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Notes > Database Systems > Locks, 2PL and Deadlocks


A transaction will use a lock to deny data access to other transactions and so prevent incorrect updates. Locks can be Read (shared) or Write (exclusive) locks. Write Locks on a data item prevent other transactions from reading that data item whereas Read Locks simply stop other transactions editing (writing to) the data item.

Two-Phase Locking (2PL)

A transaction follows the 2PL protocol if all lock requests come before the first unlock operation within the transaction. This means there are two main phases within the transaction:

- Growing phase (all locks are acquired)
- Shrinking phase (locks released - no new locks can be acquired)


Deadlock occurs when two or more transactions are left waiting for locks held by each other to be released. The only way to break a deadlock is to abort one of the transactions so that a lock is released so that the other transaction can proceed.

The DBMS can manage this process of aborting a transaction when necessary. The aborted transaction is typically restarted so that it is able to execute and commit without the user being aware of any problems occurring.

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