1. ACS-4902 R. McFadyen 1 Chapter 18 Database Concurrency Control Locking 18.1 Shared/Exclusive (aka Read/Write) Locks Lock Operations Read_lock(X) Write_lock(X) unlock(X) Lock conversion Two-phase (2PL) Guaranteeing Serializability Basic, conservative, strict, rigorous 2PL Deadlock Prevention: Wait-die, wound-wait Detection: wait-for graph Starvation

2. ACS-4902 R. McFadyen 2 Database Concurrency Control Two locks modes: (a) shared (read) (b) exclusive (write). Shared mode: shared lock (X) More than one transaction can apply share lock on X for reading its value but no write lock can be applied on X by any other transaction. Exclusive mode: Write lock (X) Only one write lock on X can exist at any time and no shared lock can be applied by any other transaction on X. Conflict/compatibility matrix

3. ACS-4902 R. McFadyen 3 Database Concurrency Control Lock Manager: Managing locks on data items on behalf of transactions Lock table: Lock manager uses it to store the identify of transaction locking a data item, the data item, lock mode.

4. ACS-4902 R. McFadyen 4 Database Concurrency Control All transactions must be well-formed. A transaction is well-formed if: It must lock the data item before it reads or writes to it. It must not lock an already locked data items and it must not try to unlock a free data item.

5. ACS-4902 R. McFadyen 5

6. 6 Consider the processing by the lock manager and the contents of the lock table

7. ACS-4902 R. McFadyen 7 Database Concurrency Control Two Phase Locking Two Phases: (a) Locking (Growing) (b) Unlocking (Shrinking). Locking (Growing) Phase: A transaction applies locks (read or write) on desired data items one at a time. Unlocking (Shrinking) Phase: A transaction unlocks its locked data items one at a time. Requirement: For a transaction these two phases must be mutually exclusive, that is, during locking phase unlocking phase must not start and during unlocking phase locking phase must not begin. Guarantees serializability

8. ACS-4902 R. McFadyen 8

9. 9 Consider the processing by the lock manager and the contents of the lock table

10. ACS-4902 R. McFadyen 10 Database Concurrency Control 2PL variations: Conservative: Prevents deadlock by locking all desired data items before transaction begins execution. (Not practical) Basic: Transaction locks data items incrementally. This may cause deadlock which is dealt with. Strict: A stricter version of Basic algorithm where exclusive locks are released after a transaction terminates (commits or rolled-back). This is the most commonly used two-phase locking algorithm. Rigorous: No locks at all are released until commit or abort

11. ACS-4902 R. McFadyen 11 Database Concurrency Control Lock conversion Lock upgrade: existing read lock to write lock if Ti has a read-lock (X) and Tj has no read-lock (X) (i  j) then convert read-lock (X) to write-lock (X) else force Ti to wait until Tj unlocks X Lock downgrade: existing write lock to read lock Ti has a write-lock (X) (*no transaction can have any lock on X*) convert write-lock (X) to read-lock (X)