Describe
the 2PL concurrency control protocol.
It's a commonly used method to
ensure serializability in a database system, which means it ensures that
transactions are executed in such a way that the result is the same as if the
transactions were executed serially, one after the other.
The 2PL protocol consists of two
distinct phases:
1.
Growing
Phase: In this phase, a transaction can
acquire locks (both read and write locks) but cannot release any locks. As the
transaction progresses, it requests and acquires all the locks it needs. This
phase continues until the transaction acquires its final lock.
· Acquiring Locks:
During this phase, a transaction can acquire locks (shared or exclusive) on the
data items it needs to access.
· No Lock Release:
However, it cannot release any locks during this phase.
· Purpose: The
purpose of this phase is to gather all the necessary locks before any changes
are made to the data.
2.
Shrinking
Phase: Once a transaction releases its
first lock, it enters the shrinking phase. In this phase, the transaction can
only release locks and cannot acquire any new locks. This ensures that once a
transaction starts releasing locks, it must continue to release all the locks
it holds until it completes.
· Releasing Locks:
Once the transaction starts releasing any lock, it enters the shrinking phase.
· No Lock
Acquisition: During this phase, a transaction can release locks, but it
cannot acquire any new locks.
· Purpose: The
purpose of this phase is to gradually release the locks once the transaction no
longer needs them.
Ensuring
Serializability:
2PL ensures serializability by
enforcing the following rules:
- Once a transaction releases its first lock, it cannot
acquire any more locks.
- This prevents conflicting operations and ensures that
the execution order of transactions is consistent and equivalent to a
serial execution.
The combination of these two phases
ensures that there are no cycles in the wait-for graph, which helps in
preventing deadlocks and ensures serializability. Here’s a brief example to
illustrate the protocol:
1.
Growing
Phase:
o
Transaction T1 requests and acquires
a lock on Resource A.
o
Transaction T1 requests and acquires
a lock on Resource B.
2.
Shrinking
Phase:
o
Transaction T1 releases the lock on
Resource A.
o
Transaction T1 releases the lock on
Resource B.
The
key rule of 2PL is that no transaction can acquire a new lock after it has
released any of its locks. This simple yet effective rule helps maintain the
integrity and consistency of the database by ensuring proper synchronization of
concurrent transactions.
There are also variations of 2PL,
such as Strict 2PL and Rigorous 2PL:
- Strict 2PL:
Transactions hold all their exclusive locks until they commit or abort.
This ensures strict serializability and recoverability but can lead to
increased waiting times.
- Rigorous 2PL:
Transactions hold all their locks (both shared and exclusive) until they
commit or abort. This further simplifies the recovery process but can also
impact performance due to longer lock durations.
2PL is widely used due to its
simplicity and effectiveness in ensuring serializability in concurrent
transaction processing.
Example Scenario:
1.
Transaction
T1: Reads and then writes to
data items A and B.
o
T1 acquires an
S-lock on A.
o
T1 reads A.
o
T1 acquires an
X-lock on A (upgrading the S-lock to an X-lock).
o
T1 writes to A.
o
T1 acquires an
S-lock on B.
o
T1 reads B.
o
T1 acquires an
X-lock on B.
o
T1 writes to B.
o
T1 releases locks
on A and B (enters the shrinking phase).
2.
Transaction
T2: Wants to read data item A
while T1 is running.
o
T2 requests an
S-lock on A but must wait until T1 releases its X-lock on A.
o
Once T1 commits
and releases the lock, T2 can acquire the S-lock and proceed.
By following the 2PL protocol, the database ensures that transactions are executed in a safe and consistent manner, preventing issues like dirty reads, lost updates, and non-repeatable reads.
