Editing Two-phase commit protocol (section)

{{Short description|Computer science transaction algorithm}}
{{Redirect|2PC|the play in American and Canadian football|Two-point conversion|the cryptographic protocol|Commitment scheme|the concurrency control|Two-phase locking}}

[[File:Two phase commit seq diagram success 01.png|thumb|A typical sequence of a Two-phase commit.]]
In [[transaction processing]], [[database]]s, and [[computer networking]], the '''two-phase commit protocol''' ('''2PC''', ''tupac'')  is a type of [[Atomic commit|atomic commitment protocol]] (ACP). It is a [[distributed algorithm]] that coordinates all the processes that participate in a [[Distributed transaction|distributed atomic transaction]] on whether to [[Commit (data management)|commit]] or abort (roll back) the transaction. This protocol (a specialised type of [[Consensus (computer science)|consensus]] protocol) achieves its goal even in many cases of temporary system failure (involving either process, network node, communication, etc. failures), and is thus widely used.<ref name="bernstein1987">[[Phil Bernstein|Philip A. Bernstein]], Vassos Hadzilacos, Nathan Goodman (1987): [http://research.microsoft.com/en-us/people/philbe/ccontrol.aspx  ''Concurrency Control and Recovery in Database Systems''], Chapter 7, Addison Wesley Publishing Company, {{ISBN|0-201-10715-5}}</ref><ref name="weikum2001">[[Gerhard Weikum]], Gottfried Vossen (2001): [http://www.elsevier.com/wps/find/bookdescription.cws_home/677937/description#description  ''Transactional Information Systems''], Chapter 19, Elsevier, {{ISBN|1-55860-508-8}}</ref><ref name=Bern2009>Philip A. Bernstein, Eric Newcomer (2009): [http://www.elsevierdirect.com/product.jsp?isbn=9781558606234 ''Principles of Transaction Processing'', 2nd Edition] {{Webarchive|url=https://web.archive.org/web/20100807151625/http://www.elsevierdirect.com/product.jsp?isbn=9781558606234 |date=2010-08-07 }}, Chapter 8, Morgan Kaufmann (Elsevier), {{ISBN|978-1-55860-623-4}}</ref>
However, it is not resilient to all possible failure configurations, and in rare cases, manual intervention is needed to remedy an outcome. To accommodate recovery from failure (automatic in most cases) the protocol's participants use [[Server log|logging]] of the protocol's states. Log records, which are typically slow to generate but survive failures, are used by the protocol's [[recovery procedure]]s. 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 participants, the coordinator decides whether to commit (only if all have voted "Yes") or abort the transaction (otherwise), and notifies the result to all the participants. The participants 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).

The two-phase commit (2PC) protocol should not be confused with the [[two-phase locking]] (2PL) protocol, a [[concurrency control]] protocol.