Matakuliah
Tahun
Versi
: M0184 / Pengolahan Data Distribusi
: 2005
:
Session - 14
CONCURRENCY CONTROL
CONCURRENCY TECHNIQUES
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.
TIMESTAMP METHOD
• Fundamental goal of timestamping method
is to order transactions globally in such a
way that older transaction, transaction with
smaller timestamps, get priority in the
event of conflict
• Timestamps method produce serializable
schedules, which are equivalent to the
serial schedule defined by the timestamps
of successfully committed transactions
TIMESTAMPS IN A DISTRIBUTED
ENVIRONMENT
• No central system clock, each node in the
network has its own and there’s no guarantee
that these clocks are synchronized with each
other
• Two or more transactions can start
simultaneously at different sites in the network.
• Simple approach to supporting the notion of
global time in distributed system is to define
global time as concatenation of the local site
clock time with the site identifier, that is :
<site clock, site identifier>
Basic Timestamping
• To implement basic timestamping, the following
variables are required :
• For each data item x
ts (read x) = the timestamp of the transaction
which last read data item x
ts (write x) = the timestamp of the transaction
which last update data item x
• For each transaction T
ts (T) = the timestamp assigned to transaction T,
when it is launched
Write Operation using Basic
Timestamping
Begin
Ti attemps to update (write) data item x
if there is an update pending on X (ts(Tj) <
ts(Ti)) then
Ti wait until Tj is committed or restartes
else
ti commits update and sets ts(write x) = ts(ti)
end if
end
Conservative Timestamping
• A modification of basic timestamping
• Reduce the degree of concurrency but
eliminate the need for restart
• To transaction wait until system knows id
can not receive a conflicting request from
an older transaction
OPTIMISTIC METHODS
• Optimistic method of concurrency control
are based on premise that conflict are rare
and that the best approach is to allow
transaction to proceed.
• When transaction wishes to commit,
system will check for conflict. If conflict
detected, restart the transaction
• Optimistically allow transaction to proceed
as far as possible.
OPTIMISTIC METHODS
Transaction are executed in three phase :
1. Read phase; represent the body of the
transaction up to commit (no write to
database during this phase)
2. Validation; the result (updates) of the
transaction are examined for conflict
3. Write phase; if the transaction validated, the
update are propagated from the transactionlocal copy to the database. If during
validation, conflict which would result in a
loss of integrity are detected, then the
transaction is rolled back and restarted
Concurrency in replicated
database
• Where a DDBMS support replication, the
system must ensure that all copies of the
replicated data items are consistent.
• This can be done by treating the update of
replicated data items as a distributed,
atomic transaction which can be handle by
scheduler in the normal way
Concurrency in replicated
database
Transaction can be executed un three ways
:
1. Read phase, during which updates are
made to local copies of the data only
2. Validate phase, during which the
proposed update is checked for conflict at
all sites
3. Write phase, during which the transaction
is committed.