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How to use the meta data to decouple a program from the data format --- Physical Data Independence

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• Physical Data Independence
  • Physical Data Independence:

    Physical Data Independence = the property where a computer program is independent of the physical storage structure (= record format) of the data files that the computer program is using

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• Achieving Physical Data Independence with meta data
  • The technique to overcome the problem of Physical Data Dependence is as follows:

    Create a "meta data" file that contains the description of the structure/format of the data file The format of the meta data file is fixed: Format of the Meta data: +-----------+-----------+-----------+ | fieldName | fieldType | fieldSize | +-----------+-----------+-----------+ fieldName = the name of a field (This is a String) fieldType = the data type of the filed "C" = Characters (String) "I" = Integer "F" = Floating point number or double fieldSize = the number of bytes used to store the value Example: In order to describe the structure/format of the student data file with the following format: Format of the Data (record) file: +-------------+--------------------------------+-------+ | ID | Name | level | +-------------+--------------------------------+-------+ 10 chars 30 chars 4 chars we use a meta data file with the following entries: Meta data used to describe the format: fieldName fieldType fieldSize ========================================== ID C 10 <---- field 1 of record name C 30 <---- field 2 level C 4 <---- field 3 A program that achieve physical data independence is written as follows: The program must first read the meta data file and process the structure/format description I.e.: The program must first learn about the structure of the data file that it will access !!! Using the information on the structure, the program accesses the actual data file by reading the fields in the format and size (# bytes) given in the meta data file

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  • Schematically:

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  • I will now show you an example on how to use meta data to access a file

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  • The implementation is a bit convoluted... with lots of IF-statements....

    (But the principle is very simple)

    The gist of the algorithm:

    We must first find out what is the data type of the next data item (using if-statements) Then we can read (and process it) using the correctly typed variable

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  • The file that contains the description of the format/structure of the data is called Meta Data

    (Meta data is "data about data")