1. Concurrency Control Presented by Khadke, Suvarna CS 257
Chapter 18
Section 18.5
Presented by
Khadke, Suvarna
CS 257
(Section II) Id 213

2. Overview Assume knowledge of:
Lock
Two phase lock
Lock modes: shared, exclusive, update
A simple scheduler architecture based on following principle :
Insert locks for transaction
Release locks when the transaction manager tells it

3. Scheduler That Inserts Lock Actions

4. Scheduler That Inserts Lock Actions
Part I: Select & insert appropriate lock modes to db operations (read, write, or update)
Part II: Take actions (a lock or db operation) from Part I and executes it appropriately.
Determine the transaction (T) that action belongs and status of T (delayed or not)
If T is not delayed then
Database access action is transmitted to the database and executed

5. Scheduler That Inserts Lock Actions
If lock action is received, then L Table checks whether lock can be granted or not
i> Granted, the L Table is modified to include granted lock
ii>Not G. then update L Table about requested lock
When T commits or aborts, Part I is notified by the transaction manager and releases all locks. If any transactions are waiting PartI notifies PartII.
Part II when notified, determines next transaction T’ to get lock to continue.

6. The Lock Table
A relation that associates database elements with locking information
Implemented with a hash table using database elements as the hash key
Size is proportional to the number of lock elements only, not to the size of the entire database
DB element A
Lock information for A

7. Lock Table Entries Structure
Group modes: a summary of the most stringent conditions that transaction requesting a new lock on A face.
SXU scheme:
S: only shared locks
X: one exclusive
lock and no other locks
U: one update lock and zero or
shared locks
wait : one transaction waiting

8. Handling Lock Requests
SL1(A):
GM=S;W=N
UL2(A):
GM=U;W=N
XL3(A):
GM=U;W=Y

9. Handling Unlock Requests
Now suppose transaction T unlocks A
Delete T’s entry on the list for A
T’s lock is not the same as the group mode, no need to change group mode
Otherwise check entire list for new group mode
S: GM(S) or nothing
U: GM(S) or nothing
X: nothing

10. Handling Unlock Requests
If the value of waiting is “yes" need to grant one or more locks using following approaches
First-Come-First-Served:
Grant longest waiting request.
No starvation (waiting forever for lock)
Priority to Shared Locks:
Grant all S locks waiting, then one U lock.
Grant X lock if no others waiting
Priority to Upgrading:
If there is a U lock waiting to upgrade to an X lock, grant that first.

11. Reference List
ULLMAN, J. D., WISDOM J. & HECTOR G., DATABASE SYSTEMS THE COMPLETE BOOK, 2nd Edition, 2008.

12. Thank You