In databases and transaction processing two-phase locking, (2PL) is a concurrency control method that guarantees serializability.[1][2] It is also the name of the resulting set of database transaction schedules (histories). The protocol utilizes locks that block (interpreted as signals to stop) other transactions from accessing the same data during a transaction's life.
By the 2PL protocol locks are applied and removed in two phases:
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),[3] also called rigorousness,[4] 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[1] 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,[3] 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):
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.
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A lock is a system object associated with a shared resource such as a data item of an elementary type, a row in a database, or a page of memory. In a database, a lock on a database object (a data-access lock) may need to be acquired by a transaction before accessing the object. Correct use of locks prevents undesired, incorrect or inconsistent operations on shared resources by other concurrent transactions. When a database object with an existing lock acquired by one transaction needs to be accessed by another transaction, the existing lock for the object and the type of the intended access are checked by the system. If the existing lock type does not allow this specific attempted concurrent access type, the transaction attempting access is blocked (according to a predefined agreement/scheme). In practice a lock on an object does not directly block a transaction's operation upon the object, but rather blocks that transaction from acquiring another lock on the same object, needed to be held/owned by the transaction before performing this operation. Thus, with a locking mechanism, needed operation blocking is controlled by a proper lock blocking scheme, which indicates which lock type blocks which lock type.
Two major types of locks are utilized:
The common interactions between these lock types are defined by blocking behavior as follows:
Several variations and refinements of these major lock types exist, with respective variations of blocking behavior. If a first lock blocks another lock, the two locks are called incompatible; otherwise the locks are compatible. Often lock types blocking interactions are presented in the technical literature by a Lock compatibility table. The following is an example with the common, major lock types:
| Lock type | read-lock | write-lock |
|---|---|---|
| read-lock | X | |
| write-lock | X | X |
According to the two-phase locking protocol a transaction handles its locks in two distinct, consecutive phases during the transaction's execution:
The serializability property is guaranteed for a schedule with transactions that obey the protocol. The 2PL schedule class is defined as the class of all the schedules comprising transactions with data access orders that could be generated by the 2PL protocol (or in other words, all the schedules that the 2PL protocol can generate).
Typically, without explicit knowledge in a transaction on end of phase-1, it is safely determined only when a transaction has entered its ready state in all its processes (processing has ended, and it is ready to be committed; no additional data access and locking are needed and can happen). In this case phase-2 can end immediately (no additional processing is needed), and actually no phase-2 is needed. Also, if several processes (two or more) are involved, then a synchronization point (similar to atomic commitment) among them is needed to determine end of phase-1 for all of them (i.e., in the entire distributed transaction), to start releasing locks in phase-2 (otherwise it is very likely that both 2PL and Serializability are quickly violated). Such synchronization point is usually too costly (involving a distributed protocol similar to atomic commitment), and end of phase-1 is usually postponed to be merged with transaction end (atomic commitment protocol for a multi-process transaction), and again phase-2 is not needed. This turns 2PL to SS2PL (see below). All known implementations of 2PL in products are SS2PL based, and whenever 2PL (or Strict 2PL, S2PL) practical utilization has been mentioned in the professional literature, the intention has been SS2PL.
The strict two-phase locking (S2PL) class of schedules is the intersection of the 2PL class with the class of schedules possessing the Strictness property.
To comply with the S2PL protocol a transaction needs to comply with 2PL, and release its write (exclusive) locks only after it has ended, i.e., being either committed or aborted. On the other hand, read (shared) locks are released regularly during phase 2. Implementing general S2PL requires explicit support of phase-1 end, separate from transaction end, and no such widely utilized product implementation is known.
S2PL is a special case of 2PL, i.e., the S2PL class is a proper subclass of 2PL.
or Rigorousness, or Rigorous scheduling, or Rigorous two-phase locking
To comply with strong strict two-phase locking (SS2PL) the locking protocol releases both write (exclusive) and read (shared) locks applied by a transaction only after the transaction has ended, i.e., only after both completing executing (being ready) and becoming either committed or aborted. This protocol also complies with the S2PL rules. A transaction obeying SS2PL can be viewed as having phase-1 that lasts the transaction's entire execution duration, and no phase-2 (or a degenerate phase-2). Thus, only one phase is actually left, and "two-phase" in the name seems to be still utilized due to the historical development of the concept from 2PL, and 2PL being a super-class. The SS2PL property of a schedule is also called Rigorousness. It is also the name of the class of schedules having this property, and an SS2PL schedule is also called a "rigorous schedule". The term "Rigorousness" is free of the unnecessary legacy of "two-phase," as well as being independent of any (locking) mechanism (in principle other blocking mechanisms can be utilized). The property's respective locking mechanism is sometimes referred to as Rigorous 2PL.
SS2PL is a special case of S2PL, i.e., the SS2PL class of schedules is a proper subclass of S2PL (every SS2PL schedule is also an S2PL schedule, but S2PL schedules exist that are not SS2PL).
SS2PL has been the concurrency control protocol of choice for most database systems and utilized since their early days in the 1970s. It is proven to be an effective mechanism in many situations, and provides besides Serializability also Strictness (a special case of cascadeless Recoverability), which is instrumental for efficient database recovery, and also Commitment ordering (CO) for participating in distributed environments where a CO based distributed serializability and global serializability solutions are employed. Being a subset of CO, an efficient implementation of distributed SS2PL exists without a distributed lock manager (DLM), while distributed deadlocks (see below) are resolved automatically. The fact that SS2PL employed in multi database systems ensures global serializability has been known for years before the discovery of CO, but only with CO came the understanding of the role of an atomic commitment protocol in maintaining global serializability, as well as the observation of automatic distributed deadlock resolution (see a detailed example of Distributed SS2PL). As a matter of fact, SS2PL inheriting properties of Recoverability and CO is more significant than being a subset of 2PL, which by itself in its general form, besides comprising a simple serializability mechanism (however serializability is also implied by CO), in not known to provide SS2PL with any other significant qualities. 2PL in its general form, as well as when combined with Strictness, i.e., Strict 2PL (S2PL), are not known to be utilized in practice. The popular SS2PL does not require to mark "end of phase-1" as 2PL and S2PL do, and thus is simpler to implement. Also unlike the general 2PL, SS2PL provides, as mentioned above, the useful Strictness and Commitment ordering properties.
Many variants of SS2PL exist that utilize various lock types with various semantics in different situations, including cases of lock-type change during a transaction. Notable are variants that use Multiple granularity locking.
Comments:
| Lock type | read-lock | write-lock |
|---|---|---|
| read-lock | ||
| write-lock | X | X |
Between any two schedule classes (define by their schedules' respective properties) that have common schedules, either one contains the other (strictly contains if they are not equal), or they are incomparable. The containment relationships among the 2PL classes and other major schedule classes are summarized in the following diagram. 2PL and its subclasses are inherently blocking, which means that no optimistic implementations for them exist (and whenever "Optimistic 2PL" is mentioned it refers to a different mechanism with a class that includes also schedules not in the 2PL class).
Locks block data-access operations. Mutual blocking between transactions results in a deadlock, where execution of these transactions is stalled, and no completion can be reached. Thus deadlocks need to be resolved to complete these transactions' executions and release related computing resources. A deadlock is a reflection of a potential cycle in the precedence graph, that would occur without the blocking. A deadlock is resolved by aborting a transaction involved with such potential cycle, and breaking the cycle. It is often detected using a wait-for graph (a graph of conflicts blocked by locks from being materialized; conflicts not materialized in the database due to blocked operations are not reflected in the precedence graph and do not affect serializability), which indicates which transaction is "waiting for" lock release by which transaction, and a cycle means a deadlock. Aborting one transaction per cycle is sufficient to break the cycle. Transactions aborted due to deadlock resolution are executed again immediately.
In a distributed environment an atomic commitment protocol, typically the Two-phase commit (2PC) protocol, is utilized for atomicity. When recoverable data (data under transaction control) are partitioned among 2PC participants (i.e., each data object is controlled by a single 2PC participant), then distributed (global) deadlocks, deadlocks involving two or more participants in 2PC, are resolved automatically as follows:
When SS2PL is effectively utilized in a distributed environment, then global deadlocks due to locking generate voting-deadlocks in 2PC, and are resolved automatically by 2PC (see Commitment ordering (CO), in Exact characterization of voting-deadlocks by global cycles; No reference except the CO articles is known to notice this). For the general case of 2PL, global deadlocks are similarly resolved automatically by the synchronization point protocol of phase-1 end in a distributed transaction (synchronization point is achieved by "voting" (notifying local phase-1 end), and being propagated to the participants in a distributed transaction the same way as a decision point in atomic commitment; in analogy to decision point in CO, a conflicting operation in 2PL cannot happen before phase-1 end synchronization point, with the same resulting voting-deadlock in the case of a global data-access deadlock; the voting-deadlock (which is also a locking based global deadlock) is automatically resolved by the protocol aborting some transaction involved, with a missing vote, typically using a timeout).
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