51 terms

System analysis and design Ch 5

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a logical model shows
what the system must do, regardless of how it will be implemented physically.
A physical model
describes how the system will be constructed
A data flow diagram (DFD)
uses various symbols to show how the system transforms input data into useful information
A process
receives input data and produces output that has a different content, form, or both.
Processes contain the business logic
also called business rules, that transform the data and produce the required results.
A data flow
is a path for data to move from one part of the information system to another.
A data flow in a DFD
represents one or more data items
Spontaneous generation process
it has no input
black hole
process that has input, but produces no output
gray hole
at least one input and one output, but the input obviously is insufficient to generate the output shown.
the symbol for an entity
is a rectangle, which may be shaded to make it look three-dimensional. The name of the entity appears inside the symbol.
A DFD shows
only external entities that provide data to the system or receive output from the system.
DFD entries also are called
terminators because they are data origins or final destinations.
Systems analysts call an entity that supplies data to the system a ____, and an entity that receives data from the system a _____.
source, sink
A context diagram
is a top=level view of an information system that shows the system's boundaries and scope.
To draw a context diagram
you start by placing a single process symbol in the center of the page. The symbol represents the entire information system, and you identify it as process 0.
When you expand the context diagram into DFD diagram 0
you must retain all the connections that flow into and out of process 0.
If processes must be performed in a specific sequence
you document the information in the process descriptions, not in the DFD.
A diverging data flow
is a data flow in which the same data travels to two or more different locations. In that situation, a diverging data flow is the best way to show the flow rather than showing two identical data flows, which could be misleading.
When you explode a DFD
the higher-level diagram is called the parent diagram, and the lower level diagram is referred to as the child diagram.
A functional primitive
is a process that consists of a single function that is not exploded further.
Deciding whether to explode a process further or determine that it is a functional primitive is a matter of
experience, judgment, and interaction with programmers who must translate the logical design into code.
Leveling also is called
exploding, partitioning, or decomposing.
The DFDs also are leveled
because each internal process is numbered to show that it is a child of the parent process
A data dictionary, or data repository
is a central storehouse of information about the system's data.
An analyst uses the data dictionary
to collect, document, and organize specific facts about the system, including the contents of data flows, data stores, entities, and processes.
A data element
also caled a data item or field, is the smallest piece of data that has meaning within an information system.
Data elements are combined
into records, also called data structures. A record is a meaningful combination of related data elements that is included in a data flow or retained in a data store.
The more complex the system, the more difficult it is to
maintain full and accurate documentation. Fortunately, modern CASE tools simplify the task.
A CASE repository ensures
data consistency
Alias
any names other than the standard data element name
specifications of the data element's
domain, which is the set of values permitted for the data element; these values either can be specifically listed or referenced in a table, or can be selected from a specified range of values.
each data flow represents a group of
related data elements called a record or data structure.
Volume and frequency
describes the expected number of occurrences for the data flow per unit of time.
You must document every
DFD data store in the data dictionary
In addition to describing each data element, data flow, data store, record, entity, and process
the data dictionary documents the relationships among these components. You can obtain many valuable reports from a data dictionary.
A process description
documents the details of a functional primitive, and represents a specific set of processing steps and business logic.
Typical process description tools include
structured english, decision tables, and decision trees. when you analyze a functional primitive, you break the processing steps down into smaller units in a process called modular design.
Each logical structure must
have a single entry and exit point
The three structures are called
sequence, selection, and iteration
Sequence
the completion of steps in sequential order
Selection
The completion of one of two or more process steps based on the results of a test or condition
Iteration
the completion of a process step that is repeated until a specific condition changes.
Iteration is also called
looping
Structured english might look familiar to programming students because it resembles
Pseudocode
A decision table is a
logical structure that shows every combination of conditions and outcomes
If a process has a single condition, there are only two possibilites
yes or no
Decision trees show the
logic structure in a horizontal form that resembles a tree with the roots at the left and the branches to the right
during the systems design phase
you create a physical model of the new information system that follows from the logical model and involves operational tasks and techniques.
Many analysts follow a
four-model approach, which means that they develop a physical model of the current system, a logical model of the current system, a logical model of the new system, and a physical model of the new system.
the only disadvantage of the four-model approach is the
added time and cost
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