Global Concurrency Control Research Articles

NASEEB: An Escrow-Based Approach for Ensuring Data Correctness Over Global Clouds

Ensuring data correctness over global clouds using classical pessimistic concurrency control approaches degrades the overall performance. To overcome such problem, a new breed of modern scalable approaches that adopt optimistic global concurrency control and/or non-serializable consistency is proposed. Such approaches cannot prevent conflicts as they allow replicas to diverge. Hence, they trade data correctness for performance. To overcome the limitations of the existing approaches, we propose an alternative novel approach (known as NASEEB) that uses the notion of a “Datacenter Escrow” to ensure data correctness over the cloud datacenters while maintaining good performance. NASEEB distributes the capacities of the data objects’ attributes among the datacenters such that every datacenter has its own non-overlapping quota (i.e., the datacenter escrow). Each datacenter consumes its escrow locally without the need for any global checks. NASEEB allows datacenters to borrow quota from each other to meet incoming high demand. Experimental results show that NASEEB successfully maintains the data correctness as in the classical approaches and provides a good performance as in the modern approaches as long as the quota borrowing rate is kept low.

고 가용성 분산 시스템을 위한 효율적인 하이브리드 복제 프로토콜

분산 시스템에서 가용성을 높이고 전체 시스템의 성능을 향상시키기 위해 데이터는 여러 노드에 중복하여 저장된다. 여기서는 전역적 접근제어를 위해서 읽기/쓰기 동작을 수행하는데 필요한 노드의 집합을 정의하는 Quorum 프로토콜이 존재한다. Quorum 프로토콜을 사용하는 대표적인 복제 프로토콜인 Tree Quorum 프로토콜은 트리의 높이가 증가할수록 노드의 수가 기하급수적으로 증가하고, Grid 프로토콜은 노드에 장애가 발생하지 않아도 언제나 같은 읽기/쓰기 비용을 갖는다는 단점을 갖고 있다. 따라서, 본 논문에서는 기존 프로토콜의 장점을 가지면서 단점을 해결할 수 있는 새로운 하이브리드 프로토콜을 제안한다. 제안된 하이브리드 프로토콜은 전체적으로는 트리 구조를 가지면서 각 레벨에서는 그리드의 열과 같은 구조를 가짐으로써 노드에 장애가 없을 때에는 Tree Quorum 프로토콜과 같이 적은 동작 비용을 요구하며, 노드에 장애가 존재할 경우에도 기존 프로토콜에 비해 상대적으로 적은 동자 비용과 높은 가용성을 보인다. 그러므로 높은 데이터 가용성이 요구되는 서바이벌 스토리지 시스템에 효율적으로 적용 가능하다. 본 논문에서는 수학적 모델링을 통하여 제안된 프로토콜의 비용과 가용성을 평가하고, 시뮬레이션을 통해 응답시간과 처리율을 기존의 Tree quorum프로토콜과 비교한다. In distributed systems data are replicated and stored at several nodes to increase the availability and overall performance. Here Quorum protocol doffing a certain set of replicas required for read/write operation exists for global concurrency control. One of the representative replication Protocols - the Tree Quorum protocol - has a drawback of rapidly growing number of replicas as the level increases, while the Grid protocol requires the same operation cost even without any failure. In this paper, thus, we propose a new replication protocol called hybrid protocol which capitalizes the merits of the existing protocols and solves the problems of them at the same time. The proposed hybrid protocol has very low operation cost in the absence of failure like the tree quorum protocol, and has relatively lower operation cost and higher availability than existing protocols when failure occurs by employing tree architecture as the overall organization while each level of the tree is organized as a row of a grid architecture. It is thus effective to be applied to survival storage system. We conduct cost and availability analysis of the proposed protocol through mathematical modeling, and response time and throughput are compared with those of the Tree Quorum protocol through computer simulation.

Global scheduling for flexible transactions in heterogeneous distributed database systems

A heterogeneous distributed database environment integrates a set of autonomous database systems to provide global database functions. A flexible transaction approach has been proposed for the heterogeneous distributed database environments. In such an environment, flexible transactions can increase the failure resilience of global transactions by allowing alternate (but in some sense equivalent) executions to be attempted when a local database system fails or some subtransactions of the global transaction abort. We study the impact of compensation, retry, and switching to alternative executions on global concurrency control for the execution of flexible transactions. We propose a new concurrency control criterion for the execution of flexible and local transactions, termed F-serializability, in the error-prone heterogeneous distributed database environments. We then present a scheduling protocol that ensures F-serializability on global schedules. We also demonstrate that this scheduler avoids unnecessary aborts and compensation.

Achieving high degree of concurrency in multidatabase transaction scheduling: MTOS

Abstract A multidatabase system (MDBS), which is also called a heterogeneous distributed database system (HDDBS) or federated database system (FDBS), is a facility that allows users or applications to access data located in multiple local database systems (LDBSs), each of which is autonomously operated. Several practical MDBS concurrency control schemes have been proposed whilst maintaining global consistency without compromising local autonomy. However, they could degrade either local concurrency of local transactions or global concurrency of global transactions for the purpose of ensuring the global serializability. In this paper, we propose a new ticket-based global concurrency control scheme that employs multiple tickets at each site. The basic idea is that, with proper consideration of the potential cyclic global serialization orders, subtransactions with the possibility of the local indirect conflicts must access the same ticket at their sites, otherwise they are allowed to access the different tickets. This scheme alleviates the blockage on local resources, and, as a result, increases both global concurrency and local concurrency, whilst preserving the local autonomy.

Protocols for Integrity Constraint Checking in FederatedDatabases

A federated database is comprised of multiple interconnected database systems that primarily operate independently but cooperate to a certain extent. Global integrity constraints can be very useful in federated databases, but the lack of global queries, global transaction mechanisms, and global concurrency control renders traditional constraint management techniques inapplicable. This paper presents a threefold contribution to integrity constraint checking in federated databases: (1) The problem of constraint checking in a federated database environment is clearly formulated. (2) A family of protocols for constraint checking is presented. (3) The differences across protocols in the family are analyzed with respect to system requirements, properties guaranteed by the protocols, and processing and communication costs. Thus, our work yields a suite of options from which a protocol can be chosen to suit the system capabilities and integrity requirements of a particular federated database environment.

A theory of global concurrency control in multidatabase systems

This article presents a theoretical basis for global concurrency control to maintain global serializability in multidatabase systems. Three correctness criteria are formulated that utilize the intrinsic characteristics of global transactions to determine the serialization order of global subtransactions at each local site. In particular, two new types of serializability, chain-conflicting serializability and sharing serializability, are proposed and hybrid serializability, which combines these two basic criteria, is discussed. These criteria offer the advantage of imposing no restrictions on local sites other than local serializability while retaining global serializability. The graph testing techniques of the three criteria are provided as guidance for global transaction scheduling. In addition, an optimal property of global transactions for determinating the serialization order of global subtransactions at local sites is formulated. This property defines the upper limit on global serializability in multidatabase systems.

Distributed management of transactions in heterogeneous distributed database systems

In the two phase commit protocol, the coordinator decides to commit the distributed transaction if all its subtransactions are in a state called the “prepared state”. We have increased the semantics of the two phase commit protocols by replacing the prepared state by more restrictive states called the “source state” and the “serializable state”. The source state ensures that the execution of committed distributed transactions is not only atomic but also serializable. It also ensures that distributed transactions cannot interfere with other distributed transactions through local transactions, but it does not ensure a serializable view of the distributed database if local transactions are taken into account. The serializable state ensures that the execution is serializable also if local transactions are taken into account. Which of the two correctness criteria is appropriate for global concurrency control depends on the distributed applications of the system. In the proposed mechanism, each interconnected site supports the source state or the serializable state. Various methods of supporting these states are presented. Which method is appropriate for a site depends on the concurrency control method and the autonomy requirements of the site.

A non-two-phase locking protocol for global concurrency control in distributed heterogeneous database systems

A concurrency control method is proposed for global transactions in a distributed heterogeneous database system. This method is applicable when the database sites are interconnected in a rooted tree fashion. It guarantees deadlock freedom in addition to serializability. A general architecture of a heterogeneous system is given. The global transaction manager (GTM) decomposes the global transactions initiated at a site and the subtransactions received from other sites into smaller subtransactions, some of which are sent to the GTMs of the other sites, and those remaining, called g-local transactions, are to be executed by the local transaction manager (LTM) at that site. A concurrency control mechanism ensures serializability among: the local transactions (including the g-local transactions of the global ones) at each site, the global transactions and the global and local transactions together.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>