DBMS Q1L4

Lesson 4: DATA FLOW DIAGRAM

Data Flow Diagram

A graphic picture of a logical system from the viewpoint of its data.

Uses of a Data Flow Diagram

It helps Analyst organize information about a system.

Physical Data Flow Diagrams

Show how the current system flows.

Logical Data Flow Diagrams

Show the data flow, structure, and requirements of a new system.

System

A group of interrelated procedures used for a business function, with an identifiable boundary, working together for some purpose.

Analysis

Separation of a whole into its component parts.

Design

To create, fashion, execute, or construct according to plan.

Data Flow Diagrams Symbols

Consists of symbols for process, data store, source/sink, and data flow.

Yourdon/DeMarco Symbols

Circle (Process), Up-Down Lines (Data Store), Rectangle (Source/Sink), Arrow (Data Flow)

Gane & Sarson Symbols

Box with titular space (Process), Up-down Lines with titular space (Data Store), Rectangle (Source/Sink), Arrow (Data Flow)

Entity

Helps to establish the boundaries of the system.

Source

Identifies the origin of data inflow to the system.

Sink

Identifies the outflow of a system, many times as information.

Process

Activities (manual or automated) that transform the inputs, transport data from process to process, stores the data, and produce the outputs of a system.

Law of conservation of data

1. Data stays at rest unless moved by a process.

2. Processes cannot consume or create data.

   • Must have at least 1 input data flow (to avoid miracles)

   • Must have at least 1 output data flow (to avoid black holes)

   • Should have sufficient inputs to create outputs (to avoid gray holes)

Logical process models

Omit any processes that do nothing more than move or route data, thus leaving the data unchanged.

Data Store

The resting place of the data in a system.

Data Flow

The data is in motion.

Creating DFD

Iterative process of gradual refinement.

Process Rules

• No process can have only outputs (a miracle)

• No process can have only inputs (black hole)

• A process has a VERB phrase label

Data Store Rules

• Data cannot be moved directly from one store to another

• Data cannot move directly from an outside source to a data store

• Data cannot move directly from a data store to a data sink

• Data store has a NOUN phrase label

Entity Rules

• Data cannot move directly from a source to a sink

• A source/sink has a NOUN phrase label

Data Flow Rules

• A data flow has only one direction of flow between symbols.

• A data flow cannot go directly back to the same process it leaves.

• A data flow to a data store means update.

• A data flow from a data store means retrieve or use.

• A data flow has a NOUN phrase label.

Basic Rules that Apply to all DFDs

• Inputs to a process are always different than outputs.

• Objects always have a unique name.

• In order to keep the diagram uncluttered, you can repeat data stores and sources/sinks on a diagram.

Types of Diagrams

Context Diagram, Level 0, Level 1

Context Diagram

A data flow diagram (DFD) of the scope of an organizational system that shows the system boundaries, external entities that interact with the system and the major information flows between the entities and the system.

Level-0 Diagram

A data flow diagram (DFD) that represents a system’s major processes, data flows and data stores at a high level of detail.

Steps in Creating Data Flow Diagrams

1. Create a list of activities.

2. Construct Context Level DFD (identifies sources and sink).

3. Construct Level 0 DFD (identifies manageable sub processes).

4. Construct Level 1- n DFD (identifies actual data flows and data stores).