Instance-Independent View Serializability for Semistructured Databases

Semistructured databases require tailor-made concurrency control mechanisms since traditional solutions for the relational model have been shown to be inadequate. Such mechanisms need to take full advantage of the hierarchical structure of semistructured data, for instance allowing concurrent updates of subtrees of, or even individual elements in, XML documents. We present an approach for concurrency control which is document-independent in the sense that two schedules of semistructured transactions are considered equivalent if they are equivalent on all possible documents. We prove that it is decidable in polynomial time whether two given schedules in this framework are equivalent. This also solves the view serializability for semistructured schedules polynomially in the size of the schedule and exponentially in the number of transactions

A Concurrency Control Method Based on Commitment Ordering in Mobile Databases

Disconnection of mobile clients from server, in an unclear time and for an unknown duration, due to mobility of mobile clients, is the most important challenges for concurrency control in mobile database with client-server model. Applying pessimistic common classic methods of concurrency control (like 2pl) in mobile database leads to long duration blocking and increasing waiting time of transactions. Because of high rate of aborting transactions, optimistic methods aren`t appropriate in mobile database. In this article, OPCOT concurrency control algorithm is introduced based on optimistic concurrency control method. Reducing communications between mobile client and server, decreasing blocking rate and deadlock of transactions, and increasing concurrency degree are the most important motivation of using optimistic method as the basis method of OPCOT algorithm. To reduce abortion rate of transactions, in execution time of transactions` operators a timestamp is assigned to them. In other to checking commitment ordering property of scheduler, the assigned timestamp is used in server on time of commitment. In this article, serializability of OPCOT algorithm scheduler has been proved by using serializability graph. Results of evaluating simulation show that OPCOT algorithm decreases abortion rate and waiting time of transactions in compare to 2pl and optimistic algorithms.Comment: 15 pages, 13 figures, Journal: International Journal of Database Management Systems (IJDMS