Backups : Real-time Processing

The consequences of failure in many real-time systems (e.g. flying a missile/passenger plane, running a nuclear power station) are immense. There is not a lot of point in keeping backup copies of data in a real-time system as there is unlikely to be time to restore backups if anything goes wrong. When a real-time system is developed the emphasis is placed on ensuring that the system will not go wrong rather than on designing a backup method to restore the system if it does go wrong.

Therefore most real-time systems incorporate a lot of spare hardware. Having much more hardware than you need to do a job means that if some hardware fails you can carry on doing the job. For example three identical computers may be used to decide which direction a missile should fly in. The computers each have a "vote" on which direction the missile should fly in. Because the computers are identical all three will normally agree on which direction to take. If one computer goes wrong then it will be outvoted by the two working computers so the missile will still function properly despite the failure. If two computers go wrong you are in trouble.

Sometimes backing up data to disk can be useful in real-time systems. Although this data could not be used to put the system back into its proper state, it could be used later to analyse why a system failed. The "black box" that records flight data in an aircraft is an example of this type of system.

GCSE ICT Companion 04 - (C) P Meakin 2004