Matakuliah
Tahun
Versi
: M0184 / Pengolahan Data Distribusi
: 2005
:
Session - 13
CONCURRENCY CONTROL
CONCURRENCY TECHNIQUE
OBJECTIVES
• Concurrency control techniques, which are
Locking method, timestamp method and
optimistic methods
• Both locking and timestamping are essentially
conservative approach
• Optimistic method based on the premise that
conflict is rare and allow transaction to proceed
unsynchronized and only check for conflicts at
the end when a transaction commits.
Taxonomy of Concurrency Control
Mechanism
Concurrency Control
Optimistic
Pessimistic
Locking
Timestamp
Ordering
Hybrid
Centralized
Basic
Distributed
Multi Ver.
Primary
Copy
Conservative
Locking
Timestamp
Ordering
LOCKING METHODS
• Locking method are the most widely used
approach to handling concurrent control in
DBMS
• A transaction must claim a read (shared) or
write (exclusive) lock on a data item prior to
the execution of the corresponding read or write
operation.
• Read operation  it is permissible for more than
one transaction to hold read lock simultaneously
• Write Operation  it is transaction exclusive
access
• As long as transaction hold write lock, no other
transaction can read or update
LOCKING METHODS Cont’d
The most common locking protocol known
as two-phase locking (2PL).
The 2PL operate in two distinct phase :
Growing phase during which the transaction
acquires Locks and
A Shrinking phase during which it release
those lock
LOCKING METHODS Cont’d
The rules for transaction which obey 2PL are:
• Transaction are well-performed, must acquire a
lock on data object before operating on it and
release lock when the transaction is finished
• Compatibility rules for locking are observed, thus
no conflicting locks are held.
• Once the transaction has released lock, no new
locks are acquired
• All write locks are released together when
transaction is commit
DEADLOCK
xi
T1
T2
xj
Simple Deadlock
T1 is waiting for a lock on data item Xi which is currently
held by transaction T2, while T2 is waiting for a lock on data
item Xj which is currently held by transaction T1
G = T1  T2  T1
Distributed DeadLock
Site A
Site B
T1(A)
T1(B)
Xj
Xi
T2(A)
T2(B)