What is file organization? What are the various file organizational methods?

File organization refers to the relationship of the key of the record to the physical location of that record in the computer file.
File organization may be either physical file or a logical file. A physical file is a physical unit, such as magnetic tape or a disk.
A logical file on the other hand is a complete set of records for a specific application or purpose.
A logical file may occupy a part of physical file or may extend over more than one physical file.

The objectives of computer based file organization:

Ease of file creation and maintenance

Efficient means of storing and retrieving information.

The various file organization methods are:

Sequential access.

Direct or random access.

Index sequential access.

The selection of a particular method depends on:

Type of application.

Method of processing.

Size of the file.

File inquiry capabilities.

File volatility.

The response time.

1. Sequential access method: Here the records are arranged in the ascending or descending order or chronological order of a key field which may be numeric or both. Since the records are ordered by a key field, there is no storage location identification. It is used in applications like payroll management where the file is to be processed in entirety, i.e. each record is processed. Here, to have an access to a particular record, each record must be examined until we get the desired record.
Sequential files are normally created and stored on magnetic tape using batch processing method.

Advantages:

Simple to understand.

Easy to maintain and organize

Loading a record requires only the record key.

Relatively inexpensive I/O media and devices can be used.

Easy to reconstruct the files.

The proportion of file records to be processed is high.

Disadvantages:

Entire file must be processed, to get specific information.

Very low activity rate stored.

Transactions must be stored and placed in sequence prior to processing.

Data redundancy is high, as same data can be stored at different places with different keys.

Impossible to handle random enquiries.

2. Direct access files organization: (Random or relative organization). Files in his type are stored in direct access storage devices such as magnetic disk, using an identifying key. The identifying key relates to tits actual storage position in the file. The computer can directly locate the key to find the desired record without having to search through any other record first. Here the records are stored randomly, hence the name random file. It uses online system where the response and updation are fast.

Advantages:

Records can be immediately accessed for updation.

Several files can be simultaneously updated during transaction processing.

Transaction need not be sorted.

Existing records can be amended or modified.

Very easy to handle random enquiries.

Most suitable for interactive online applications.

Disadvantages:

Data may be accidentally erased or over written unless special precautions are taken.

Risk of loss of accuracy and breach of security. Special backup and reconstruction procedures must be established.

Les efficient use of storage space.

Expensive hardware and software are required.

High complexity in programming.

File updation is more difficult when compared to that of sequential method.

3. Indexed sequential access organization: Here the records are stored sequentially on a direct access device i.e. magnetic disk and the data is accessible randomly and sequentially. It covers the positive aspects of both sequential and direct access files.
The type of file organization is suitable for both batch processing and online processing.
Here, the records are organized in sequence foe efficient processing of large batch jobs but an index is also used to speed up access to the records.
Indexing permit access to selected records without searching the entire file.

Advantages:

Permits efficient and economic use of sequential processing technique when the activity rate is high.

Permits quick access to records, in a relatively efficient way when this activity is a fraction of the work load.

Disadvantages:

Slow retrieval, when compared to other methods.

Does not use the storage space efficiently.

Hardware and software used are relatively expensive.

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