فایل ورد کامل یک رویکرد خوشبینانه در کنترل همروندی پایگاه داده توزیع شده

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تعداد صفحات این فایل: ۱۴ صفحه

در خاتمه، جدید بودن و تازگی روش پیشنهادی در تخصیص یک فیلداضافی از تیپ عدد صحیح به هر رکورد در DBMS بلادرنگ به منظور آدرس دهی نشان داده شده است که در قسمت بالا به عنوان فیلد مهر یا استامپ خوانده شد، این مهر می تواند رقم اولیه صفر را نگهداشته و با هر بار به روزرسانی موفق روی رکورد، یک واحد افزایش یابد. با این حال مسائل پیچیدگی در صورتی بروز می کند که نشان داده شود فرایند بعد از هر ارتکاب مجبور است مهرش را ازسمت کاربر به سرور به روز رسانی نماید که این مسئله باعث ایجاد سربار می شود. از طرف دیگر، روش پیشنهادی، زمان های شروع مجدد مورد نیاز را کاهش و کارایی را بهبود می بخشد.
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عنوان انگلیسی:An Optimistic Approach in Distributed Database Concurrency Control~~en~~

The Concurrency Control Problem is the activity of coordinating concurrent accesses to a Database in a multi user Management System (DBMS). Concurrency control authorizes users to access a database in a multiple programmed fashion while preserving the illusion that each user is executing alone on a dedicated system [1, 3]. The main technical difficulty in achieving this goal is to prevent database updates performed by one user from interfering with database retrievals and updates performed by another user. The concurrency control problem is worsened in a distributed (DDBMS) because (1) users may access data stored in many different computers in a distributed system, and (2) a concurrency control mechanism at one computer cannot instantly be acknowledged. As Distributed database concurrency control is one of the most complex and challenging areas in database systems [2]. It is also of great significance, where real-time software is necessary to all reliable and critical applications of concurrency control in distributed database. Real-time systems play a dominant role here, where too many applications involve real-time tasks, which often carry significant drawbacks in terms of cost and loss time. In recent studies, a lot of research work has been dedicated to the concurrency controls methods and mechanisms as they are highly required to maintain Real-time distributed database reliability and stability as they also chain several features from real-time applications used in distributed database that assist (1) the description of data (metadata), (2) the preservation of correctness and integrity of the data, (3) efficient access to the data, and (4) the correct accomplishments of transactions despite concurrency and failures [4,5,6 and 12]. Traditionally, databases deal with persistent data. Main transactions that access data while preserving its consistency. The goal of transaction and query processing methods chosen in databases is to get a good throughput or response time in order to guarantee a better performance. On the contrary, realtime database systems can also deal with temporary data, (i.e., data timed out after a certain time). The important difference is that the goal of real-time database systems is to meet the time constraints of the transactions. It is also worth to mention here that real-time does not necessarily mean fast [9, 10 and 13]. Real-time means the need to manage categorical time constraints in a predictable manner, that is, to use timeconscious methods to deal with deadlines or periodicity constraints associated with tasks and transactions [11, 18]. The work magnifies the need for sophisticated concurrency control methods in real-time distributed database systems [36]. Hence, a new optimistic concurrency control approach is presented where it is based on reworking to the current workload. Every database transaction conforms the ACID rules; Atomicity, Consistency, Isolation, and Durability rules. Depending on the transaction types, the level of parallelism, and other factors, different concurrency control mechanisms are implemented. a) Pessimistic concurrency control: if more than one transaction intended to update the same data at the same time, then lock the data and prevents the overlapping update operations of the transactions to access the same data but permit the read operations to access the locked data. b) Optimistic concurrency control: Delay checking whether the transaction meets the ACID rules to the commit state, to prevent violation. This paper has been organized as follows. Chapter 2 presents a literature description of concurrency control studies over the years. Meanwhile, Chapter 3 presents concurrency control in real-time databases. And Chapter 4 presents a new optimistic concurrency control mechanism to lessen overhead and improve efficiency. Chapter 5 draws the conclusions of this study. II. LITRATURE In this section, we present some well-known pessimistic concurrency control methods. Most of these methods are based on Two Phase Locking (2PL). 2PL High Priority In the 2PLHP (2PL High Priority) concurrency control method [14, 15, 22, 24] conflicts are resolved approving of the higher priority transactions. If the priority of the lock requester is higher than the priority of the lock holder, the lock holder is aborted, rolled back and restarted. The lock is granted to this requester and the requester can continue its execution. If the priority of the lock requester is lower than that of the lock holder, the requesting transaction blocks to wait for the lock holder to finish and release its locks [18, 20, and 29]. High Priority concurrency control may lead to the cascading blocking problem, a deadlock situation, and priority inversion. 2PL Wait Promote In 2PL-WP (2PL Wait Promote) [14,17] the analysis of concurrency control method is developed from [28]. The mechanism presented uses shared and exclusive locks. Shared locks permit multiple concurrent readers. A new definition is made – the priority of a data object, which is defined to be the highest priority of all the transactions holding a lock on the data object. If the data object is not locked, its priority is undefined [23, 31, 42]. A transaction can join in the read group of an object if and only if its priority is higher than the maximum priority of all transactions in the write group of an object. Thus, conflicts arise from incompatibility of locking modes as usual.

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