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

Dynamics and Topology of S-gap Shifts

Let $S=\{s_i\in\mathbb N\cup\{0\}:0\leq s_i<s_{i+1}\}$ and let $d_{0}=s_{0}$ and $\Delta(S)=\{d_{n}\}_{n}$ where $d_{n}=s_{n}-s_{n-1}$. In this note, we show that an $S$-gap shift is subshift of finite type (SFT) if and only if $S$ is finite or cofinite, is almost-finite-type (AFT) if and only if $\Delta(S)$ is eventually constant and is sofic if and only if $\Delta(S)$ is eventually periodic. We also show that there is a one-to-one correspondence between the set of all $S$-gap shifts and $\{r \in \mathbb R: r \geq 0\}\backslash \{\frac{1}{n}: n \in {\mathbb N}\}$ up to conjugacy. This enables us to induce a topology and measure structure on the set of all $S$-gaps. By using this, we give the frequency of certain $S$-gap shifts with respect to their dynamical properties.Comment: This paper has been withdrawn due to a flaw in Theorem 3.2. The correct version with some minor results will be replace

Abrasion Resistance of Ag/SiO2/PA6 Nanocomposite Fabrics

In recent decades, polymeric nanocomposites have been widely used in chemical, automobile and aero industries due to their low weight and smoothness. Abrasion resistance is a vital property in variable ap-plications of polymeric composites. Here, to investigate the wear resistance of nylon 6 nanocomposite fab-rics, melt spun yarns were filled with different concentrations of Ag/SiO2 nanoparticles and the wear prop-erties of weft knitted nanocomposite and pure samples were evaluated. Results disclosed that the wear re-sistance of composite fabrics containing nano-particles increased considerably in comparison to pure one. Therefore, nanoparticles can play an important role in improving the nanocomposite wear resistance. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3563