, external entities that interact with the system, and major information flows between entities and the system.
• Example: Order system that a company uses to enter orders and apply payments against a customer’s balance