File access methods.54

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File Concept, File Management, File

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Recap
In the last class, you have learnt:
•File and File operations

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Objective(s)
On completion of this period, you would be able to:

•Understand the different file access methods
• Introduction to directory

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Introduction
•Files store information
•This information must be accessed and read into
computer memory for use
•There are different access methods
•Some systems support only one access method
•Other systems support different access methods

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Types of Access Methods
• Sequential Access
• Direct Access
• Other Access Methods

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Sequential Access
•Simplest access method
•Information processed in order
•Most common
•Used by editors and compilers
•Based on a tape model of a file
•Works well on sequential access devices and
random access devices

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Sequential Access
beginning
current position
end
read or write
rewind
Fig : Sequential Access File

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Most Common Operations on Files
•Read
•Write

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File Pointer
• Whenever a file is opened for read or write
operation a file pointer is maintained to keep
track of the current position in the file

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Read operation
• Reads the next portion of the file
• Automatically advances file pointer

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Write operation
•Appends data to the end of the file
•Advances pointer to the new end of file

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Reset
•Reset to beginning

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Skip
•Skip n records forwards or backwards
( supported by few systems )

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Applications for sequential access
•Payroll of employees
•Student data processing

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Direct Access
•Also known as Relative Access
•Fixed length logical records
•No particular order for read and write
•Based on disk model of a file
•File viewed as a numbered sequence of blocks or
records
•Arbitrary block read or written

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Direct Access
•Useful for immediate access to large volumes of
data
•Most suitable for database applications
•Easy to read, write and delete a record

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•Address of arbitrary record N
Let length of logical record = L
Address of record N = L*(N-1)
Direct Access

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Simulate Sequential Access
•Easy to simulate sequential access on direct
access file
•Keep a variable cp i.e. Current position

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Sequential AccessImplementation for
direct access
reset cp := 0
read next read cp
cp := cp + 1
write next write cp
cp := cp + 1
Fig : Simulation of sequential access on a direct access file
Simulate Sequential Access

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Applications for Direct access
•Airline reservation system
•Customer account processing in a bank

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Other Access Methods
•Built on top of direct access method
•Require the construction of an index for the file
•Index is a small table stored in memory
•The index contains pointers to various blocks

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To find an entry in the file…
•Search the index
•Use the pointer to access the file directly
for the desired entry

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To find an entry in the file
Adams
Arthur
Asher
.
.
.
Smith
Smith, John SSN
Age
last name
logical record
number
index file
relative file

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Primary index and Secondary index
•With large files index file itself too large to
be in memory
•Solution is create index for the index file
•Primary index points to secondary index
•Secondary index points to actual data

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