THREE-PHASE LOCKING (3PL)
by jasim qazi [121]
Problems with 2PL

2PL has performance issues when dealing with
frequently-changing data in multi user
environments:
 Deadlocks:
multiple users so concurrency a major
issue. Added to the number of data access, deadlocks
occur.
Problems with 2PL(UPDATE)

Cascading Rollbacks: in this
case if an error occurs after
the Read(A,a) operation of
T1, then T1 will have to
rollbackand start over. Since
T2 and T3 are waiting for the
result of T1, these two will
also be rolled back till T1
executes correctly.
T1
T2
T3
Lock(A)
Read(A,a)
Lock(B)
Read(B,b)
Write(A,a)
Unlock(A)
Lock(A)
Read(A,a)
Write(A,a)
Unlock(A)
Lock(A)
Read(A,a)
3PL




Not a necessity, not used a lot.
Procedures available to solve 2PL’s problems.
Dependent on the nature of the data and the
environment: changing data, many users.
Types of locks used: Read, Write, Write Intent.
Scenario




User signs in and requests data from the
database.
Read lock is applied to the data.
Once the data is read, the Read lock is
dropped.
There can be multiple Read locks by multiple
users on the same data/tuple.
Scenario





When the user indicates that he wishes to edit the data,
take out a WRITE-INTENT lock.
UPDATE:Write Intent Lock also known as Change Lock or
Protect Lock.
Other users can still obtain a Read Lock on the data.
Write Lock allowed only to the user who has the Write
Intent Lock.
If data locked with Write Intent Lock, then no further Write
Intent Locks can be applied on it.
Scenario


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When user finishes editing the data and submits
the changes, immediately Write Lock is applied on
the data.
Write Intent Lock is unlocked.
Transaction is committed and Write Lock is
unlocked.
3PL



All locks are subject to timeouts, with appropriate actions
(unlocks, error/warning messages to user etc) taken in the
event of lock failure. This prevents deadlocks.
All access to the data in question should use the same
locking protocol. No other protocol should be applied or
considered as it may disrupt the flow of operations.
UPDATE:Only WRITE-INTENT locks can be held for any
length of time, typically because the record of interest is at
the mercy of the user in edit mode...
Final words…



This is actually not dissimilar to the way many
"modern" relational databases handle locking,
The average RDBMS can't support the degree of
control involved here
3PL developed out of need.