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).
What is rigorous phase locking?
Rigorous 2-PL –
This requires that in addition to the lock being 2-Phase all Exclusive
(X) and Shared(S) locks held
by the transaction be released until after the Transaction Commits.
What benefit does rigorous two phase locking provide?
Answer: Rigorous two-phase locking has the advantages of
strict 2PL
. In addi- tion it has the property that for two conflicting transactions, their commit order is their serializability order. In some systems users might expect this behavior.
What is the difference between two-phase locking and strict two-phase locking in transactions?
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.
How does the two-phase locking protocol ensures serializability?
This is a protocol which ensures conflict-serializable schedules. The protocol assures serializability. It can be proven that the
transactions can be serialized in
the order of their lock points (i.e., the point where a transaction acquired its final lock). Two-phase locking does not ensure freedom from deadlocks.
What is Multiversion two-phase locking explain with an example?
Multiversion Two-Phase Locking
Update transactions
acquire read and write locks, and hold all locks up to the end of the transaction
. That is, update transactions follow rigorous two-phase locking (all locks are held to the end of the transaction).
What is growing and shrinking phase?
Growing Phase:
New locks on data items may be acquired but none can be released
. Shrinking Phase: Existing locks may be released but no new locks can be acquired.
What is phantom read problem?
The phantom read problem occurs
when a transaction reads a variable once but when it tries to read that same variable again
, an error occurs saying that the variable does not exist. Example: … Thus, when transaction 2 tries to read X, it is not able to it.
What is conflict Serializability?
Conflict Serializable: A schedule is called conflict serializable if
it can be transformed into a serial schedule by swapping non-conflicting operations
. Conflicting operations: Two operations are said to be conflicting if all conditions satisfy: They belong to different transactions. They operate on the same data item.
Which among the following 2 phase locking protocol is a deadlock free?
2pl
is a deadlock free protocol but it is difficult to use in practice.
What benefit does strict two phase locking provide what are the disadvantages of it?
16.3 What benefit does strict two-phase locking provide? What disadvantages re- sult? Answer: Because it
produces only cascadeless schedules, recovery is very easy
. But the set of schedules obtainable is a subset of those obtainable from plain two phase locking, thus concurrency is reduced.
Shared lock can be placed on objects that do not have an exclusive lock already placed on them. Exclusive lock can only be placed on objects that do no have any other kind of lock. … Any number of transaction can hold shared lock on an item.
Exclusive lock can be hold by only one transaction
.
How Serializability is used in concurrency control?
Serializability is the classical concurrency scheme. It ensures that
a schedule for executing concurrent transactions is equivalent to one that executes the transactions serially in some order
. It assumes that all accesses to the database are done using read and write operations.
Does two-phase locking protocol ensures conflict serializability justify your answer with appropriate examples?
Answer: Rigorous two-phase locking has the advantages of strict 2PL. In addition it has the property that
for two conflicting transactions
, their commit order is their serializability order. … locks, or an update transaction, in which case it can request only exclusive locks.
How does locking protocol work?
When a locking protocol is being used,
a lock held by one transaction can block a lock request from another transaction
. If there are no circular waits for a lock, then the lock will ultimately be granted. If there are circular waits, then deadlock occurs.
What are the limitations of two-phase locking protocol?
The greatest disadvantage of the two-phase commit protocol is that it is
a blocking protocol
. If the coordinator fails permanently, some participants will never resolve their transactions: After a participant has sent an agreement message to the coordinator, it will block until a commit or rollback is received.
