FILE ACCESS

 FILE ACCESS

• File organization is static.
• File access is dynamic.

Sequential Access

Terms

sequential access

• Access of data in order. 

• Accessing data from a file whose records are organized on the basis of their successive physical positions.

Remarks

• Sequential access processes a file from its beginning.
  
• All operating systems support sequential access of files.
• Sequential access is the fastest way to read or write all of the records in a file.
• Sequential access is slow when reading a singlt random record, since all the preceeding records must be read.

Direct Access

Terms

direct access

Access of data in arbitrary order, with variable access time.

Accessing data from a file by record position with the file, without accessing intervening records.

relative record number

An ordinal number indicating the position of a record within a file. 

Remarks

• Direct access processes single records at a time by position in the file.
  
• Mainfreme and midrange operating systems support direct access of files.
• The Windows, DOS, UNIX, and Linux operating systems do not natively support direct access of files.
• When using Windows, DOS, UNIX, and Linux operating systems applications must be programmed to use direct access.
• Direct access is slower than sequential when reading or writing all of the records in a file.
• Direct access is fast when reading a singlt random record, since the preceeding records are ignored.
• Direct access allows individual records to be read from different locations in the file without reading intervening records.
• When files are organized with fixed length records, the location of a record in a file can be calculated from its relative record number, and the file can be accessed using the seek functions.
  

•         
     ByteOffset = (RRN - 1) × RecLen
          
  
  

• When files have variable length records supported by an index, the records can be accessed directly through the index, with the use of the seek function.
  
• For direct access to be useful, the relative record number of the record of interest must be known.
• Direct access is often used to support keyed access.

Keyed Access

Terms

keyed access

Accessing data from a file by an alphanumeric key associated with each record.

key

A value which is contained within or associated with a record and which can be used to identify the record. 

Remarks

• Keyed access processes single records at a time by record key.
  
• Mainfreme and midrange operating systems support keyed access of files.
• The Windows, DOS, UNIX, and Linux operating systems do not natively support keyed access of files.
• When using Windows, DOS, UNIX, and Linux operating systems applications must be programmed to use keyed access.
• Keyed access will be covered in more detail in later chapters.