Concurrency control mechanisms ensure transactions execute reliably in a distributed database system. Lock-based protocols require transactions to acquire locks before accessing data items to ensure serializability. Transactions can request shared or exclusive locks. Timestamp-based protocols assign timestamps to transactions to determine their execution order. Weak consistency models relax serializability for increased concurrency.

1. CONCURRENCY
Concurrency of Issues of Distributed
Advance Transaction
Presented by : ABDEL-HAFIZ AHMAD KHOUDOUR
Email : abdelhafiiz@yahoo.com

2. Concurrency Control
 when several transaction execute
concurrently in the database The
fundamental properties of the transaction
is isolation.
 The system must control the interaction
among the concurrent transaction.

3.  The control achieved through one of a
variety of mechanism called concurrency
control schemes .
 This concurrency – control schemes based
on the serializability property .

4. Lock based Protocol
 A locking protocol is a set of rules that
state when a transaction may lock and
unlock .
 To ensure serializability is to require that
data items be accessed in mutually
exclusive manner.

5.  While one transaction is accessing a data
item, no other transaction can modify that
data.
 The most common method used to
implement requirements is to allow a
transaction to access data item only if it is
currently holding a lock on that item.

6.  Two modes of data item may be locked
 Shared-mode (s) lock.
 Executive-mode (X) lock
Lock compatibility matrix
S X
S true false
X false false

7. • Shared mode is compatible with shared mode,
several shared mode lock can be held at the
same time on a particular data item .
• Every transaction request a lock depending on
the types of operations.

8. • A transaction will make by the concurrency –
control manager.
• Both of the transaction and the operation
continue after the concurrency-manager
grants the lock to the transaction.

9. Granting of Locks
The concurrency-control manager grants the lock
provided :
• If there is no other transaction holding a lock in
a conflicting mode.
• Also if there is no other transaction that is
waiting for a lock .
• A lock request made later will never get blocked
a lock request.

10. Implementation of locking
• Receive messages from transaction and sends
messages in reply.
• Replay to lock-request messages with lock-grants
messages.
• Unlock-messages require only and acknowledgment
In response, but may result in a grant messages to
another waiting transaction.

11. Graph-Based Protocols
• It impose restrictions on the order in which items are
accessed.
• in that way we can ensure serializability without
requiring of two-phase locking and can also ensure
deadlock freedom.

12. Timestamp – Based Protocols
• A method that to select an ordering among
transaction in advance.
• The most common method for doing so is to use a
timestamp-ordering scheme

13. Timestamps
The timestamp of the transactions determine the
serializability order.
There are two simple methods for implementing
this scheme:
1. Use the value of the system clock as the
timestamp, a transaction’s timestamp is equal
to the value of the clock when the transaction
enters the system.

14. Use a logical counter : is incremented when after a new
timestamp has been assigned
We can associate with each data items two values
W-timestamp
R-timestamp

15. Weak level of consistency
Serializability is a useful concept because it allows
programmers to ignore issues related to concurrency
when they code transaction.
For example SQL allows a transaction to specify that it
may executed in a way that becomes nonserializable
with respect to other transaction .