The copar service: Combining optimism and pessimism in accessing replicas

In order to increase availability in a distributed system some or all of the data items are replicated and stored at separate sites. This is an issue of key concern especially since there is such a proliferation of wireless technologies and mobile users. 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. We have built a distributed service that manages updates to widely deployed counter-like replicas. The service is built on our distributed concurrency control scheme which combines optimism and pessimism in the processing of transactions. The service 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 to ensure mutual consistency.