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In transaction processing, databases, and computer networking, the two-phase commit protocol (2PC) is a type of atomic commitment protocol (ACP). It is a distributed algorithm that coordinates all the processes that participate in a distributed atomic transaction on whether to ''commit'' or ''abort'' (''roll back'') the transaction (it is a specialized type of consensus protocol). The protocol achieves its goal even in many cases of temporary system failure (involving either process, network node, communication, etc. failures), and is thus widely utilized.〔Philip A. Bernstein, Vassos Hadzilacos, Nathan Goodman (1987): ( ''Concurrency Control and Recovery in Database Systems'' ), Chapter 7, Addison Wesley Publishing Company, ISBN 0-201-10715-5〕〔Gerhard Weikum, Gottfried Vossen (2001): ( ''Transactional Information Systems'' ), Chapter 19, Elsevier, ISBN 1-55860-508-8〕〔Philip A. Bernstein, Eric Newcomer (2009): (''Principles of Transaction Processing'', 2nd Edition ), Chapter 8, Morgan Kaufmann (Elsevier), ISBN 978-1-55860-623-4〕 However, it is not resilient to all possible failure configurations, and in rare cases, user (e.g., a system's administrator) intervention is needed to remedy an outcome. To accommodate recovery from failure (automatic in most cases) the protocol's participants use logging of the protocol's states. Log records, which are typically slow to generate but survive failures, are used by the protocol's recovery procedures. Many protocol variants exist that primarily differ in logging strategies and recovery mechanisms. Though usually intended to be used infrequently, recovery procedures compose a substantial portion of the protocol, due to many possible failure scenarios to be considered and supported by the protocol. In a "normal execution" of any single distributed transaction, i.e., when no failure occurs, which is typically the most frequent situation, the protocol consists of two phases: #The ''commit-request phase'' (or ''voting phase''), in which a ''coordinator'' process attempts to prepare all the transaction's participating processes (named ''participants'', ''cohorts'', or ''workers'') to take the necessary steps for either committing or aborting the transaction and to ''vote'', either "Yes": commit (if the transaction participant's local portion execution has ended properly), or "No": abort (if a problem has been detected with the local portion), and #The ''commit phase'', in which, based on ''voting'' of the cohorts, the coordinator decides whether to commit (only if ''all'' have voted "Yes") or abort the transaction (otherwise), and notifies the result to all the cohorts. The cohorts then follow with the needed actions (commit or abort) with their local transactional resources (also called ''recoverable resources''; e.g., database data) and their respective portions in the transaction's other output (if applicable). Note that the two-phase commit (2PC) protocol should not be confused with the two-phase locking (2PL) protocol, a concurrency control protocol. ==Assumptions== The protocol works in the following manner: one node is designated the coordinator, which is the master site, and the rest of the nodes in the network are designated the cohorts. The protocol assumes that there is stable storage at each node with a write-ahead log, that no node crashes forever, that the data in the write-ahead log is never lost or corrupted in a crash, and that any two nodes can communicate with each other. The last assumption is not too restrictive, as network communication can typically be rerouted. The first two assumptions are much stronger; if a node is totally destroyed then data can be lost. The protocol is initiated by the coordinator after the last step of the transaction has been reached. The cohorts then respond with an agreement message or an abort message depending on whether the transaction has been processed successfully at the cohort. 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「Two-phase commit protocol」の詳細全文を読む スポンサード リンク
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