Editing Two-phase locking (section)

{{Short description|Concurrency control method}}
{{About|[[concurrency control]]|commit consensus within a distributed transaction|Two-phase commit protocol}}
In [[database]]s and [[transaction processing]], '''two-phase locking''' ('''2PL''') is a pessimistic [[concurrency control]] method that guarantees [[Database transaction schedule#Conflict-serializable|conflict-serializability]].<ref name=Bern1987>[[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''], 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''], Elsevier, {{ISBN|1-55860-508-8}}</ref> It is also the name of the resulting set of [[database transaction]] [[database transaction schedule|schedule]]s (histories). The protocol uses [[lock (computer science)|locks]], applied by a transaction to data, which may block (interpreted as signals to stop) other transactions from accessing the same data during the transaction's life.

By the 2PL protocol, locks are applied and removed in two phases:
# Expanding phase: locks are acquired and no locks are released.
# Shrinking phase: locks are released and no locks are acquired.
Two types of locks are used by the basic protocol: ''Shared'' and ''Exclusive'' locks. Refinements of the basic protocol may use more lock types. Using locks that block processes, 2PL, S2PL, and SS2PL may be subject to [[deadlock (computer science)|deadlock]]s that result from the mutual blocking of two or more transactions.