Computing systems are becoming more and more complex and assuming more and more responsibilities in all sectors of human activity. Applications do not run locally on a single computer any more. A lot of today's applications are built as distributed system; with services on different computers communicating with each other. Distributed systems arise everywhere. The Internet is one of the best-known distributed systems and used by nearly everyone today. It is obvious that we are more and more dependant on computer services. Many people expect to be able to buy things like clothing or electronic equipment even at night on the Internet. Computers are expected to be operational and available 7 days a week, 24 hours a day. Downtime, even for maintenance, is no longer acceptable.