DEPENDABLE SYSTEMS ARCHITECTURES:
The simplest realization of a dependable architecture is in replicated servers, where two or more servers carry out the same task. Requests for processing are channeled through a server management component that routes each request to a particular server. This component also keeps track of server responses. In the event of server failure, which is usually detected by a lack of response, the faulty server is switched out of the system. Unprocessed requests are resubmitted to other servers for processing. This replicated server approach is widely used for transaction processing systems where it is easy to maintain copies of transactions to be processed. Transaction processing systems are designed so that data is only updated once a transaction has finished correctly so delays in processing do not affect the integrity of the system.
It can be an efficient way of using hardware if the backup server is one that is normally used for low-priority tasks. If a problem occurs with a primary server, its processing is transferred to the backup server, which gives that work the highest priority. Replicated servers provide redundancy but not usually diversity. The hardware is usually identical and they run the same version of the software. Therefore, they can cope with hardware failures and software failures that are localized to a single machine. They cannot cope with software design problems that cause all versions of the software to fail at the same time. To handle software design failures, a system has to include diverse software and hardware.
