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Data DictionaryWhat does “Backordered item” mean?
What does “New Customer info.” contain?
How does the “account receivable report” look like?
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Data Dictionary
• A list of names used in the system modelsarranged alphabetically and their meaningand information about them
• Types of information provided: – Description of data
– If composite data, describe elements
– Data representation or type – Creator, creation date, user contacts
– Other names for the data (aliases)
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Data Dictionary
• Data dictionary is a metadata
• Created in parallel with DFD
• Created in top-down approach
• Data dictionary should be kept up-to-datewith the DFD model
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Data Dictionary
• Advantages: – Documentation for the whole system
– Eliminate redundancy in a system which has been
created by different people – Identify aliases
– Provide a starting point to develop reports andscreens
– Validates the data flow diagrams for completenessand accuracy
– Develop the logic for DFD processes
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Data Dictionary Contents
Data dictionaries contain:
– Data flow.
– Data structures.
– Elements.
– Data stores.
– Process
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Data flow Description
• ID number
• Name (as it appears in DFD)
• Description
• Source• Destination
• Type (file, screen, report, form, internal)
• Data structure name• How frequently produced
• Comments
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Defining Data Flow (Continued)
– The source of the data flow
• This could be an external entity, a process, or adata flow coming from a data store.
– The destination of the data flow – Type of data flow, either:
• A record entering or leaving a file.
• Containing a report, form, or screen.
• Internal - used between processes.
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Data Store
• ID number• Name• Alias name
• Description• Type (manual, computerized)• File format (database, sequential, Indexed)• Maximum/ average number of recording
– This helps the analyst to predict the amount of diskspace required.
• File name• Data structure
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Data Structure
• For compositional data flow and data storeelements
• Describe the elements of the data structureusing algebraic notation
= Composed of
{ } Repetition
( ) Optional+ and
[ / ] or
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Data Structure
• Example:Student record= student ID +
Student name+
Address +Date of birth +
Gender +
Nationality +
Telephone number +
(mobile telephone number)+
{ course record}
Course record= course name +Course number+Grade
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Data Elements
• Each element in the data structure should bedescribed – ID number
– Name
– Alias
– Description
– Type (Base: entered from user, derived: computed)
– Length (in characters) [max or fixed]
– Data type (date, numeric, alphanumeric)
– Input/Output format
– Validation criteria
– Default value
– Comments
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Defining Elements
– Aliases, which are synonyms or other namesfor the element
– These are names used by different users within
different systems – Example, a Customer Number may be called a:
• Receivable Account Number.
• Client Number.
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Defining Elements
• A base element is one that has beeninitially keyed into the system.
– Input from an external entity in the DFD
• A derived element is one that is created bya process, usually as the result of acalculation or some logic.
– Output from a process in the DFD
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Data Store Definition
• If the file is computerized, the file formatdesignates whether the file is a database fileor the format of a traditional flat file.
• The maximum and average number ofrecords on the file
• The growth per year
– This helps the analyst to predict the amount ofdisk space required.
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Data Store Definition
• The data set name specifies the table orfile name, if known.
– In the initial design stages, this may be left
blank.
• The data structure should use a namefound in the data dictionary.
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Logical and Physical data
• Logical data structures is what the usersees
• Physical data structure is what thesoftware needs for processing
– (e.g. flags, key fields for search, ..)
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Using the Data Dictionary
Data dictionaries may be used to:
– Create reports, screens, and forms.
– Generate computer program source code.
– Analyze the system design for completion andto detect design flaws.
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Creating Reports, Screens,Forms
To create screens, reports, and forms:
– Use the element definitions to create fields.
– Arrange the fields in an aesthetically pleasing
screen, form, or report, using designguidelines and common sense.
– Repeating groups become columns.
– Structural records are grouped together onthe screen, report, or form.
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Data Dictionary Analysis
• The data dictionary may be used inconjunction with the data flow diagram toanalyze the design, detecting flaws and
areas that need clarification.
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Data Dictionary Analysis(Continued)
• Some considerations for analysis are:
– All base elements on an output data flow mustbe present on an input data flow to the
process producing the output. – Base elements are keyed and should never
be created by a process.
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Process Data• Process number
• Process name
• Description
• Input
• Output• Type (batch, online, manual)
• Prewritten code: ready made functional libraries
• Process logic (Business rules)
• Reference to description tables or structures
• Unresolved issues: (notes to check with users)
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Process Specifications
• Process specifications are created for primitiveprocesses on a data flow diagram.
• They are also called minispecs.
• Advantages – Understand how the process works and go back to
users if not clear
– Describe what should happen for designer and
programmer
– Validate the DFD
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Structured English
• based on structured logic and Simple Englishstatements
• Appropriate technique for analyzing the system
when structured decisions are not complex. – Express all logic in terms of sequential structures,decision structures, case structures, or iterations.
– Use and capitalize accepted keywords such as IF,THEN, ELSE, DO, and PERFORM.
– Indent blocks of statements to show their hierarchy. – Underline words that have been described in the data
dictionary
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Describing Processes
• Example: Bonus monthly payment (StructuredEnglish)
For each customer
Read Customer recordFor each item rented between 1/10 to 1/11
add total
End for
If total is greater than or equal to 50$ then
send bonus and thank you letter
End if
End for
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Automatic Data Dictionary
• Using CASE tools:
– Easy to change and add
– Created and integrated with other models
– Checks that the DFD is valid
• e.g. all data needed for a process is input to it
• Data store contains data input/ output to/from it
• Derived data should be output from process• Checks data source and destination in DFD
– Use data structures to create reports andscreens
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