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Commitment ordering (CO) is a class of interoperable ''serializability'' techniques in concurrency control of databases, transaction processing, and related applications. It allows optimistic (non-blocking) implementations. With the proliferation of multi-core processors, CO has been also increasingly utilized in concurrent programming, transactional memory, and especially in software transactional memory (STM) for achieving serializability optimistically. CO is also the name of the resulting transaction schedule (history) property, which was originally defined in 1988 with the name ''dynamic atomicity''.〔Alan Fekete, Nancy Lynch, Michael Merritt, William Weihl (1988): (''Commutativity-based locking for nested transactions'' (PDF) ) MIT, LCS lab, Technical report MIT/LCS/TM-370, August 1988.〕 In a CO compliant schedule the chronological order of commitment events of transactions is compatible with the precedence order of the respective transactions. CO is a broad special case of ''conflict serializability'', and effective means (reliable, high-performance, distributed, and scalable) to achieve global serializability (modular serializability) across any collection of database systems that possibly use different concurrency control mechanisms (CO also makes each system serializability compliant, if not already). Each not-CO-compliant database system is augmented with a CO component (the commitment order coordinator—COCO) which orders the commitment events for CO compliance, with neither data-access nor any other transaction operation interference. As such CO provides a low overhead, general solution for global serializability (and distributed serializability), instrumental for global concurrency control (and distributed concurrency control) of multi database systems and other transactional objects, possibly highly distributed (e.g., within cloud computing, grid computing, and networks of smartphones). An atomic commitment protocol (ACP; of any type) is a fundamental part of the solution, utilized to break global cycles in the conflict (precedence, serializability) graph. CO is the most general property (a necessary condition) that guarantees global serializability, if the database systems involved do not share concurrency control information beyond atomic commitment protocol (unmodified) messages, and have no knowledge whether transactions are global or local (the database systems are ''autonomous''). Thus CO (with its variants) is the only general technique that does not require the typically costly distribution of local concurrency control information (e.g., local precedence relations, locks, timestamps, or tickets). It generalizes the popular ''strong strict two-phase locking'' (SS2PL) property, which in conjunction with the ''two-phase commit protocol'' (2PC) is the de facto standard to achieve global serializability across (SS2PL based) database systems. As a result CO compliant database systems (with any, different concurrency control types) can transparently join such SS2PL based solutions for global serializability. In addition, locking based ''global deadlocks'' are resolved automatically in a CO based multi-database environment, an important side-benefit (including the special case of a completely SS2PL based environment; a previously unnoticed fact for SS2PL). Furthermore, strict commitment ordering (SCO; Raz 1991c), the intersection of ''Strictness'' and CO, provides better performance (shorter average transaction completion time and resulting better transaction throughput) than SS2PL whenever read-write conflicts are present (identical blocking behavior for write-read and write-write conflicts; comparable locking overhead). The advantage of SCO is especially significant during lock contention. Strictness allows both SS2PL and SCO to use the same effective ''database recovery'' mechanisms. Two major generalizing variants of CO exist, extended CO (ECO; Raz 1993a) and multi-version CO (MVCO; Raz 1993b). They as well provide global serializability without local concurrency control information distribution, can be combined with any relevant concurrency control, and allow optimistic (non-blocking) implementations. Both use additional information for relaxing CO constraints and achieving better concurrency and performance. Vote ordering (VO or Generalized CO (GCO); Raz 2009) is a container schedule set (property) and technique for CO and all its variants. Local VO is a necessary condition for guaranteeing global serializability, if the atomic commitment protocol (ACP) participants do not share concurrency control information (have the ''generalized autonomy'' property). CO and its variants inter-operate transparently, guaranteeing global serializability and automatic global deadlock resolution also together in a mixed, heterogeneous environment with different variants. ==Overview== The ''Commitment ordering'' (CO; Raz 1990, 1992, 1994, 2009) schedule property has been referred to also as ''Dynamic atomicity'' (since 1988〔), ''commit ordering'', ''commit order serializability'', and ''strong recoverability'' (since 1991). The latter is a misleading name since CO is incomparable with ''recoverability'', and the term "strong" implies a special case. This means that a schedule with a strong recoverability property does not necessarily have the CO property, and vice versa. In 2009 CO has been characterized as a major concurrency control method, together with the previously known (since the 1980s) three major methods: ''Locking'', ''Time-stamp ordering'', and ''Serialization graph testing'', and as an enabler for the interoperability of systems using different concurrency control mechanisms.〔Philip A. Bernstein, Eric Newcomer (2009): (''Principles of Transaction Processing'', 2nd Edition ), Morgan Kaufmann (Elsevier), June 2009, ISBN 978-1-55860-623-4 (pages 145, 360)〕 In a federated database system or any other more loosely defined multidatabase system, which are typically distributed in a communication network, transactions span multiple and possibly Distributed databases. Enforcing global serializability in such system is problematic. Even if every local schedule of a single database is serializable, still, the global schedule of a whole system is not necessarily serializable. The massive communication exchanges of conflict information needed between databases to reach conflict serializability would lead to unacceptable performance, primarily due to computer and communication latency. The problem of achieving global serializability effectively had been characterized as open until the public disclosure of CO in 1991 by its inventor Yoav Raz (Raz 1991a; see also Global serializability). Enforcing CO is an effective way to enforce conflict serializability globally in a distributed system, since enforcing CO locally in each database (or other transactional object) also enforces it globally. Each database may use any, possibly different, type of concurrency control mechanism. With a local mechanism that already provides conflict serializability, enforcing CO locally does not cause any additional aborts, since enforcing CO locally does not affect the data access scheduling strategy of the mechanism (this scheduling determines the serializability related aborts; such a mechanism typically does not consider the commitment events or their order). The CO solution requires no communication overhead, since it uses (unmodified) ''atomic commitment'' protocol messages only, already needed by each distributed transaction to reach atomicity. An atomic commitment protocol plays a central role in the distributed CO algorithm, which enforces CO globally, by breaking global cycles (cycles that span two or more databases) in the global conflict graph. CO, its special cases, and its generalizations are interoperable, and achieve global serializability while transparently being utilized together in a single heterogeneous distributed environment comprising objects with possibly different concurrency control mechanisms. As such, ''Commitment ordering'', including its special cases, and together with its generalizations (see CO variants below), provides a general, high performance, fully distributed solution (no central processing component or central data structure are needed) for guaranteeing global serializability in heterogeneous environments of multidatabase systems and other multiple transactional objects (objects with states accessed and modified only by transactions; e.g., in the framework of transactional processes, and within Cloud computing and Grid computing). The CO solution scales up with network size and the number of databases without any negative impact on performance (assuming the statistics of a single distributed transaction, e.g., the average number of databases involved with a single transaction, are unchanged). With the proliferation of Multi-core processors, Optimistic CO (OCO) has been also increasingly utilized to achieve serializability in software transactional memory, and numerous STM articles and patents utilizing "commit order" have already been published (e.g., Zhang et al. 2006〔). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Commitment ordering」の詳細全文を読む スポンサード リンク
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