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Two-Phase Locking

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Encyclopedia of Database Systems

Synonyms

Locking protocol; Isolation; Conflict serializability; Pessimistic scheduler

Definition

A locked transaction is a transaction which, in addition to read and write actions, contains lock and unlock operations to the data items. Lock and unlock operations enable a database system to control the order of read and write actions of a concurrent set of transactions. A locking policy is a set of rules which restrict the possible ways to introduce lock and unlock operations into a transaction. A locking policy is safe, if, whenever all the transactions conform to the policy, any history of the transactions is guaranteed to be serializable. Two-Phase Locking is a safe locking policy which is based on the simple rule saying a transaction is not allowed to further lock a data item once it has already unlocked some data item.

Historical Background

Two-Phase Locking was first described in [6]. Later the basic policy has been extended into several directions. In [1] ordered sharing of...

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Recommended Reading

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Author information

Authors and Affiliations

  1. University of Freiburg, Freiburg, Germany

    Georg Lausen

Authors
  1. Georg Lausen
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Editor information

Editors and Affiliations

  1. College of Computing, Georgia Institute of Technology, 266 Ferst Drive, 30332-0765, Atlanta, GA, USA

    LING LIU (Professor) (Professor)

  2. Database Research Group David R. Cheriton School of Computer Science, University of Waterloo, 200 University Avenue West, N2L 3G1, Waterloo, ON, Canada

    M. TAMER ÖZSU (Professor and Director, University Research Chair) (Professor and Director, University Research Chair)

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© 2009 Springer Science+Business Media, LLC

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Lausen, G. (2009). Two-Phase Locking. In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_832

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