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ABSTRACT

In a distributed system some or all of the data items are replicated and stored at separate sites. This increases the availability of these items and it is then possible to complete transactions faster than in a single site system. However, the concurrent processing of transactions at separate sites can generate inconsistencies in the stored information. Several replica control techniques have been proposed to deal with the processing of transactions . Some of these techniques include certain consistency checks before processing the transaction, i.e. they are pessimistic in nature. Other techniques process the transaction and do the consistency checking afterwards, i.e. they are optimistic.

The system discussed here is an attempt to show how the increased processing capabilities provided by (i) multi-threaded object-oriented programming, and by (ii) powerful computing nodes in the distributed system can be used to combine optimism and pessimism into one system. The system allows a transaction to be processed immediately (optimistically) at any individual replica as long as the transaction satisfies a cost bound. All transactions are also processed in a concurrent pessimistic manner. Only the changes made to the replicas via pessimistic processing are made permanent.

Keywords: Distributed system, replica management, concurrency control, availability. Optimistic processing, pessimistic processing.

INTRODUCTION

The system under study contains a collection of computing nodes connected by a communications network. The information that transactions access can have any level of replication across the nodes. Transactions can originate at any node and the transaction processing system attempts to treat all transactions in a uniform manner through cooperation among the nodes.

One of the main reasons for replicating the data in distributed systems is to increase availability. However, this replication increases the need for effective control measures to preserve some level of mutual consistency. Several replica control techniques have been proposed to deal with this issue and these techniques are described to

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