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

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

Scheduler That Inserts Lock Actions

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

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.

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

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

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

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

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.

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

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