Transaction management for mobile objects using optimistic concurrency control

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994.Includes bibliographical references (p. 89-93).by Atul Adya.M.S

Hermes: a Fast, Fault-Tolerant and Linearizable Replication Protocol

Today's datacenter applications are underpinned by datastores that are responsible for providing availability, consistency, and performance. For high availability in the presence of failures, these datastores replicate data across several nodes. This is accomplished with the help of a reliable replication protocol that is responsible for maintaining the replicas strongly-consistent even when faults occur. Strong consistency is preferred to weaker consistency models that cannot guarantee an intuitive behavior for the clients. Furthermore, to accommodate high demand at real-time latencies, datastores must deliver high throughput and low latency. This work introduces Hermes, a broadcast-based reliable replication protocol for in-memory datastores that provides both high throughput and low latency by enabling local reads and fully-concurrent fast writes at all replicas. Hermes couples logical timestamps with cache-coherence-inspired invalidations to guarantee linearizability, avoid write serialization at a centralized ordering point, resolve write conflicts locally at each replica (hence ensuring that writes never abort) and provide fault-tolerance via replayable writes. Our implementation of Hermes over an RDMA-enabled reliable datastore with five replicas shows that Hermes consistently achieves higher throughput than state-of-the-art RDMA-based reliable protocols (ZAB and CRAQ) across all write ratios while also significantly reducing tail latency. At 5% writes, the tail latency of Hermes is 3.6X lower than that of CRAQ and ZAB.Comment: Accepted in ASPLOS 202

Transaction Management for Mobile Objects Using Optimistic Concurrency Control

We present the design of a new transaction mechanism for an object-oriented database system called Thor. We also describe a mechanism that allows objects to migrate from one server to another. Our transaction management approach is di erent from other systems because we use optimistic concurrency control to provide serializability. Optimistic schemes have been suggested in the literature but they do not address issues such as space and logging overheads. In this thesis, we consider these problems and propose a scheme that has low space overhead per object and also has low delays. We take advantage of system characteristics such as loosely synchronized clocks and high availability to achieve these goals. We also present anovel mechanism that allows applications to increase the transaction throughput by overlapping the commit of a transaction with the execution of the next transaction. Our work on object migration is di erent from previous work because we provide transaction semantics with respect to movement of objects � if a user moves a set of objects, our scheme guarantees that either all or none of the objects are moved to their destination sites. I

Weak Consistency: A Generalized Theory and Optimistic Implementations for Distributed Transactions

Current commercial databases allow application programmers to trade off consistency for performance. However, existing definitions of weak consistency levels are either imprecise or they disallow efficient implementation techniques such as optimism. Ruling out these techniques is especially unfortunate because commercial databases support optimistic mechanisms. Furthermore, optimism is likely to be the implementation technique of choice in the geographically distributed and mobile systems of the future. This thesis presents the first implementation-independent specifications of existing ANSI isolation levels and a number of levels that are widely used in commercial systems, e.g., Cursor Stability, Snapshot Isolation. It also specifies a variety of guarantees for predicate-based operations in an implementation-independent manner. Two new levels are defined that provide useful consistency guarantees to application writers; one is the weakest level that ensures consistent reads, while the other captures some useful consistency properties provided by pessimistic implementations. We use a graph-based approach to define different isolation levels in a simple and intuitive manner. The thesis describes new implementation techniques for supporting different weak consistenc