Ch 5: Data & Processing Models

Chapter 5: Data & Process Modeling

** EXAM CONTENT: **

• Errors: spontaneous generation, black hole, gray hole
• Processes, data flows, data stores, entities (section 5.3)
• Context diagrams, level-0, lower-level diagrams (sections 5.5, 5.6 & 5.7)
• Decision tables (5.9.4)
• Decision trees (5.9.5)

Logical vs Physical Models

• Logical Model: illustrates what a system must do, regardless of physical implementation
• Physical Model: presents how the system will be built
• Mainly follows a four-model approach, w/c develops:
  • A physical model of the current system
  • A logical model of the current system
  • A logical model of the new system
  • A physical model of the new system

Data Flow Diagrams (DFDs)

• Employ/Make use of symbols to depict how a system transforms input data into useful information
• A DFD displays the movement of data within an information system, excluding program logic or processing steps
• A set of DFDs provides a logical model, illustrating what a system does rather than how it does it.

Data Flow Diagram Symbols

• Data flow diagrams utilize four fundamental symbols that represent processes, data flows, data stores, and entities
• Several versions of DFD symbols exist
• DFD examples use Gane and Sarson symbols
• Yourdon symbols are another popular set

Process Symbols

• Processes contain business logic to transform data to the required results
• Process names identify a function by using a verb followed by a clear noun
• A process called "black box" has its inputs, output and functions known, but not the underlying logic

Data Flow Symbols

• A data flow is a path for data, allowing movement from one part of an information system to another
• A singular noun, and an adjective (if needed), should be used for the name
• A data flow in a DFD signifies data items
• Data flow is symbolized by a line, which could have a single or double arrowhead

Data Store

• A data store is used in a DFD to illustrate data that is stored by the system, which can be used later by processes
• In a DFD, the Gane and Sarson symbol for a data store is a flat rectangle open on the right and closed on the left
• A data store name uses a plural noun, adding adjectives where necessary, and must be associated with a process

Entity Symbols

• Rectagles may be shaded to look three-dimensional represent entities
• DFDs include solely the external entities providing data to the system or receiving output from it
• Entities in DFDs, also known as terminators, represent origins or final destinations of data
• An entity is called a source if supply to the system; whereas, it is a sink if it receives data

Drawing Data Flow Diagrams

• Guidelines to drawing a data flow diagram:
• Ensure the context diagram fits on one page.
• Use the information system's name as the process name in the context diagram.
• Employ names that are unique within each set of symbols.
• Avoid crossing lines.
• Provide a unique name and a reference number for each process.
• Ensure that the model is accurate, easy to understand, and addresses the needs of its intended users.

Context Diagram

• A context diagram offers a high-level view of the information system, outlining its scope and boundaries.
• Draw a context diagram, start with a single process symbol at the center representing the entire information system with the idenifier process 0
• Then place the entities around the perimeter of the page and link them to the central process using data flows.

Diagram

• It provides an overview of the components which interact to form the entire system
• It zooms in on the system, highlighting the major internal processes, data flows, and data stores, copying the entities and data flows from the context diagram
• The connections that flow into, and out of, process 0 must be retained
• A diverging data flow occurs if the same data travels to two or more locations

Lower-Level DFDs

• Leveling involves drawing increasingly detailed diagrams until functional primitives are apparent
• Balancing seeks to ensure consistency throughout DFDs by ensuring alignment of input and output data flows

Data Dictionary

• A data dictionary contains information about data, such as meaning, usage, origin, format and its connections to other data
• Data dictionaries typically include:
• Data element descriptions (also called a data item or field)
• Attribute/property definitions
• Relationship descriptions
• Rules and constraints
• Other metadata
• Each piece of data in the data dictionary requires documentation to provide comprehensive and clear information about data and processes within a system
• Data element attributes usually include what is documented in the data dictionary: name, alias, default value, type, source, security, and description
• The name or label of the data flow should have documentation in the data dictionary
• A data store should have documentation in the data dictionary

Process Description Tools in Modular Design

• A process description outlines the specifics of a functional primitive, denoting a certain sequence of processing steps and business logic
• Process descriptions in object-oriented development:
• O-O analysis synthesizes data and the processes acting on it into objects so similar objects can be grouped into classes
• O-O processes are called methods
• Modular design constructs a process from a combination of three logical structures: sequence, selection, and iteration
• Structured English uses a limited subset of standard English to clearly define a process with readability from using indentation
• A decision table shows the various conditions and outcomes systematically and creates actions based on numbered possibilities of credit approval