02/27/2008Loc Nguyen1 Chapter 18.5: An Architecture for a Locking Scheduler Loc Nguyen Class ID: 213 Feb 27, 2008

02/27/2008Loc Nguyen2  Assume knowledge of: Lock Two phase lock Lock modes: shared, exclusive, update  Consider a simple scheduler that: Insert locks for transaction Release locks when told Overview

02/27/2008Loc Nguyen3 Two-part Scheduler Scheduler, Part I Scheduler, Part II DB Lock table transactions R(A); W(B); COMMIT(T); … LOCK(A); R(A); … R(A); W(B); …

02/27/2008Loc Nguyen4 Semantics of Two-part Scheduler  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 Determine the transaction (T) that action belongs and status of T (delayed or not)

02/27/2008Loc Nguyen5 Semantics (Cont’d)  If T is delayed, action is delayed and added to wait list  If not, Action is db operation, to be executed Is lock, check lock table  Granted, update lock table w. granted entry  Else, update lock table w. lock request; delay further actions for T until lock granted

02/27/2008Loc Nguyen6 Semantics (Cont’d)  When T is done (commits or aborts), transaction manager (belongs to T) notify Part I to release all locks, if exists waiting lock, Part I notifies Part II  Part II when notified, determines next transaction T’ to get lock to continue.

02/27/2008Loc Nguyen7 The Lock Table Group modes: - S: only shared locks - U: one update lock and zero or shared locks - X: one exclusive lock and no other locks A Uno T2 X yes T3 Sno T1 Tran Mode Wait Tnext Next Sno T1 B Group mode: U Waiting: yes List:

02/27/2008Loc Nguyen8 Handling Lock Requests A Uno T2 X yes T3 Sno T1 Tran Mode Wait Tnext Next Sno T1 B Group mode: U Waiting: yes List:  SL1(A): GM=S;W=N  UL2(A): GM=U;W=N  XL3(A): GM=U;W=Y

02/27/2008Loc Nguyen9 Handling Unlock Requests  Remove entry  Update group mode Same as group mode, no action Diff; then check entire list for new group mode  S: GM(S) or nothing  U: GM(S) or nothing  X: nothing  Update wait if “yes” 1 st come 1 st serve Prio. shared locks Prio. upgrading A Uno T2 X yes T3 Sno T1 Tran Mode Wait Tnext Next Sno T1 B Group mode: U Waiting: yes List:

02/27/2008Loc Nguyen10 Q & A  Thank you!