Studying fo the Efficiency Lmproving Technique of the Highly Available System.

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It is very important to implement highly available systems which make users unaware of temporal stoppages generated by software errors and hardware failures. Generally, hardware mechanisms are used to obtain highly availabilities. By using software techniques, following advantages are obtained.1. Temporal stoppages caused by software bugs can be handled.2. We can construct highly available systems economically because no special hardware are required.As there are some overheads which are required to synchronous redundant computation and to compare the result of each computation, the system efficiency is dropped. In this research, we have developed the concurrency control mechanism to improve the efficiency of the highly available system by using redundant computations. The following shows our results.1. Concurrency control for redundant computations. We recuce the overhead to synchronize redundant computations by executing redundant computations asynchronously as much as possible. We h … More ave shown that deadlocks and rollbacks are reduced by executing sequence of operations in the later system in the same order which is executed in the faster system, and we have developed Multi-Wait Two- Phase Locking Mechanism by extending this idea.2. Reduction of the degree of redundancy by dual computation systems. It is possible to detect only errors of computations by dual computation and the system efficiency can be improved by knowing the sequence of operations. Thus we have implemented the concurrency control mechanism for the following dual computation system. First, operations are executed in one of two systems and the system sends the sequence of valid operations to another system. Second, another system executes the sequence of operations sent in the first phase.3. Recovery from the failure state. Normally, wer use recovery mechanisms used in the most serious failure for all type of failures in order to simplify the recovery algorithms. We have improved the system efficiency by selecting recovery mechanisms depending on type of failures. Less