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X X X indicates incompatibility, i. An object typically has a queue of waiting requested by transactions operations with respective locks.
The first blocked lock for operation in the queue is acquired as soon as the existing blocking lock is removed from the object, and then its respective operation is executed.
If a lock for operation in the queue is not blocked by any existing lock existence of multiple compatible locks on a same object is possible concurrentlyit is acquired immediately. In some publications, the table entries are simply marked "compatible" or "incompatible", or respectively "yes" or "no".
Two-phase locking and its special cases[ edit ] Two-phase locking[ edit ] According to the two-phase locking protocol, a transaction handles its locks in two distinct, consecutive phases during the transaction's execution: Expanding phase aka Growing phase: Shrinking phase aka Contracting phase: The two phase locking rule can be summarized as: The serializability property is guaranteed for a schedule with transactions that obey this rule.
Typically, without explicit knowledge in a transaction on end of phase-1, it is safely determined only when a transaction has completed processing and requested commit.
In this case, all the locks can be released at once phase This is to ensure that a transaction that already holds some locks will not block waiting for other locks. Conservative 2PL prevents deadlocks.
Strict two-phase locking[ edit ] 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. On the other hand, read shared locks are released regularly during phase 2.
This protocol is not appropriate in B-trees because it causes Bottleneck while B-trees always starts searching from the parent root.
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 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.
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 has been the concurrency control protocol of choice for most database systems and utilized since their early days in the s. It is proven to be an effective mechanism in many situations, and provides besides Serializability also Strictness a special case of cascadeless Recoverabilitywhich is instrumental for efficient database recoveryand 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 DLMwhile 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 COin not known to provide SS2PL with any other significant qualities.
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.Your resume summary can go by any of several names, such as Summary of Qualifications, Highlights, Professional Summary, or Profile. You might even want to title it with your own personal headline.
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