1. By Donavon Norwood Ankit Patel 112 Aniket Mulye 111
Concurrency Control
By Donavon Norwood
Ankit Patel 112
Aniket Mulye 111

2. INTRODUCTION Enforcing serializability by locks
Locking scheduler
Two phase locking
Locking systems with several lock modes
Shared and exclusive locks
Compatibility matrices
Upgrading/updating locks
Incrementing locks

3. Locks It works like as follows : A request from transaction
Scheduler checks in the lock table
Generates a serializable schedule of actions.

4. Consistency of transactions
Actions and locks must relate each other
Transactions can only read & write only if has a lock and has not released the lock.
Unlocking an element is compulsory.
Legality of schedules
No two transactions can aquire the lock on same element without the prior one releasing it.

5. Locking scheduler
Grants lock requests only if it is in a legal schedule.
Lock table stores the information about current locks on the elements.

6. The locking scheduler (contd.)
A legal schedule of consistent transactions but unfortunately it is not a serializable.

7. Locking schedule (contd.)
The locking scheduler delays requests that would result in an illegal schedule.

8. Two-phase locking
Guarantees a legal schedule of consistent transactions is conflict-serializable.
All lock requests proceed all unlock requests.
The growing phase:
Obtain all the locks and no unlocks allowed.
The shrinking phase:
Release all the locks and no locks allowed.

9. Working of Two-Phase locking
Assures serializability.
Two protocols for 2PL:
Strict two phase locking : Transaction holds all its exclusive locks till commit / abort.
Rigorous two phase locking : Transaction holds all locks till commit / abort.
Possible to find a transaction Tj that has a 2PL and a schedule S for Ti ( non 2PL ) and Tj that is not conflict serializable.

10. Failure of 2PL.
2PL fails to provide security against deadlocks.

11. Locking Systems with Several Lock Modes
Locking Scheme
Shared/Read Lock ( For Reading)
Exclusive/Write Lock( For Writing)
Compatibility Matrices
Upgrading Locks
Update Locks
Increment Locks

12. Shared & Exclusive Locks
Consistency of Transactions
Cannot write without Exclusive Lock
Cannot read without holding some lock
This basically works on 2 principles
A read action can only proceed a shared or an exclusive lock
A write lock can only proceed a exclusice lock
All locks need to be unlocked before commit

13. Shared and exclusive locks (cont.)
Two-phase locking of transactions
Must precede unlocking
Legality of Schedules
An element may be locked exclusively by one transaction or by several in shared mode, but not both.

14. Compatibility Matrices
Has a row and column for each lock mode.
Rows correspond to a lock held on an element by another transaction
Columns correspond to mode of lock requested.
Example :
LOCK
REQUESTED S X
YES NO
HOLD

15. Upgrading Locks Suppose a transaction wants to read as well as write :
It aquires a shared lock on the element
Performs the calculations on the element
And when its ready to write, It is granted a exclusive lock.
Transactions with unpredicted read write locks can use UPGRADING LOCKS.

16. Upgrading locks (cont.)
Indiscriminating use of upgrading produces a deadlock.
Example : Both the transactions want to upgrade on the same element

17. Update locks Solves the deadlock occurring in upgrade lock method.
A transaction in an update lock can read but cant write.
Update lock can later be converted to exclusive lock.
An update lock can only be given if the element has shared locks.

18. Update locks (cont.)
An update lock is like a shared lock when you are requesting it and is like a exclusive lock when you having it.
Compatibility matrix :

19. Increment Locks Used for incrementing & decrementing stored values.
E.g. - Transfer money from one bank to another, Ticket selling transactions in which number seats are decremented after each transaction.

20. Increment lock (cont.)
A increment lock does not enable read or write locks on element.
Any number of transaction can hold increment lock on element
Shared and exclusive locks can not be granted if an increment lock is granted on element

21. Refrences Database systems : The complete book
Hector Garcia, Jeffrey Ullman, Jennifer Widom
Topics 18.3 , 18.4

22. Any questions ?

23. Thank you