Two-phase Locking

In databases and transaction processing, two-phase locking (2PL) is a concurrency control method that guarantees serializability. It is also the name of the resulting set of database transaction schedules (histories). The protocol utilizes 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:

1. Expanding phase: locks are acquired and no locks are released.
2. Shrinking phase: locks are released and no locks are acquired.

Two types of locks are utilized by the basic protocol: Shared and Exclusive locks. Refinements of the basic protocol may utilize more lock types. Using locks that block processes, 2PL may be subject to deadlocks that result from the mutual blocking of two or more transactions.

2PL is a superset of strong strict two-phase locking (SS2PL), also called rigorousness, which has been widely utilized for concurrency control in general-purpose database systems since the 1970s. SS2PL implementations have many variants. SS2PL was called strict 2PL but this name usage is not recommended now. Now strict 2PL (S2PL) is the intersection of strictness and 2PL, which is different from SS2PL. SS2PL is also a special case of commitment ordering, and inherits many of CO's useful properties. SS2PL actually comprises only one phase: phase-2 does not exist, and all locks are released only after transaction end. Thus this useful 2PL type is not two-phased at all.

Neither 2PL nor S2PL in their general forms are known to be used in practice. Thus 2PL by itself does not seem to have much practical importance, and whenever 2PL or S2PL utilization has been mentioned in the literature, the intention has been SS2PL. What has made SS2PL so popular (probably the most utilized serializability mechanism) is the effective and efficient locking-based combination of two ingredients (the first does not exist in both general 2PL and S2PL; the second does not exist in general 2PL):

1. Commitment ordering, which provides both serializability, and effective distributed serializability and global serializability, and
2. Strictness, which provides cascadelessness (ACA, cascade-less recoverability) and (independently) allows efficient database recovery from failure.

Additionally SS2PL is easier, with less overhead to implement than both 2PL and S2PL, provides exactly same locking, but sometimes releases locks later. However, practically (though not simplistically theoretically) such later lock release occurs only slightly later, and this apparent disadvantage is insignificant and disappears next to the advantages of SS2PL.

Thus, the importance of the general Two-phase locking (2PL) is historic only, while Strong strict two-phase locking (SS2PL) is practically the important mechanism and resulting schedule property.