Optimistic concurrency control for abstract data types

Abstract

A concurrency control technique is optimistic if it allows transactions to execute without synchronization, relying on commit-time validation to ensure serializability. This paper describes several new optimistic concurrency control techniques for objects in distributed systems, proves their correctness and optimality properties, and characterizes the circumstances under which each is likely to be useful. These techniques have the following novel aspects. First, unlike many methods that classify operations only as reads or writes, these techniques systematically exploit type-specific properties of objects to validate more interleavings. Necessary and sufficient validation conditions are derived directly from an object's data type specification. Second, these techniques are modular: they can be applied selectively on a per-object (or even per-operation) basis in conjunction with standard pessimistic techniques such as two-phase locking, permitting optimistic methods to be introduced exactly where they will be most effective. Third, when integrated with quorum-consensus replication, these techniques circumvent certain tradeoffs between concurrency and availability imposed by comparable pessimistic techniques. Finally, the accuracy and efficiency of validation are further enhanced by some technical improvements: distributed validation is performed as a sideeffect of the commit protocol, and validation takes into account the results of operations, accepting certain interleavings that would have produced delays in comparable pessimistic schemes.

Local atomicity properties: modular concurrency control for abstract data types

“…It is important to state the basic definitions of correctness abstractly enough that they apply to a wide variety of synchronization and recovery mechanisms; we believe we have achieved this in our definition of atomicity. Support for this claim can be found in the analysis of locking implementations of dynamic atomicity, including recovery, in [42], [44], and [45], and a similar analysis of pessimistic hybrid atomic algorithms in [19] and of optimistic hybrid atomic algorithms in [17]. Second, our model places no interpretation on the order in which operations are scheduled to run.…”

“…For example, the choices of how much concurrency to provide, whether to use a locking or nonlocking implementation, or whether to use a pessimistic or optimistic approach can all be made locally as part of implementing a single object. (Nonlocking implementations of dynamic atomicity are discussed in [42]; optimistic implementations of hybrid atomicity are discussed in [17].) As a result, these decisions can be changed fairly easily, since only a single object is affected.…”

“…By encapsulating the synchronization and recovery needed to support atomicity in the implementations of the shared objects, we can enhance modularity; in addition, by using information about the specifications of the shared objects, we can increase concurrency among transactions. (Examples illustrating how specifications can be used to enhance concurrency will be given in the body of this paper; further examples, including detailed algorithms, can be found in a number of papers, including [17]- [19], [37], [42], and [44]- [46]. )…”

Consistency without borders

“…To address this, several researchers have explored concurrency control techniques for objects or abstract data types [33,39,47,51,54,60]. Recently, Conflict-Free Replicated Data Types (CRDTs) have been proposed as a way to encode a common class of useful semantic properties: data values that change in an associative, commutative, and idempotent fashion, which guarantees that replicas of such values will eventually converge [55].…”

Transactional Support for Cooperative Applications

“…Other (non-symmetric) semantic relations have been proposed in the literature that relax the requirements on the state equivalence of the data objects and enable an even higher degree of concurrency control. In [Her86,Her90], the invalidated-by relationship has been introduced for optimistic concurrency. Recoverability [BR92] allows non-commuting but recoverable operations to be executed concurrently.…”