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Lock based protocols
This document discusses lock-based protocols for concurrency control. It describes that locks can be requested in exclusive or shared mode to control concurrent access to data items. A lock compatibility matrix is used to determine if a requested lock is compatible with existing locks held by other transactions. The Two Phase Locking protocol is introduced to ensure conflict serializable schedules by restricting transactions to an growing phase where they only acquire locks and a shrinking phase where they only release locks.
In this document
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Introduction to lock-based protocols managing concurrent data access, including lock types, compatibility, and examples.
Detailed description of the Two Phase Locking (2PL) protocol ensuring serializability through growing and shrinking phases.
Closing remarks on the presented lock-based protocols.
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Lock based protocols
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- 2.LOCK BASED PROTOCOLSA lock is a mechanism to control concurrent access to a data item Data items can be locked in two modes :1. exclusive (X) mode. Data item can be both read as well aswritten. X-lock is requested using lock-X instruction.2. shared (S) mode. Data item can only be read. S-lock isrequested using lock-S instruction. Lock requests are made to the concurrency-control manager by theprogrammer. Transaction can proceed only after request is granted.
- 3.LOCK BASED PROTOCOL(CONTD)Lock-compatibility matrix A transaction may be granted a lock on an item if the requested lock is compatiblewith locks already held on the item by other transactions Any number of transactions can hold shared locks on an item, But if any transaction holds an exclusive on the item no other transaction may holdany lock on the item. If a lock cannot be granted, the requesting transaction is made to wait till allincompatible locks held by other transactions have been released. The lock is thengranted.
- 4.LOCK BASED PROTOCOL(CONTD)Example of a transaction performing locking:T2: lock-S(A);read (A);unlock(A);lock-S(B);read (B);unlock(B);display(A+B) Locking as above is not sufficient to guarantee serializability — if A and B get updated in-between the read of A and B, the displayed sum would be wrong. A locking protocol is a set of rules followed by all transactions while requesting and releasinglocks. Locking protocols restrict the set of possible schedules.
- 5.TWO PHASE LOCKINGPROTOCOL This protocol ensures conflict-serializable schedules. Phase 1: Growing Phase Transaction may obtain locks Transaction may not release locks Phase 2: Shrinking Phase Transaction may release locks Transaction may not obtain locks The protocol assures serializability. It can be proved that the transactions canbe serialized in the order of their lock points (i.e., the point where atransaction acquired its final lock).
- 6.TWO PHASE LOCKINGPROTOCOL(CONTD)There can be conflict serializable schedules that cannot be obtained if two-phase locking is used. However, in the absence of extra information (e.g., ordering of access todata), two-phase locking is needed for conflict serializability in the followingsense: Given a transaction Ti that does not follow two-phase locking, we can find atransaction Tj that uses two-phase locking, and a schedule for Ti and Tj that is notconflict serializable.
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