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مقاله حاضر بر تکنیک های کنترل همروندی اخیر مثل قفل گذاری احتکاری سنکرون توزیع شده، قفل گذاری احتکاری غیر سنکرون توزیع شده، رویکرد صف ارزیابی و رویکرد مبتنی بر مهر تاکید کرده است. از میان آنها، رویکرد مبتنی بر مهر برای ارزیابی تراکنش در سمت سرور، از لحاظ زمان انتظار، تاخیر، عملکرد، تعداد تبادلات پیام به نظر بهترین می رسد. همچنین این روش، زمان شروع مجدد را کاهش و کارایی را بهبود می بخشد. 

عنوان انگلیسی:Comparative Study of Concurrency Control Techniques in Distributed Databases~~en~~

In distributed databases, there are multiple sites involved on which the data is stored. Hence concurrency control is necessary to ensure reliability and consistency of transactions on these databases [1]. Database is inconsistent when transactions are in deadlock. Therefore concurrency control is needed to maintain database in consistent state. For concurrency control serializability is the most important criterion. The serializability ensures the correctness of the database by converting conflict equivalent schedule to a serial schedule [1]. The basic concurrency control methods in distributed system are: two phase locking (2PL), where transaction obtain lock on data item when they read and convert this lock to write when they need to update it. In wound wait (WW) rather than waiting for the information from all sites deadlocks are prevented by use of timestamps. The third method is Basic Timestamp Ordering. Like WW it employ transaction startup timestamp but use it differently. Distributed certification is operated by exchanging certification information. In all above methods there is some point of deadlock situation formed between the executions of operations. Hence some advance concurrency control methods are proposed like Speculative locking [2], Validation Queue [3], Stamp based [4]. In validation queue transactions are validated on client side and also on server side. In Stamp based, validation is done by matching stamp values. In speculative locking, validation is based on status of preceding transaction. But these methods vary in terms of performance, delay, waiting time and number of message exchanges. Rest of the paper is organized as follows section II describes related work; Section III shows promises of distributed databases; section IV describes distributed concurrency control algorithms; section V gives comparative analysis; finally conclusion is done in section VI. II. RELATED WORK There are many techniques proposed for controlling the concurrent transactions in distributed databases apart from the basic techniques. In [2] Mohit Goyal, T. Ragunathan and P. Krishna Reddy proposed the Speculative locking protocols for distributed environment. Fahren Bukhari and Santosh Shrivastava [3] proposed ROCC (Read Commit Order Concurrency Control) scheme. There are two validation queues, one CVQ (Cache validation queue) which is located at the client side and maintained by local cache manager and another is SVQ (Server validation queue) located at server and maintained by scheduler component. In [4], priority protocol is explained by implementing a timestamp where another value of flag is added and this value doesn’t change unless transaction update has been successfully committed. Atul Adya, Robert Gruber Barbara, Liskov Umesh Maheshwari [5] describes an efficient optimistic concurrency control scheme for use in distributed database systems in which objects are cached and manipulated at client machines while persistent storage and transactional support are provided by servers. The scheme provides both serializability and external consistency for committed transactions; it uses loosely synchronized clocks to achieve global serialization. In [6] T. Ragunathan, P. Krishna Reddy have given semantics-based high performance asynchronous speculation based protocol to improve parallelism among Update transactions and read only transactions. In [7] the approach reduces waiting time for read only transactions and improves its performance. Speculation based locking along with synchronous approach is suggested in [8]. In [9] T. Ragunathan, P. Krishna Reddy, and Mohit Goyal propose semantics- based high performance asynchronous speculation based protocol to improve parallelism among Update transactions and read only transactions. In [10] general concurrency control algorithms in the distributed environment is proposed. These includes the locking algorithms, time stamp algorithm and optimistic algorithm. Arun Kumar Yadav and Ajay Agarwal also proposed the transaction processing in distributed environment. Kamal Solaiman, Graham Morgan [11] suggested optimistic algorithm for transactions resides on resource constraint system. III. PROMISES OF DDBS There are many of the advantages of DDBS. These are basic for achieving concurrency in databases. All of these can be viewed as promises of DDBS [1]. These are: A. Transparent Management of relational and distributed data . Transparency ranges from higher system semantics to lower implementation issues. Its advantage is to provide higher level support in the development of complex applications. B. Reliability through distributed transaction. DDBS improves reliability of data by the use of data replication and thereby reducing risk of single point of failure. In DDBS some of data may be unreachable in this case with proper care user are permitted to access data from other part of distributed database. C. Improved performance. Distributed databases first fragment conceptual database thereby enabling data stored in closed proximity to its point of use.

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