Editing Two-phase locking (section)

=== Conservative two-phase locking ===
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Conservative two-phase locking (C2PL) differs from 2PL in that transactions obtain all the locks they need before the transactions begin. This is to ensure that a transaction that already holds some locks will not block waiting for other locks. C2PL ''prevents'' [[deadlock (computer science)|deadlock]]s. 

In cases of heavy lock [[Resource contention|contention]], C2PL reduces the time locks are held on average, relative to 2PL and Strict 2PL, because transactions that hold locks are never blocked. In light lock contention, C2PL holds more locks than is necessary, because it is difficult to predict which locks will be needed in the future, thus leading to higher overhead. 

A C2PL transaction will not obtain any locks if it cannot obtain all the locks it needs in its initial request. Furthermore, each transaction needs to declare its read and write set (the data items that will be read/written), which is not always possible. Because of these limitations, C2PL is not used very frequently.