Transcript Chapter 18.5: An Architecture for a Locking Scheduler

Chapter 18.5: An Architecture
for a Locking Scheduler
Loc Nguyen
Class ID: 213
Feb 27, 2008
02/27/2008
Loc Nguyen
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Overview
 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
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Two-part Scheduler
transactions
R(A); W(B); COMMIT(T); …
Scheduler, Part I
LOCK(A); R(A); …
Lock
table
Scheduler, Part II
R(A); W(B); …
DB
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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)
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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
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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.
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The Lock Table
Group mode: U
Waiting: yes
List: <pointer>
A
B
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Tran Mode Wait Tnext Next
T1
S
no
T2
U
no
T3
X
yes
T1
S
no
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Group modes:
- S: only shared locks
- U: one update lock
and zero or
shared locks
- X: one exclusive lock
and no other locks
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Handling Lock Requests
Group mode: U
Waiting: yes
List: <pointer>
A
B
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Tran Mode Wait Tnext Next
T1
S
no
T2
U
no
T3
X
yes
T1
S
no
 SL1(A):
 GM=S;W=N
 UL2(A):
 GM=U;W=N
 XL3(A):
 GM=U;W=Y
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Handling Unlock Requests
Group mode: U
Waiting: yes


T1
S
no
Same as group
mode, no action
Diff; then check
entire list for new
group mode
T2
U
no


List: <pointer>
A
B
Remove entry
Update group mode

Tran Mode Wait Tnext Next


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T3
X
yes
T1
S
no

Update wait if “yes”



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S: GM(S) or
nothing
U: GM(S) or
nothing
X: nothing
1st come 1st serve
Prio. shared locks
Prio. upgrading
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Q&A
 Thank you!
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