Database partitioning is a technique for improving the performance of distributed OLTP databases, since “single partition” transactions that access data on one partition do not need coordination with other partitions. For workloads that are amenable to partitioning, some argue that transactions should be executed serially on each partition without any concurrency at all. This strategy makes sense for a main memory database where there are no disk or user stalls, since the CPU can be fully utilized and the overhead of traditional concurrency control, such as two-phase locking, can be avoided. Unfortunately, many OLTP applications have some transactions which access multiple partitions. This introduces network stalls in order to coordinate distributed transactions, which will limit the performance of a database that does not allow concurrency. In this paper, we compare two low overhead concurrency control schemes that allow partitions to work on other transactions during network stalls, yet have little cost in the common case when concurrency is not needed. The first is a light-weight locking scheme, and the second is an even lighter-weight type of speculative concurrency control that avoids the overhead of tracking reads and writes, but sometimes performs work that eventually must be undone. We quantify the range of workloads over which each technique is beneficial, showing that speculative concurrency control generally outperforms locking as long as there are few aborts or few distributed transactions that involve multiple rounds of communication. On a modified TPC-C benchmark, speculative concurrency control can improve throughput relative to the other schemes by up to a factor of two.

Software Transactional Memories (STMs) are emerging as a potentially disruptive programming model. In this paper we are address the issue of how to enhance dependability of STM systems via replication. In particular we present AGGRO, an innovative Optimistic Atomic Broadcast-based (OAB) active replication protocol that aims at maximizing the overlap between communication and processing through a novel AGGRessively Optimistic concurrency control scheme. The key idea underlying AGGRO is to propagate dependencies across uncommitted transactions in a controlled manner, namely according to a serialization order compliant with the optimistic message delivery order provided by the OAB service. Another relevant distinguishing feature of AGGRO is of not requiring a-priori knowledge about read/write sets of transactions, but rather to detect and handle conflicts dynamically, i.e. as soon (and only if) they materialize. Based on a detailed simulation study we show the striking performance gains achievable by AGGRO (up to 6x increase of the maximum sustainable throughput, and 75 % response time reduction) compared to literature approaches for active replication of transactional systems. 1

Abstract—In this paper we investigate the problem of speculative processing in a replicated transactional system layered on top of an optimistic atomic broadcast service. We consider a realistic model in which transactions ’ read/write sets are not known a-priori, and transactions ’ data access patterns may vary depending on the observed snapshot. We formalize a set of correctness and optimality properties aimed at ensuring that transactions are not activated on inconsistent snapshots, as well as the minimality and completeness of the set of explored serialization orders. Finally, an optimal speculative transaction replication protocol is presented. I.

A read-only transaction (ROT) does not modify any data. Efforts are being made in the literature to improve the performance of ROTs without correctness and data currency issues. The widely used twophase locking protocol (2PL) processes the transactions without any correctness and data currency issues. However, the performance of 2PL deteriorates with data contention. Snapshot isolation (SI)-based protocols proposed in the literature improve the performance of ROTs, but they compromise on correctness and data currency issues. Speculative locking (SL) protocols are proposed in the literature for improving the performance of ROTs by carrying out speculative executions only for ROTs and following 2PL for update transactions. In SL-based protocols, update transactions are blocked if they conflicting with ROTs. In this paper, we have proposed an improved approach to improve parallelism among update transactions and ROTs by exploiting a new notion called “compensatability”. In this protocol, an ROT which can be “compensatable ” can complete the execution and carry out compensating operation to incorporate the effect of conflicting update transactions. As a result, the parallelism is improved over SL protocols as the update transactions which are conflicting with ‘compensatable ’ ROTs need not block. In this paper, we have proposed a protocol by exploiting both “compensatability” property of ROTs and speculation. The simulation results show that the proposed protocol improves the performance by carrying out less number of speculative executions. Further, the proposed protocol does not violate serializability criteria. 1

Abstract: Transaction processing in a Mobile Database System (MDS) is more complex because of unlimited mobility of the Mobile Unit (MU). The handoff and frequent failure of mobile unit makes it tricky to store log records and access it for recovery. In this paper, we present a new log management scheme, which uses a mobile-agent-based framework to facilitate efficient transaction processing during handoff or MU failure. Instead of executing the transaction in mobile unit, we are giving it to base agent which is in the base station. By doing so, we save power in the mobile unit. Moreover, we are using announcement scheme during handoff which minimizes traffic of Home Location Register Database. Also we use Speculative Locking (SL) protocol, which improves the performance of fixed network transaction by trading extra processing power. In SL, a transaction releases the lock on the data object whenever it produces corresponding after-image during its execution. By accessing both before image and after images, the waiting transaction carries out speculative execution retains one execution based on the termination (commit or abort) mode of the preceding transactions. We have also compared waiting time of ordinary locking protocol and speculative locking protocol.

Whenever distributed transaction processing in MANETs or other unreliable networks has to guarantee atomicity and isolation, a major challenge is how long-term blocking of resources can be avoided in case the mobile device looses connection to other participants of the transaction. We present a new technique for treating blocked data of transaction participants that wait for a coordinator’s commit decision. Our technique, Bi-State-Termination (BST), gives participants that have moved during transaction execution the possibility to continue transaction processing before they know the coordinator’s decision on transaction commit. The key idea of our technique is to consider both possible outcomes (commit and abort) of unknown transaction decisions. Within this paper, we describe a fast implementation of the fundamental relational database operations for a DBMS supporting the BST transaction synchronization protocol that avoids longterm transaction blocking.