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WJEC CG3 1.4

Topic 4: File Organisation

Hashing algorithm finds the position on the disc from the key field (input); uses an algorithm to calculate / determine disc address.
Overflow area is necessary if this address is already occupied by data; data in the overflow area is usually stored and searched in linear order.
Indexed sequential file An indexed sequential file stores records in key sequence order and an index allows data to be accessed directly. The key field and index are used to locate the correct position, allowing faster access as the data can be accessed directly.
Archive file Archiving is the process of storing data/files which are no longer in current/frequent use. It is held for security/legal/ historical reason / as backup. Frees up resources on the main computer system / allows faster access to in-use data.
Multilevel index A multilevel index arises where this index is a main index which itself contains a range of addresses and the location of the next level index. This process may extend to several levels, with the last index containing the physical address of the record.
3 Generation file backup system A (three) generation file backup system involves storage of several (three) of the most recent versions of master file (condone file / data) (+ transaction file). Useful if one version is corrupted: the previous version(s) is still available.
Encrypted data Encrypted data is more secure – cannot easily be read (without the decryption key/process).
Encryption key The key is the secure binary number (known only to the sender and recipient); Can only decrypt data if this key is known.
Random access file is one where the physical location of the record is calculated from the data in the key field. Sometimes, a data collision occurs. In these circumstances, there needs to be overflow areas where the latest data is stored.
Sequential file (additional) Make a new copy of the records until in the correct place to add the new record; Add the new record to the new copy; Continue until the end of the file; If multiple records to be added, these should preferably be sorted before the above.
Sequential file (Deletion) Make a new copy of the records until in the correct place for deletion; Do not copy the record to be deleted; Continue until the end of the file; If multiple records to be deleted, these should preferably be sorted before the above.
Transaction log is used with on-line updating - stores all the update data; It can be used in case of failure - could restore data by being combined with previous master/backup file, with minimal data loss.
What is an Algorithm? An algorithm is a finite set of rules or instructions to solve a specific problem.
Created by: mcounsell