Software Engineering-The Data Dictionary

The analysis model encompasses representations of data objects, function, and control. In each representation data objects and/or control items play a role. Therefore, it is necessary to provide an organized approach for representing the characteristics of each data object and control item. This is accomplished with the data dictionary. The data dictionary has been proposed as a quasi-formal grammar for describing the content of objects defined during structured analysis. This important modeling notation has been defined in the following manner :

The data dictionary is an organized listing of all data elements that are pertinent to the system, with precise, rigorous definitions so that both user and system analyst will have a common understanding of inputs, outputs, components of stores and [even] intermediate calculations.

Today, the data dictionary is always implemented as part of a CASE "structured analysis and design tool." Although the format of dictionaries varies from tool to tool, most contain the following information:

• Name—the primary name of the data or control item, the data store or an external entity.
• Alias—other names used for the first entry.
• Where-used/how-used—a listing of the processes that use the data or control item and how it is used (e.g., input to the process, output from the process, as a store, as an external entity.
• Content description—a notation for representing content.
• Supplementary information—other information about data types, preset values (if known), restrictions or limitations, and so forth.

Once a data object or control item name and its aliases are entered into the data dictionary, consistency in naming can be enforced. That is, if an analysis team member decides to name a newly derived data item xyz, but xyz is already in the dictionary, the CASE tool supporting the dictionary posts a warning to indicate duplicate names. This improves the consistency of the analysis model and helps to reduce errors.

“Where-used/how-used” information is recorded automatically from the flow models. When a dictionary entry is created, the CASE tool scans DFDs and CFDs to determine which processes use the data or control information and how it is used. Although this may appear unimportant, it is actually one of the most important benefits of the dictionary. During analysis there is an almost continuous stream of changes. For large projects, it is often quite difficult to determine the impact of a change. Many a software engineer has asked, "Where is this data object used? What else will have to change if we modify it? What will the overall impact of the change be?" Because the data dictionary can be treated as a database, the analyst can ask "where used/how used" questions, and get answers to these queries.

The notation used to develop a content description is noted in the following table:

Data Construct              Notation                 Meaning
                                          =                      is composed of
Sequence                            +                       and
Selection                           [ | ]                     either-or
Repetition                          { }n                    n repetitions of
                                         ( )                     optional data
                                       * ... *                  delimits comments

The notation enables a software engineer to represent composite data in one of the three fundamental ways that it can be constructed:

1. As a sequence of data items.
2. As a selection from among a set of data items.
3. As a repeated grouping of data items. Each data item entry that is represented as part of a sequence, selection, or repetition may itself be another composite data item that needs further refinement within the dictionary.

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