J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of Real-Time Systems Symposium, pages 94--103, 1990.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....data contention. Considerable research has been devoted to designing concurrency control algorithms for RTDBSs and to evaluating their performance Most of these algorithms use serializability as correctness criteria and are based on one of the two basic concurrency control mechanisms: 2PL [4, 6, 12, 13, 16, 21] or optimistic concurrency control (OCC) 9, 7, 3, 2, 13, 10, 11] However, This work is partially funded by Nokia Telecommunications, Solid Information Technology Ltd. and the National Technology Agency Finland. 2PL has some inherent problems such as the possibility of deadlocks as well as ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of the 11th Real-Time Symposium, pages 94--103, 1990.

....they are non blocking and deadlock free. Therefore, in recent years, numerous optimistic concurrency control methods have been proposed for real time databases (e.g. 16, 20, 21] Although optimistic approaches have been shown to be better than locking methods for real time database systems [9, 10], they have the problem of unnecessary restarts and heavy restart overhead. This is due to the late conflict detection that increases the restart overhead since some near to complete transactions have to be restarted. Because conflict resolution between the transactions is delayed until a ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the 11th IEEE Real-Time Systems Symposium, pages 94--103, Lake Buena Vista, Florida, USA, 1990. IEEE Computer Society Press.

....data contention. Considerable research has been devoted to designing concurrency control algorithms for RTDBSs and to evaluating their performance Most of these algorithms use serializability as correctness criterion and are based on one of the two basic concurrency control mechanisms: 2PL [4, 6, 12, 13, 16, 19] or optimistic concurrency control (OCC) 9, 7, 3, 2, 13, 10, 11] However, 2PL has some inherent problems such as the possibility of deadlocks as well as long and unpredictable blocking times. These problems appear to be serious in real time transaction processing since real time transactions ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of the 11th Real-Time Systems Symposium, pages 94--103.

....blocking times. These problems appear to be serious in real time transaction processing since real time transactions need to meet their timing constraints, in addition to consistency requirements. Many alternatives to two phase locking in real time systems have been proposed and studied [4, 11, 17]. Among them is a class of concurrency control schemes based on the optimistic approach [10, 19] Optimistic concurrency control protocols have the nice properties of being non blocking and deadlock free. These properties make them especially attractive for real time database systems (RTDBS) ....

Haritsa, J. R., Carey, M. J., and Livny, M. (1990). Dynamic real-time optimistic concurrency control. In Proc. of the 11th Real-Time Symposium, pages 94--103, IEEE Computer Society Press.

....collection distinguish our work. Our work on making read only transactions completely non blocking and their execution times more predictable is complementary to the time cognizant transaction processing schemes proposed in the real time database systems literature [SZ88, AGM88, AGM89, HCL90b, HCL90a, HSRT89, HSRT91, HCL93, Ram93] Our use of main memory technology and version based concurrency control schemes enable transaction running times to be estimated more accurately. Consequently, scheduling algorithms can generate better schedules, that is, schedules in which more transactions meet ....

J. Haritsa, M. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the IEEE Real-Time Systems Symposium, 1990.

....was that transaction blocking in lock based protocols results in unpredictable delays causing transactions to miss their deadlines. The authors later developed an optimistic protocol, called WAIT 50, which allows for the use of priorities to improve decision making in resolving conflicts [24]. The protocol uses a 50 percent rule as follows: If half or more of the transactions conflicting with a committing transaction are of higher priority, the transaction is made to wait for the high priority transactions to complete; otherwise, it is allowed to commit while the conflicting ....

....scheme was found to perform better than the locking scheme only under low data contention. When data contention was high, the situation was reversed due to the overhead of large number of transaction restarts. Those experimental results do not agree with the simulation results of Haritsa et al. [23, 24]. The differences are contributed to the different types of systems involved in evaluations and the different degree of protocol implementation [30] The priority inversion problem that was defined for locking protocols can also exist in a RTDBS that maintains data consistency through use of a ....

J.R. Haritsa, M.J. Carey, M. Livny `Dynamic Real-Time Optimistic Concurrency Control', Proceedings of the 11th Real-Time Systems Symposium, 1990, pp.94-103.

.... studied, by simulation, the relative performance of two well known classes of concurrency control algorithms (locking protocols and optimistic techniques) in a RTDBS environment [19, 22] They presented and evaluated a new real time optimistic concurrency control protocol through simulations in [20]. Son and Chang [40] investigated methods to apply the priority ceiling protocol as a basis for real time locking protocol in a distributed environment. Agrawal et al. 5] proposed a new locking approach, referred to as ordered sharing, which attempts to eliminate blocking of read and write ....

....the executing transactions. If half or more of the transactions conflicting with a committing transaction are of higher priority, the transaction is made to wait for the high priority transactions to complete; otherwise, it is allowed to commit while the conflicting transactions are aborted [20]. Although most of the previous works involving distributed database models assumed either noreplication [6, 28] or full replication [15, 16, 31, 38, 39, 40, 41] some performance evaluation studies of partially replicated database systems were also provided [7, 12, 14, 29] The impact of the ....

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J.R.Haritsa, M.J.Carey, M.Livny, "Dynamic Real-Time Optimistic Concurrency Control", 11th Real-Time Systems Symposium, 1990, pp.94-103.

....in priority order. Even at the network, it is easy to ensure that messages of high priority transactions are transmitted before those of other lower priority requesters. With respect to data access also, prioritybased pre emptive concurrency control algorithms such as 2PL HP [AGM88] and OPTWAIT [HCL90] have been developed. Removing priority inversion in the commit protocol, however, is not fully feasible. This is because, once a cohort reaches the prepared state, it has to retain all its data locks until it receives the global decision from the master this retention is fundamentally ....

Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic Real-Time Optimistic Concurrency Control. In Proceedings of the 11th IEEE Real-Time Systems Symposium, December 1990.

....in priority order. Even at the network, it is easy to ensure that messages of high priority transactions are transmitted before those of other lower priority requesters. With respect to data access also, prioritybased pre emptive concurrency control algorithms such as 2PL HP [AGM88] and OPTWAIT [HCL90] have been developed. Removing priority inversion in the commit protocol, however, is not fully feasible. This is because, once a cohort reaches the prepared state, it has to retain all its data locks until it receives the global decision from the master this retention is fundamentally ....

Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic Real-Time Optimistic Concurrency Control. In Proceedings of the 11th IEEE Real-Time Systems Symposium, December 1990.

.... these papers include the modeling of real time transactions and real time database systems [Abbo88, Buch89, Care89, Daya88, Hari90, Huan89, Kort90, Liu88, Sha88, Stan88] scheduling of real time transactions [Abbo88, Abbo89, Huan89, Stan88, Sha88] concurrency control and data conflict resolution [Abbo88, Abbo89, Agra92, Buch89, Hari90, Hari90b, Huan89, Huan91b, Huan91c, Lee93, Lee93b, Lee93c, Lee93d, Lin90, Sha88, Son90, Son90b, Song90, You93], processing of queries with real time constraints [Hou89] buffer management [Care89, Huan90] and I O scheduling [Abbo90, Care89, Chen91] A number of papers on related issues have also appeared. These include work on a protocol for timed atomic commitment [Davi91] fast recovery protocols for ....

.... for data consistency other than serializability that can provide a higher level of concurrency by sacrificing data consistency temporarily to some degree [Lin89, Kuo92, Pu92] Considerable work has been devoted to transaction scheduling and concurrency control in real time database systems [Abbo88, Abbo89, Agra92, Buch89, Hari90, Hari90b, Huan89, Huan91b, Huan91c, Lee93, Lee93b, Lee93c, Lee93d, Lin90, Sha88, Son90, Song90, You93]. Most of these studies use the serializability for maintaining data consistency, because there is no general purpose consistency criterion available that is less stringent than serializability, and yet as obviously correct and implementable as serializability. Most of the proposed algorithms for ....

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Haritsa, J. R., M. J. Carey, and M. Livny, "Dynamic Real-Time Optimistic Concurrency Control," Proceedings of the 11th IEEE Real-Time Systems Symposium, Orlando, Florida, December 1990.

....optimistic protocols in detail. OPT BC [HCL90b] extends classical optimistic protocol with Broadcast Commit [MN82, Rob87] protocol. In Broadcast Commit protocol the transaction notifies other running transactions that conflict with it. The conflicting transactions are restarted. OPT SACRIFICE [HCL90a] is an optimistic protocol which uses a priority driven abort for conflict resolution. When a transaction reaches its validation phase, algorithm checks for conflicts with currently executing transactions. If conflicts are detected and at least one of the transactions in the conflict set is a ....

....executing transactions. If conflicts are detected and at least one of the transactions in the conflict set is a higher priority transaction, then the validating transaction is restarted. It is sacrificed in an effort to help the higher priority transactions make their deadlines. OPT WAIT [HCL90a] algorithm incorporates a priority wait mechanism. When transaction reaches its validation phase, if its priority is not the highest among the conflicting transactions, it waits for the conflicting transactions with higher priority to complete. This gives the higher 6 priority transactions a ....

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J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the 11th Real-Time Symposium, pages 94--103, Los Alamitos, Calif., 1990. IEEE, IEEE Computer Society Press.

....to be correct, if it is serialisible, that is if it produces the same results as any sequential execution of the same transactions. A pessimistic strategy to handle this task is the so called two phase locking protocol [15, 32] optimistic strategies work without locks and are not regarded here [22]) in an acquisition phase, a lock has to be acquired for each data item to be accessed by the transaction. This lock may be a shared lock which can be granted to more than one transactions simultaneously, if and only if the transaction merely intends to read the data item. It must be an exclusive ....

J. R. Haritsa, M. J. Carey, and M. Linvy. "Dynamic Real-Time Optimistic Concurrency Control". In Prooceedings of the 11th Real-Time Systems Symposium, December 1990.

....concurrency control protocol, called OCC DATI. In this paper we concentrate on firm and non real time transaction models. OCC DATI is a fully optimistic protocol and uses forward adjustment. With the new protocol, the number of transaction restarts is smaller than with OCC BC, OPTWAIT, or WAIT 50 [7, 8, 9], because the serialization order of the conflicting transactions is adjusted dynamically. On the other hand, the overhead for supporting dynamic adjust ment is much smaller than the one in OCC DA [15, 16] We also present methods to relax serializability for telecommunication applications. The ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of the 11th RealTime Symp., pp 94--103, Los Alamitos, Calif., 1990. IEEE Computer Society Press.

....to reduce the number of transaction restarts and we propose a new optimistic concurrency control protocol, called OCC DATI. OCC DATI is fully optimistic protocol and uses forward adjustment. With the new protocol, the number of transaction restarts is smaller than with OCC BC, OPT WAIT, or WAIT 50 [HCL90b, HCL90a, HSRT91], because the serialization order of the conflicting transactions is adjusted dynamically. On the other hand, the overhead for supporting dynamic adjustment is much smaller than the one in OCC DA [LLH95a, LLH95b] or modified OCC TI [KLR97] Remaining parts of the paper is organized as follows. ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the 11th Real-Time Symposium, pages 94--103, Los Alamitos, Calif., 1990. IEEE, IEEE Computer Society Press.

....OCC DATI and OCC PDATI protocols. In this paper we concentrate on firm real time transaction models. OCC DATI and OCC PDATI are a fully optimistic protocols and they use forward adjustment. With the new protocol, the number of transaction restarts is smaller than with OCC BC, OPTWAIT, or WAIT 50 [5 7], because the serialization order of the conflicting transactions is adjusted dynamically. On the other hand, the overhead for supporting dynamic adjustment is much smaller than the one in OCC DA [10, 11] The rest of the paper is organized as follows. Section 2 introduces the revised OCC TI. ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of the 11th Real-Time Symposium, pages 94--103, Los Alamitos, Calif., 1990. IEEE, IEEE Computer Society Press.

....protocols in detail. OPT BC [HCL90b] extends classical optimistic protocol with the Broadcast Commit [MN82, Rob87] protocol. In the Broadcast Commit protocol the transaction notifies other running transactions that conflict with it. The conflicting transactions are restarted. OPT SACRIFICE [HCL90a] is an optimistic protocol which uses a priority driven abort for conflict resolution. When a transaction reaches its validation phase, the algorithm checks for conflicts with currently executing transactions. If conflicts are detected and at least one of the transactions in the conflict set is a ....

....executing transactions. If conflicts are detected and at least one of the transactions in the conflict set is a higher priority transaction, then the validating transaction is restarted. It is sacrificed in an effort to help the higher priority transactions make their deadlines. OPT WAIT [HCL90a] algorithm incorporates a priority wait mechanism. When a transaction reaches its validation phase, if its priority is not the highest among the conflicting transactions, it waits for the conflicting transactions with higher priority to complete. This gives the higher priority transactions a ....

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J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the 11th Real-Time Symposium, pages 94--103, Los Alamitos, Calif., 1990. IEEE, IEEE Computer Society Press.

....blocking times. These appear to be serious problems for real time transaction 1 processing, since real time transactions need to meet their time constraints, as well as consistency requirements. Many alternatives to two phase locking for real time systems have been proposed and studied [3, 11, 16]. Among them is a class of concurrency control schemes based on the well know optimistic approach [18, 10] Optimistic concurrency control protocols have the nice properties of being non blocking and deadlock free. These properties make them especially attractive for RTDBSs. As conflict ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of the 11th Real-Time Symposium, pages 94--103, Los Alamitos, Calif., 1990. IEEE, IEEE Computer Society Press.

....considers a real time context. The pioneering work in RTDBS performance evaluation was reported in [1, 2, 3] where the authors simulated a number of CC protocols based on the two phase locking algorithm. However, this work was not examined in an active context. A subset of notable RTDBS work [23, 22, 25, 29, 44, 5, 14]. Again, all this work has been performed without considering the effects of triggering. Having identified some important work in ADBSs as well as RTDBSs, let us reiterate that not much exists in the synthesis of the two areas. An exciting and very recent development was the International ....

J.R. Haritsa, M.J. Carey, and M. Livny. Dynamic Real-Time Optimistic Concurrency Control. In Proceedings of the IEEE Real-Time Systems Symposium, 1990.

....2PL s long and unpredictable blocking times damage its appeal for real time environments, where the primary performance criterion is meeting time constraints and not just preserving consistency requirements. Over the last few years, several alternatives to 2PL for RTDBMS have been explored [18, 14, 13, 16, 17, 20, 30]. In a recent study [5] we proposed a categorically different approach to concurrency control for RTDBMS. Our approach relies on the use of redundant computation to start on alternative schedules, once conflicts that threaten the consistency of the database are detected. These alternative ....

Jayant R. Haritsa, Michael J. Carey, and Miron Linvy. "Dynamic real-time optimistic concurrency control." In Prooceedings of the 11th Real-Time Systems Symposium, December 1990.

....do not directly address the question of time constrained scheduling at each local site. Instead, we consider issues pertinent to the distributed environment where the local scheduling is handled through the use of methods that have been previously explored [AGM88, AGM89, BMHD89, C 89, HCL90b, HCL90a, KSS90] Thus, a transaction that accesses data at several sites, does so by the use of a subtransaction at each concerned site, and each such subtransaction is subject to the local concurrency control mechanism at its particular site. We adopt such an approach since the key feature that ....

J.R. Haritsa, M.J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the Eleventh Real-Time Systems Symposium, Lake Buena Vista, Florida, pages 94--103, December 1990.

....X to Y, which is confusing. In fact, the global execution schedule (Q 1 U 2 Q 3 U 4 Q 1 ) is not serializable and should not be allowed. 4. Optimistic Concurrency Control in Broadcast Environments In this section, we adapt the optimistic concurrency control with forward validation (OCC FV) [8,9,12] protocol to solve the late restart problem for read only transactions at the mobile clients in broadcast environments. At the server, validation of a transaction is done against currently running transactions. This process is based on the assumption that the validating transaction is ahead of ....

Haritsa, J. R., Carey, M. J., and Livny, M., "Dynamic RealTime Optimistic Concurrency Control," Proc. of the 11 th IEEE Real-Time Systems Symposium, 1990.

....is enforced by using a real time version of either the two phase locking protocol or optimistic concurrency control. Optimistic concurrency control has been shown to perform better than two phase locking when integrated with priority driven CPU scheduling in real time database systems [48, 49, 53]. In addition to timing constraints, in many real time database applications, each transaction imparts a value to the system, which is related to its criticalness and to when it completes execution (relative to its deadline) In general, the selection of a value function depends on the ....

J. R. Haritsa, M.J. Carey and M. Livny, "Dynamic Real-Time Optimistic Concurrency Control," Proceedings of the 11th Real-Time Systems Symposium, December 1990.

....time critical scheduling methods to observe the timing requirements of transactions [30] A considerable amount of RTDBS research has been devoted to performance evaluation of time cognizant concurrency control protocols. However, the performance studies were either based on simulation (e.g. [1, 2, 3, 10, 11, 20, 23, 27, 28, 29]) or carried out on a RTDBS testbed (e.g. 14, 15] To the best of our knowledge, no analytic performance study has been reported so far involving the evaluation of concurrency control protocols in RTDBSs, which is the main contribution of this paper. 1 The behavior of concurrency control ....

J. R. Haritsa, M. J. Carey, M. Livny `Dynamic Real-Time Optimistic Concurrency Control', 11th Real-Time Systems Symposium, pp.94-103, 1990.

.... were proposed; these protocols provide bounded blocking times imposed by accessing data objects and schedulability tests for scheduling a set of periodic tasks in uniprocessor RTDBS [SRSC91, Nak93] Haritsa et al. proposed a priority based optimistic concurrency control algorithm WAIT 50 [HCL90] which performs better than the traditional optimistic algorithm, OPT BC [MN82, Rob82] They concluded that priority information can improve the performance of optimistic concurrency control in RTDBS. Ulusoy proposed several locking and timestamp based concurrency control protocols for RTDBS ....

....as follows. When a transaction commits, it notifies all the transactions with which it conflicts and restarts all the conflicted transactions immediately. A transaction is restarted once it is found to be in conflict with a validating transaction. Such restart is useful, not wasted restart [HCL90], because a validating transaction is guaranteed to commit when it restarts all the transactions in its conflict set. Wasted restart occurs when a transaction restarts another one and later it misses its deadline. Mutual restart problem 1 [HCL90] will not happen in OPT BC, because a validating ....

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J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the Real Time Systems Symposium, pages 94--103, 1990.

....finishes successfully, the result is known. Thus several versions of the computation can be run, with different time bounds, guaranteeing an upper bound on the time to produce some result. Mena [Mena82] has classified concurrency control algorithms into two classes: optimistic algorithms (e.g. [Hari90b, Hari90a, Huan90, Kim91, Lin90, Son92]) and pessimistic algorithms (e.g. Abbo88, Stan88, Huan90, Sha91] Bestavros has proposed (in [Best92] a new approach, Speculative Concurrency Control (SCC) which incorporates redundant computation into concurrency control algorithms. Speculative concurrency control redundantly executes the ....

....a so that it can get the WriteLock on b. None of the transaction can proceed (deadlock) Furthermore, the resource conservation nature of pessimistic algorithms becomes a draw back in the real time environment where meeting the time constraint has a much higher priority than saving resources. In [Hari90b, Hari90a], Haritsa, Carey and Linvy showed that for a real time DBMS with firm deadlines (transactions which misses the deadlines are immediately discarded) optimistic algorithms outperforms the pessimistic schemes. The key result is that, if low resource utilization is acceptable (i.e. a large amount of ....

Jayant R. Haritsa, Michael J. Carey, and Miron Linvy. "Dynamic real-time optimistic concurrency control." In Prooceedings of the 11th Real-Time Systems Symposium, December 1990.

....of firm real time transactions is measured in terms of miss percentage, soft real time class uses mean tardiness and non real time class uses throughput and response time to measure performance. Additionally, optimistic protocols have been shown to work well for firm real time transactions [HCL90, LS93, HSRT91], whereas blocking protocols suit the needs of soft [AGM89] and non real time jobs [AD85, ACL87, CS84] Consider a stock market environment [Voe87] where real time transactions are submitted by brokers who are buying and selling shares and need their transactions to complete before the current ....

.... soft RTDBS, 2PL HP adopted priority based High Priority (HP) conflict resolution method in locking algorithm and showed better performance than 2PL Wait [AGM89] 2PL HP has been compared to OCC variants and 2PL HP showed better performance than OCC for soft deadline systems with finite resources [HCL90]. ffl For firm RTDBS, OCC always outperforms 2PL HP in the simulation study of [HCL90, LS93] while OCC performs better than 2PL HP only when data contention is low in the different study [HSRT91] In general, the advantage of 2PL HP over OCC is derived from its early stage blocking validation ....

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Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic real-time optimistic concurrency control. In Proceedings of Real-Time System Symposium, pages 94--103, Lake Buena Vista, Florida, 1990. IEEE.

....simultaneously. There has been a considerable amount of work in the area of RTDBSs. An extensive exploration of the issues is presented in [11, 19, 21] Most of the work in the RTDBS area has focused on the development and evaluation of priority cognizant concurrency control protocols (e.g. [1, 2, 4, 6, 8, 10, 12, 17, 18, 20]) Many proposed concurrency protocols have transferred assumptions and results from non RTDBSs in an unchallenged form to the realm of RTDBSs. The chain of thought underlying conventional concurrency control is the following: I O is 3 4 orders of magnitude slower than a main memory access. ....

....priority. The aim is to reduce the blocking times of high priority transactions. A Real Time Concurrency Control Protocol for Main Memory Database Systems 115 Optimistic Wait 50 (OPT) Protocol: OPT is an optimistic concurrency control protocol incorporating real time priorities of transactions [6]. The validation check for a committing transaction is performed against the executing transactions and if the write set of the validating transaction intersects with the read set of one of the executing transactions, these two transactions are said to be in conflict. The proposed protocol uses a ....

J.R. Haritsa, M.J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the 11th Real-Time Systems Symposium, pp.94--103 (1990).

....obtained values. Abbott et al. 1] presented a deadline model to specify the value function of a transaction. No performance evaluation was made in their work. Huang et al. 10, 11] used a real time database testbed to evaluate the performance of several scheduling algorithms. Haritsa et al. [8] made a detailed performance evaluation of various priority assignment policies and concurrency control protocols under different system environments. Tseng et al. 16] addressed the issue of scheduling transactions with timevariant values. However, all previous work on valuebased scheduling are ....

....associated with the transactions, an appropriate scheduling algorithm is required for an RTDBS such that a high performance of the system can be achieved. A number of researches on the priority assignment policies and concurrency control protocols for real time scheduling have been proposed [1 4, 8 11, 13 16]. With the decreasing of memory costs in recent years, it becomes feasible to store the databases in main memory for improving the overall performance of an RTDBS [7] The main difference between a main memory database (MMDB) and a disk resident database (DRDB) is that no disk I O is needed for a ....

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J.R. Haritsa, M.J. Carey, and M. Livny, "Dynamic real-time optimistic concurrency control," Proc. 11th IEEE Real-Time Systems Symp., pp. 94-103, Orlando, Florida, Dec. 1990.

....time critical scheduling methods to observe the timing requirements of transactions [17, 30] A considerable amount of RTDBS research has been devoted to performance evaluation of time cognizant concurrency control protocols. However, the performance studies were either based on simulation (e.g. [1, 8, 10, 14, 15, 19, 27, 28]) or carried out on a RTDBS testbed (e.g. 13] To the best of our knowledge, no analytic performance study has been reported so far involving the evaluation of concurrency control protocols in RTDBSs, which is the main contribution of this paper. 1 The behavior of concurrency control ....

J. R. Haritsa, M. J. Carey, M. Livny `Dynamic Real-Time Optimistic Concurrency Control', 11th RealTime Systems Symposium, pp.94-103, 1990.

....algorithm in Section 5. Details on their performance can be found in [23] We simulated an extended version of the broadcast commit algorithm [20,21] an optimistic concurrency control known to work well in real time databases, especially when conflicts between transactions are infrequent [14,24,25]. The extended algorithm accommodates multiversion data and periodic transactions. Like the original broadcast commit algorithm, an update transaction goes through three phases: a read phase, a validation phase, and a possible write phase. Transactions are assigned startup and commit timestamps. ....

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proceedings of IEEE 11th Real-Time Systems Symposium, pages 94--103, December 1990.

.... soft RTDBS, 2PL HP adopted priority based High Priority (HP) conflict resolution method in locking algorithm and showed better performance than 2PL Wait [AGM89] 2PLHP has been compared to OCC variants and 2PL HP showed better performance than OCC for soft deadline systems with finite resources [HCL90a]. ffl For firm RTDBS, OCC always outperforms 2PL HP in the simulation study of [HCL90a, LS93] while OCC performs better than 2PL HP only when data contention is low in the different study [HSRT91] A major advantage of the locking algorithm over OCC is its update policy. Locking can be done with ....

.... in locking algorithm and showed better performance than 2PL Wait [AGM89] 2PLHP has been compared to OCC variants and 2PL HP showed better performance than OCC for soft deadline systems with finite resources [HCL90a] ffl For firm RTDBS, OCC always outperforms 2PL HP in the simulation study of [HCL90a, LS93] while OCC performs better than 2PL HP only when data contention is low in the different study [HSRT91] A major advantage of the locking algorithm over OCC is its update policy. Locking can be done with in place update or deferred update while OCC only can be done with deferred update. In place ....

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Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic real-time optimistic concurrency control. In Proceedings of Real-Time System Symposium, pages 94--103. IEEE, 1990.

....to a computing environment with far more resources than what would be necessary to sustain average loads, thus vanishing the advantage of PCC over OCC algorithms. In particular, OCC algorithms become attractive since computing resources wasted due to restarts do not adversely affect performance [HCL90b, HCL90a]. Real time concurrency control schemes considered in the literature could be viewed as extensions of either PCC based or OCC based protocols, whereby transactions are assigned priorities that reflect the urgency of their timing constraints. These priorities are used with PCC based techniques ....

....of their timing constraints. These priorities are used with PCC based techniques [AGM88, ACL87, SZ88, HSTR89, Sin88, SRL88, SRSC91] to make it possible for urgent transactions to abort conflicting, less urgent ones (thus avoiding the hazards of blockages) and are used with OCC based techniques [Kor90, HCL90b, HCL90a, HSRT91, KS91, LS90, SPL92] to favor urgent transactions when conflicting, less urgent ones attempt to validate and commit (thus avoiding the hazards of restarts) In a recent study [Bes92] we proposed an approach to concurrency control that combines the advantages of both OCC and PCC protocols while avoiding their ....

[Article contains additional citation context not shown here]

Jayant R. Haritsa, Michael J. Carey, and Miron Linvy. Dynamic real-time optimistic concurrency control. In Prooceedings of the 11th Real-Time Systems Symposium, December 1990.

....These appear to be serious problems for real time transaction processing, since in a real time environment, transactions need to meet their time constraints as well as consistency requirements. Recently, some alternatives to two phase locking for real time systems have been proposed and studied [16, 8, 6, 10, 7, 13]. Among them is a class of concurrency control schemes based on the well know optimistic approach [12] Ideally, optimistic concurrency control (OCC) has the properties of non blocking and deadlock freedom. These properties make the scheme especially attractive to real time transaction processing. ....

....has the properties of non blocking and deadlock freedom. These properties make the scheme especially attractive to real time transaction processing. In real time database systems, OCC may be in a better position to be integrated with priority driven CPU scheduling. Previous performance studies [6, 7] have shown that under a policy that discards transactions which have missed their deadlines, OCC outperforms 2PL over a wide range of system utilizations. The results in [6, 7] are based on simulation, where optimistic concurrency control is carried out at the logical level and detailed ....

[Article contains additional citation context not shown here]

Haritsa, J.R., M.J. Carey and M. Livny, "Dynamic Real-Time Optimistic Concurrency Control," Proceedings of the 11th Real-Time Systems Symposium, Dec. 1990.

....as they occur. However, they may suffer possible unbounded waiting due to blocking and deadlocks. The resource conservation nature of pessimistic algorithms becomes a drawback in a real time environment, where meeting time constraints has a much higher priority than conserving resources. In [6, 5], Haritsa, Carey and Linvy showed that for a real time DBMS with firm deadlines (transactions missing their deadlines are immediately discarded) optimistic algorithms outperform pessimistic schemes. The key result is that, if low resource utilization is acceptable (i.e. a large amount of wasted ....

Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic real-time optimistic concurrency control. In Proceedings of the 11th RealTime Systems Symposium, December 1990.

.... with Serializability: Concurrency control techniques for real time databases that use serializability as the correctness criteria include lock based protocols such as two phase locking and its variants [3, 1, 7, 60, 86, 88, 179, 180, 193, 214, 216] optimistic concurrency control protocols [74, 78, 88, 121], and timestamp ordering protocols [133, 195, 215] Using any of these techniques, conflicts between two real time transactions or between one real time transaction and a set of real time transactions are detected. For lock based protocols, transaction conflicts are resolved by either transaction ....

.... intervals to transactions instead of a single value, and to adjust (reduce) the timestamp intervals when conflicts arise to guarantee serializable transactions [19, 230] Optimistic concurrency control protocols validate (certify) transactions for commitment after they complete their executions [74, 77, 78, 87]. Conflicts among transactions are solved using aborts and restarts. The advantage of the optimistic concurrency control technique is that it is nonblocking and deadlock free, making it attractive for real time databases. On the other hand, transaction aborts and restarts waste resources which may ....

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Haritsa, J. R., Carey, M. J., and Livny, M., "Dynamic Real-Time Optimistic Concurrency Control," in Proceedings of the 11th Real-Time Systems Symposium, 1990.

.... transaction) priority wait (wait for higher priority transactions to complete) and opt sacrifice (restart the validating transaction if at least one of the transactions in the conflict set has a higher priority) OCC schemes display better performance for firm real time transactions [HCL90a] Lin and Son [LS90] have proposed a new concurrency control algorithm which is based on mixed integrated concurrency control method [BHG87] that adjusts the serialization order dynamically. Priority scheduling with some a priori knowledge is introduced as another approach in [AGM89, BMH89, ....

....EDF which shows good performance when the system is lightly loaded. If a system load increases the effects of deadline in the priority formula decreases due to increase of ALF. Thus in heavily loaded situations the results obtained using our priority formula is comparable to Random Priority (RP) HCL90a] since the value of Timelost override the effect of deadline in the formula. Another effect of ALF in the formula is that the conflict resolution policy changes from abortive method to nonabortive method in heavy load situations because the priorities of transactions that are conflicting with ....

Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic real-time optimistic concurrency control. In Proceedings of Real-Time System Symposium, pages 94--103. IEEE, 1990.

....introduced priority inversion which is undesirable. This led to several priority based conflict resolution policies, such as Wait, Wait Promote (WP) and High Priority (HP) AGM88] schemes. In terms of performance, 2PL HP showed better performance than OCC for soft RTDBS with finite resources [HCL90a] By definition, firm real time transactions are discarded when they miss their deadlines, as there is no value to completing them after they miss their deadlines. Unlike soft RTDBS which typically use 2PL HP as their concurrency control, most of the approaches for firm RTDBS have tried to ....

....completing them after they miss their deadlines. Unlike soft RTDBS which typically use 2PL HP as their concurrency control, most of the approaches for firm RTDBS have tried to develop a new concurrency control algorithm based on OCC which uses deferred restart policy based on local update scheme [HCL90a] For firm RTDBS, OCC always outperforms 2PL HP in the simulation studies conducted by [HCL90a, LS93] while OCC performs better than 2PL HP only when data contention is low as shown in a different study [HSRT91] In our recent study [Hon95] AVCC (Alternate Version Concurrency Control which uses ....

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Jayant R. Haritsa, Michael J. Carey, and Miron Livny. Dynamic real-time optimistic concurrency control. In Proceedings of RealTime System Symposium, pages 94--103, Lake Buena Vista, Florida, 1990. IEEE.

....ffl Soft deadline transactions have some value even after their deadlines. Typically, the value drops to zero at a certain point past the deadline. If this point is the same as the deadline, we get firm deadline transactions which impart no value to the system once their deadlines expire [21]. For example, if components of a transaction are assigned deadlines derived from the deadline of the transaction, then even if a component misses its deadline, the overall transaction might still be able to make its deadline. Hence these deadlines are soft. Another example is that of a ....

....ongoing transactions in case they conflict with the validating transaction. However, depending on the characteristics of the validating transaction and those with which it con17 flicts, we may prefer not to commit the validating transaction. Several policies have been studied in the literature [20, 21, 26]. In one, termed wait 50, a validating transaction is made to wait as long as more than half the transactions that conflict with it have earlier deadlines. This is shown to have superior performance. Time cognizant extensions to timestamp based protocols have also been proposed. In these, when ....

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J.R. Haritsa, M.J. Carey and M. Livny, "Dynamic Real-Time Optimistic Concurrency Control," Proceedings of the Real-Time Systems Symposium, Dec. 1990.

....transactions is known in advance. In this work, we do not make any such assumption. Furthermore, much research has also been devoted to designing concurrency control (CC) mechanisms geared towards improving the timeliness of transaction processing and their subsequent performance evaluation [1, 2, 3, 20, 19, 22, 21, 29, 28, 41, 6, 27]. Again, all this work has been performed without considering the effects of triggering. An important result that we draw upon in this paper is reported in [20, 19] In this set of important studies, Haritsa et al. showed that in firm or hard real time scenarios (i.e. where late transactions are ....

....improving the timeliness of transaction processing and their subsequent performance evaluation [1, 2, 3, 20, 19, 22, 21, 29, 28, 41, 6, 27] Again, all this work has been performed without considering the effects of triggering. An important result that we draw upon in this paper is reported in [20, 19]. In this set of important studies, Haritsa et al. showed that in firm or hard real time scenarios (i.e. where late transactions are worthless) optimistic concurrency control (OCC) 25] outperforms locking over a large spectrum of system loading and resource contention conditions. In particular, ....

J.R. Haritsa, M.J. Carey, and M. Livny. Dynamic Real-Time Optimistic Concurrency Control. In Proceedings of the IEEE Real-Time Systems Symposium, 1990.

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Haritsa, J., Carey, M., Livny, M., "Dynamic Real-Time Optimistic Concurrency Control,"

....RTDBS is that the system policies used to meet the performance goals of real time transactions often work poorly for standard transactions. This is especially true with regard to transaction concurrency control: Real time variants of optimistic concurrency control, such as OPT WAIT and WAIT 50 [HCL90], provide significantly better real time performance than locking based algorithms in firm deadline RTDBS [HCL92] firm deadlines means that transactions which miss their deadlines are considered to be worthless and are immediately discarded from the system without being executed to ....

....transaction is always guaranteed to commit. The broadcast commit variant detects conflicts earlier than the classical OPT algorithm, resulting in fewer wasted resources and earlier restarts. In the rest of this paper, we will refer to this variant as the basic OPT algorithm. The OPT WAIT algorithm [HCL90] is a real time variant of the OPT protocol, which aims to meet more transaction deadlines by preferentially serving urgent transactions. It incorporates a priority wait mechanism: A transaction that reaches validation and finds higher priority transactions in its conflict set is put on the ....

[Article contains additional citation context not shown here]

J. R. Haritsa, M. Carey, and M. Livny. Dynamic Real-Time Optimistic Concurrency Control. In Proc. of IEEE Real-Time Systems Symp., December 1990.

.... generally considered that hard deadline RTDBSs are infeasible since it is difficult to determine a priori the computation time and execution pattern of a transaction [Abbo88, Stan88] The studies can be divided into two general groups those that treat all transactions as being equally important [Abbo88, Abbo89, Abbo90, Hari90a, Hari90b, Huan90a], and those that incorporate the notion of transactions having different values [Huan89, Huan90b] A brief summary of the studies in these two groups is presented in the remainder of this section. The problem of scheduling transactions in a RTDBS was first addressed by Abbott and Garcia Molina ....

....enforced data consistency by using a two phase locking protocol as the underlying concurrency control mechanism. In [Hari90a] the focus was shifted to studying the performance of optimistic and pessimistic methods of concurrency control in a real time environment. This work was extended in [Hari90b] with the development of new optimistic algorithms that delivered improved performance. Algorithms for buffer allocation and buffer replacement in real time database systems were proposed and evaluated in [Huan90a] In [Abbo90] a study of algorithms for scheduling disk requests with deadlines ....

[Article contains additional citation context not shown here]

Haritsa, J., Carey, M., Livny, M., "Dynamic Real-time Optimistic Concurrency Control," Proc. of the 1990 IEEE Real-Time Systems Symposium, December 1990.

No context found.

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic real-time optimistic concurrency control. In Proc. of Real-Time Systems Symposium, pages 94--103, 1990.

No context found.

Jayant R. Haritsa, Michael J. Carey, and Miron Linvy. "Dynamic real-time optimistic concurrency control." In Prooceedings of the 11th Real-Time Systems Symposium, December 1990. 17

No context found.

Jayant R. Haritsa, Michael J. Carey, and Miron Linvy. "Dynamic real-time optimistic concurrency control." In Prooceedings of the 11th Real-Time Systems Symposium,December 1990.

No context found.

Jayant R. Haritsa, Michael J. Carey, and Miron Linvy. "Dynamic real-time optimistic concurrency control." In Prooceedings of the 11th Real-Time Systems Symposium,December 1990.

No context found.

J. R. Haritsa, M. J. Carey, and M. Livny. Dynamic Real-Time Optimistic Concurrency Control. In Real-Time Systems Symposium, 1990.

No context found.

Haritsa J.R., Carey M.J., Livny M. `Dynamic Real-Time Optimistic Concurrency Control.' Proc. 11th Real-Time Systems Symposium, 1990, pp.94-103.

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J. R. Haritsa, M. J. Carey, M. Livny `Dynamic Real-Time Optimistic Concurrency Control', 11th Real-Time Systems Symposium, pp.94-103, 1990.

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J.R. Haritsa, M.J. Carey and M. Livny, "Dynamic Real-Time Optimistic Concurrency Control," Proceedings of the Real-Time Systems Symposium, Dec. 1990.

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