1,721,048 research outputs found
Effects of broadcast errors on concurrency control in wireless broadcasting environments
No full textIn wireless broadcasting environments, mobile clients cannot receive data reliably over broadcast channels because a reliable transmission protocol is not applicable to the channels. If such broadcast errors are not properly handled by a concurrency control algorithm, it could lead to a fatal effect especially when clients are permitted to issue update transactions. However, the effects of broadcast errors on concurrency control have been little researched. In this paper, we have proposed a concurrency control algorithm to support update transactions issued by mobile clients and evaluated the performance of the algorithm by focusing the effects of broadcast errors with an analytic model. The analytic results show our algorithm is efficient in resolving the broadcast errors. (C) 2002 Elsevier Science B.V. All rights reserved
Performance of concurrency control schemes in data warehousing systems
No full textTo speed up on-line analytical processing (OLAP), data warehouse, which is usually derived from operational databases, is introduced. When the operational databases happen to change.. the data warehouse gets stale. To maintain the freshness of data warehouse, operational database changes need to be frequently and concurrently propagated into the data warehouse. However, if several update transactions are allowed to execute concurrently without an appropriate concurrency control, data inconsistency between data warehouse and operational databases could arise due to incorrect propagation of changes on the operational databases into the data warehouse. In this paper, we propose a new concurrency control scheme, which could execute a number of update transactions in a consistent way. Whenever an update transaction tries to update a data that is being used by OLAP transactions, our scheme allows the update transaction to create a new version of the data. To investigate the applicable areas of our scheme, its performance is evaluated by means of simulation approach. Our experimental results show that the proposed scheme enables OLAP transactions to continuously read a very fresh data without wasting a lot of time to find out an appropriate version of the data from the version pool
Tightly secure transaction scheduler in multi-level secure database management systems
No full textIn multi-level secure database management system (MLS/DBMS), every transaction and data has associated with a unique security. Concurrent transactions on MLS/DBMS potentially have conflicts while accessing the shared data. The conflicts result in some of the conflicting transactions to be delayed. If the delayed transactions have been cleared at low security level, an information may flow downward with the violation of security policy. This kind of communication channel is called covert channel.
Several transaction scheduling schemes for solving the covert channel problem had been presented. Although they had solved the covert channel problem, unfortunately, they had left alone other kinds of security problems, such as the availability and the integrity problems. Since computer security consists of maintaining the confidentiality, the availability, and the integrity, an attempt to acquire every characteristic is necessary.
Tightly secure transaction scheduler, presented in this paper, schedules concurrent transactions without loss of the characteristics of security. For the confidentiality and the availability, the philosophy of the scheduler is based on the concealment of uncommitted data. Besides, for the integrity, a trustworthiness together with a recentness of a data are considered for discriminating the appropriateness of the data
OPTIMIZING JOINS BETWEEN 2 FRAGMENTED RELATIONS ON A BROADCAST LOCAL-NETWORK
No full textThe problem of joining two fragmented relations on a broadcast network is dealt with. Multiple copies of a fragment are used to increase parallelism and reduce transmission cost. A bipartite graph model is used to represent the pairs of fragments from the two relations which need to be joined. We are interested in minimizing total transmission cost needed to compute a distributed join between two fragmented relations. This join problem is formulated as a linear integer program. Two heuristics are developed for the general two-way join problem. When the corresponding join graph is restricted to tree structure, an optimal algorithm can be developed. Computational experiments are performed to analyze the performances of these heuristics
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