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is strict 2pl deadlock free

by Kay Crist Published 3 years ago Updated 3 years ago

If any of the predeclared items needed cannot be locked, the transaction does not lock any of the items, instead, it waits until all the items are available for locking. Conservative 2-PL is Deadlock free and but it does not ensure a Strict schedule(More about this here!).Jun 2, 2021

Full Answer

Is deadlock possible in 2-pl?

So Deadlock is also possible in 2-PL. Two-phase locking may also limit the amount of concurrency that occurs in a schedule because a Transaction may not be able to release an item after it has used it.

What is strong strict two-phase locking (SS2PL)?

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.

How does SS2PL resolve global deadlock?

When SS2PL is effectively used 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).

What are the two types of locks used in 2PL?

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.

Is 2PL deadlock free?

In Conservative 2PL protocol, a transaction has to lock all the items it access before the transaction begins execution. To avoid deadlocks, we can use Conservative 2PL. so it is deadlock free but drawback is starvation is possible .

Can strict 2PL have deadlock?

Rigorous 2PL : There is no deadlock. Also, a younger transaction requesting an item held by an older transaction is aborted and restart with the same timestamp, starvation is avoided.

Is lock based protocol deadlock free?

In traditional transaction processing systems, a transaction can be processed by multiple threads. These threads processing a transaction marks their operations using the transaction id.

What is 2PL strict locking?

Strict Two-Phase Locking Strict-2PL holds all the locks until the commit point and releases all the locks at a time. Strict-2PL does not have cascading abort as 2PL does.

Is strict 2PL Cascadeless?

In addition to a serial/serializable schedule, a schedule should be a cascadeless schedule. But, in 2PL, cascading rollback may occur in few cases.

How does 2PL vary from strict 2PL?

In Conservative 2-PL, A transaction has to acquire locks on all the data items it requires before the transaction begins it execution. In Strict 2-PL, A transaction can acquire locks on data items whenever it requires (only in growing phase) during its execution. 2. It does not have growing phase.

What is 2 phase locking protocol compare 2PL with strict 2PL protocol?

The only difference between 2PL and strict 2PL is that Strict-2PL does not release a lock after using it. Strict-2PL waits until the whole transaction to commit, and then it releases all the locks at a time. Strict-2PL protocol does not have shrinking phase of lock release.

What are the disadvantages of lock based protocols?

Problems associated with Simple locking:Data inconsistency between multiple transactions.Deadlock, a situation where the transactions try to access lock on already locked data items.No guarantee of serializability (i.e. execution of a concurrent transaction equivalent to that of a transaction executed serially)

Which of the following concurrency control protocol ensures both conflict and free from deadlock?

(b)Time stamp – ordering concurrency protocol ensures both conflict serializability and freedom from deadlock.

Is strict 2PL recoverable?

Strict 2-PL – This requires that in addition to the lock being 2-Phase all Exclusive(X) locks held by the transaction be released until after the Transaction Commits. Following Strict 2-PL ensures that our schedule is: Recoverable.

Why is strict 2PL popular?

Strict 2PL is popular for many reasons. One reason is that it ensures only 'safe' interleaving of transactions so that transactions are recoverable, avoid cascading aborts, etc. Another reason is that strict 2PL is very simple and easy to implement.

What are the benefits and disadvantages of strict two-phase locking?

AdvantagesRecovery is very easy. DisadvantagesConcurrency is reducedExplain time stamp based protocol. Time stamp based protocolThis protocol uses either system time or logical counter to be used as a time-stamp.

Why is strict 2PL so popular?

One reason is that it ensures only 'safe' interleaving of transactions so that transactions are recoverable, avoid cascading aborts, etc. Another reason is that strict 2PL is very simple and easy to implement.

What is 2PL in database?

In databases and transaction processing, two-phase locking (2PL) is a concurrency control method that guarantees serializability. 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.

Does strict 2 phase lock release?

Strict Two-Phase Locking. But in contrast to 2PL, Strict-2PL does not release a lock after using it. Strict-2PL holds all the locks until the commit point and releases all the locks at a time. Strict-2PL does not have cascading abort as 2PL does. Click to see full answer.

What is strict 2PL?

1987, p. 59), the term "Strict 2PL" (S2PL) has been frequently defined by the locking protocol "Release all locks only after transaction end," which is the protocol of SS2PL.

How does global deadlock work in SS2PL?

When SS2PL is effectively used 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 ).

How do deadlocks work?

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. If a transaction has been aborted due to deadlock resolution, it is up to the application to decide what to do next. Usually, an application will restart the transaction from the beginning but may delay this action to give other transactions sufficient time to finish in order to avoid causing another deadlock.

What is SCO in SS2PL?

A more general property than SS2PL exists (a schedule super-class), Strict commitment ordering (Strict CO, or SCO), which as well provides both serializability, strictness, and CO, and has similar locking overhead. Unlike SS2PL, SCO does not block upon a read-write conflict (a read lock does not block acquiring a write lock; both SCO and SS2PL have the same behavior for write-read and write-write conflicts) at the cost of a possible delayed commit, and upon such conflict type SCO has shorter average transaction completion time and better performance than SS2PL. While SS2PL obeys the lock compatibility table above, SCO has the following table:

What is SS2PL in a schedule?

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 used 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 used). The property's respective locking mechanism is sometimes referred to as Rigorous 2PL .

What are the two types of locks used in the basic protocol?

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.

What is two phase locking?

According to the two-phase locking protocol, a transaction handles its locks in two distinct, consecutive phases during the transaction's execution:

Why is two phase locking considered a bargain?

Two-phase locking may also limit the amount of concurrency that occurs in a schedule because a Transaction may not be able to release an item after it has used it. This may be because of the protocols and other restrictions we may put on the schedule to ensure serializability, deadlock freedom, and other factors. This is the price we have to pay to ensure serializability and other factors, hence it can be considered as a bargain between concurrency and maintaining the ACID properties.

What are the two types of lock?

Now, recalling where we last left off, there are two types of Locks available Shared S (a) and Exclusive X (a). Implementing this lock system without any restrictions gives us the Simple Lock-based protocol (or Binary Locking ), but it has its own disadvantages, they do not guarantee Serializability. Schedules may follow the preceding rules but a non-serializable schedule may result.

What is a transaction that follows the two phase lock protocol?

A transaction is said to follow the Two-Phase Locking protocol if Locking and Unlocking can be done in two phases.

Why is two phase locking important?

Two-phase locking may also limit the amount of concurrency that occurs in a schedule because a Transaction may not be able to release an item after it has used it. This may be because of the protocols and other restrictions we may put on the schedule to ensure serializability, deadlock freedom, and other factors.

Can you rollback a skeleton schedule in 2PL?

Yes, you’re correct, because of Dirty Read in T 2 and T 3 in lines 8 and 12 respectively, when T 1 failed we have to roll back others also. Hence, Cascading Rollbacks are possible in 2-PL. I have taken skeleton schedules as examples because it’s easy to understand when it’s kept simple. When explained with real-time transaction problems with many variables, it becomes very complex.

image

Overview

Two-phase locking and its special cases

According to the two-phase locking protocol, a transaction handles its locks in two distinct, consecutive phases during the transaction's execution:
1. Expanding phase (aka Growing phase): locks are acquired and no locks are released (the number of locks can only increase).
2. Shrinking phase (aka Contracting phase): locks are released and no locks are acquired.

Data-access locks

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 transacti…

Deadlocks in 2PL

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 wit…

See also

• Serializability
• Lock (computer science)

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      • 23. /vendor/laravel/framework/src/Illuminate/Routing/ControllerDispatcher.php:45
      • 24. /vendor/laravel/framework/src/Illuminate/Routing/Route.php:261
    • select * from `nova_menu_menu_items` where `nova_menu_menu_items`.`parent_id` in (1) order by `order` asc
      300μs/vendor/outl1ne/nova-menu-builder/src/Models/Menu.php:35receivinghelpdeskask
      Metadata
      Backtrace
      • 24. /vendor/outl1ne/nova-menu-builder/src/Models/Menu.php:35
      • 25. /vendor/outl1ne/nova-menu-builder/src/helpers.php:33
      • 27. /vendor/laravel/framework/src/Illuminate/Routing/Controller.php:54
      • 28. /vendor/laravel/framework/src/Illuminate/Routing/ControllerDispatcher.php:45
      • 29. /vendor/laravel/framework/src/Illuminate/Routing/Route.php:261
    • select `id`, `post_title`, `slug` from `posts` where `status` = 'publish' and `posts`.`deleted_at` is null order by RAND() limit 10
      633ms/app/View/Composers/SidebarView.php:22receivinghelpdeskask
      Metadata
      Bindings
      • 0. publish
      Backtrace
      • 14. /app/View/Composers/SidebarView.php:22
      • 15. /app/View/Composers/SidebarView.php:12
      • 16. /vendor/laravel/framework/src/Illuminate/View/Concerns/ManagesEvents.php:124
      • 17. /vendor/laravel/framework/src/Illuminate/View/Concerns/ManagesEvents.php:162
      • 20. /vendor/laravel/framework/src/Illuminate/View/Concerns/ManagesEvents.php:177
    • select * from `fake_users` where `fake_users`.`id` = 30690 limit 1
      550μsview::2dd102cf0462e89a4d4d8bc77355d767652bf9aa:15receivinghelpdeskask
      Metadata
      Bindings
      • 0. 30690
      Backtrace
      • 21. view::2dd102cf0462e89a4d4d8bc77355d767652bf9aa:15
      • 23. /vendor/laravel/framework/src/Illuminate/Filesystem/Filesystem.php:108
      • 24. /vendor/laravel/framework/src/Illuminate/View/Engines/PhpEngine.php:58
      • 25. /vendor/livewire/livewire/src/ComponentConcerns/RendersLivewireComponents.php:69
      • 26. /vendor/laravel/framework/src/Illuminate/View/Engines/CompilerEngine.php:61
    App\Models\FakeUser
    1
    Outl1ne\MenuBuilder\Models\MenuItem
    1
    Outl1ne\MenuBuilder\Models\Menu
    1
    App\Models\JsonPostContent
    1
    App\Models\Post
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