CSR: Small: Scalable Transactional Replication: Theory, Protocols, and Middleware Systems

随着基于Web的网络应用程序呈指数级增长，其后端IT系统必须处理不断增长的数据和服务请求量。当应用程序工作负载生成对共享数据的并发访问时，获得高可扩展性是具有挑战性的，这些共享数据被复制以确保在出现故障时的数据生存和服务可用性。解决这个问题的经典事务技术-状态机复制-不能扩展：调节分布式事务的提交需要解决共识，其领导者是一个显著的可扩展性瓶颈。无领导者的共识协议只在可能的情况下提供快速决策，而付出了大量法定人数的代价。为了克服这些限制，该项目正在开发两种互补技术，用于为事务系统构建可扩展的共识协议。在第一种技术中，称为凯撒方法，共识决策总是在两个通信延迟中做出，即，使用基于建议位置的方案的快速决策：在节点上激活的事务，即，事务的协调器在事务的协调器建议的位置处的所有节点上执行，并且在执行在较小位置处选择的任何其他冲突事务之后执行。为了实现这一点，事务的协调器只需要知道提议的位置不会被快速仲裁的节点拒绝。然而，通过利用网络延迟和时钟漂移估计，位置以使得它们永远不会被拒绝的方式被调整。因此，为了利用快速决策，使用比解决共识所需的更大的快速法定人数的成本被总是以这种方式决策的能力所摊销。在第二种技术中，称为M^2Paxos方法，通过依赖于解决共识所严格需要的经典仲裁大小，通常仅在两个通信延迟中决定事务的顺序，即，大多数节点。这是通过利用应用程序的数据访问局部性来实现的。特别地，在低竞争的情况下，M^2Paxos检查提交的事务要访问的数据，并确定负责对事务进行排序的节点。该项目正在将Caesar和M^2Paxos转换为实验性的开源HyFlow事务中间件系统，这使得研究社区能够广泛采用该技术。此外，该项目正在将这些技术转移到Red Hat/JBoss的生产事务中间件Infinispan中，这使得J2EE开发人员能够广泛采用这些技术。

With the exponentially increasing popularity of web-based networked applications, their back-end IT systems must process an ever growing volume of data and service requests. Obtaining high scalability is challenging when application workloads generate concurrent accesses on shared data that is replicated to ensure data survival and service availability in the presence of failures. The classical transactional technology for solving this problem -- State Machine Replication -- does not scale: regulating the commits of distributed transactions requires solving consensus, whose leader is a significant scalability bottleneck. Leaderless consensus protocols unfruitfully pay the cost of large quorums for providing fast decisions only whenever possible. To overcome these limitations, the project is developing two complimentary techniques for building scalable consensus protocols for transactional systems. In the first technique, called the Caesar approach, consensus decisions are always made in two communication delays, i.e., fast decisions, using a scheme based on proposed positions: a transaction activated on a node, i.e., the transaction's coordinator, is executed on all nodes at a position proposed by the transaction's coordinator, and after the execution of any other conflicting transaction that was chosen at a lesser position. To achieve that, the transaction's coordinator only needs to know that the proposed position is not rejected by a fast quorum of nodes. However, by exploiting network delays and clock drift estimates, the positions are adjusted in a way such that they are never rejected. Thus, the cost of using fast quorums larger than the ones necessary to solve consensus in order to exploit a fast decision is amortized by the ability of always deciding in that way. In the second technique, called the M^2Paxos approach, the order of transactions is generally decided in only two communication delays by relying on the classical quorum size that is strictly necessary to solve consensus, i.e., a majority of nodes. This is achieved by exploiting application's data access locality. In particular, in case of low contention, M^2Paxos inspects the data to be accessed by a submitted transaction and determines the node responsible for ordering the transaction. This allows all transactions accessing the same data to be implicitly ordered by the same node.The project is transitioning Caesar and M^2Paxos into the experimental, open-source HyFlow transactional middleware system, which enables adoption of the techniques by the research community at large. Additionally, the project is transitioning the techniques into Red Hat/JBoss's production transactional middleware, Infinispan, which enables adoption of the techniques by J2EE developers at large.