Abstract-One of the weaknesses of database replication protocols, compared to centralized DBMSs, is that they are unable to manage concurrent execution of transactions at different isolation levels. In the last years, some theoretical works related to this research line have appeared but none of them has proposed and implemented a real replication protocol with support to multiple isolation levels. This paper takes advantage of our MADIS middleware and one of its implemented Snapshot Isolation protocols to design and implement SIRC, a protocol that is able to execute concurrently both Generalized Snapshot Isolation (GSI) and Generalized Loose Read Committed (GLRC) transactions. We have also made a performance analysis to show how this kind of protocols can improve the system performance and decrease the transaction abortion rate in applications that do not require the strictest isolation level in every transaction.
Abstract. Whereas the Strict Snapshot Isolation (SSI) level is nowadays offered by most of the centralized DBMSs, replicated databases do not usually provide it, since the traditionally considered approach, the pessimistic management, introduces enormous performance penalties that preclude it from production applications. Instead, distributed databases usually offer Generalized Snapshot Isolation (GSI), a more relaxed level, in which a transaction may get a snapshot older than the one that was applied on the database by the time the transaction started. This paper takes advantage of our MADIS middleware and one of its implemented Snapshot Isolation protocols (SIRC) to design, implement and evaluate the performance of an extended version of SIRC (called gB-SIRC). This protocol is able to concurrently execute Generalized Read Committed (GRC), GSI, g-Bound -a non-standard SI level limiting the outdatedness of transactions wanting to commit-and optimistic SSI transactions on top of a cluster of centralized DBMSs offering RC and SSI. This work is the first implementation and evaluation of an optimistic SSI level. Although the abort rate of g-Bounded transactions is significantly higher than the GSI ones, the performance results show that introducing transactions at more restrictive levels is not detrimental to the completion time or to the abort rate of the transactions using GSI.
Wireless Sensor Networks are a rapid grown technology trying to adopt several new applications. Such applications have brought new challenges to the table and in consequence there are services that became to be critical factors, just like authenticity in WSN. Node authenticity in WSN is a commodity service that allows verifying that the message source is a node belonging to the sensor network, providing a countermeasure against node impersonation and message's injection. However, such service is considered too expensive when public key cryptography is involved. In this paper an authentication protocol for WSN is described with an estimation of costs on operations over the base field.
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