6.1.4+Reliability+of+Systems

=**__Reliability of Systems__**=
 * The reliability depends strongly on the quality and reliability of the input data. Bad input data -> bad results. (correctness of data = integrity of data)
 * Ex.: If wrong data is fed to a X-ray machine that is supposed to treat cancer, the dose of X-rays might end up being too high and the patient could possibly die.
 * The more complex & autonomous a system is, the higher the chance of mistakes happening
 * Possible threats to various systems:
 * Hardware & other system failures
 * Ex. monitoring of nuclear power stations or air traffic control: these types of systems aren't allowed to fail under any circumstance.
 * As security against such failures two, sometimes even three systems run parallel, where one system jumps in for the other, if the other fails.
 * Hackers - could alter or remove data
 * Theft of physical storage media and hardware
 * Floods and fires
 * Other security threats such as unauthorized copying or accessing networks remotely
 * Ways to protect systems & data
 * Passwords & user privileges
 * Low-level users might only be able to read data, not write or change anything
 * Ex. check-out persons in super markets usually don't have the rights to change prices or undo errors; they have to call the supervisor whenever there is a problem
 * Passwords have to be safe for sufficient security (min. 6 characters, letters, numbers, symbols, not easy to guess)
 * Physical security
 * Servers & computer systems might be locked up in special rooms, requiring keys, key cards or special codes for access
 * Encryption
 * Data might be encrypted, especially when transmitted over networks, to increase the difficulty of reading the data.
 * Backup systems
 * Backups are usually stored on a separate physical device, for protection against hardware failures
 * **3 types of backup**
 * **Full Backup**— backup of all files in your critical file set.
 * This is usually only done in longer time intervals, since it requires the most disk space and time from all backup types.
 * **Incremental Backup**— backup of all changes since last (full or incremental) backup. The second incremental would only include changes made since the last incremental backup, and so on.
 * The advantage is that it takes up only little space and is very fast. However, when recovering the backups, all incremental backups since the last full backup have to be recovered at once, to restore the system.
 * **Differential Backup**— backup of all changes since last full backup. Saves the changes that happened on the first day, The second day saves all the changes from the first and second day since the full back up. Third day it saves the changes from the first, second, and third day.
 * The advantage is, that it is faster and requires less disk space than a full backup. Also, when restoring the system, only the latest backup has to be restored. However, over time differential backups grow in size significantly, if no full backup is created in the meanwhile.
 * Some more information can be found here: http://www.imation.com/smb/tips_backup_plans.html [Sorry, dead link :/]
 * Raid systems
 * Using several physical hard disk drives to backup data (for more information look here: http://searchstorage.techtarget.com/sDefinition/0,,sid5_gci214332,00.html)

Created by Jocbe Last update: 16. November 2010

Sources:
 * Jones, Richard. Computer Science Java Enabled. Victoria: IBID Press, 2004.