CC2

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General Overview
 Relational model - SQL
 Formal & commercial query languages
 Functional Dependencies
 Normalization
 Physical Design
 Indexing
 Query Processing and Optimization
 Transaction Processing and CC
Review: AC[I]D
 Isolation
 Concurrent xctions unaware of each other
 How?
 We discussed locking protocols:
 2PL protocol and its variants
 Graph-based locking protocols
Multiple-Granularity Locks
 Hard to decide what granularity to lock (tuples vs. pages vs.
tables).
 Shouldn’t have to decide!
 Data “containers” are nested:
Database
contains
Tables
Pages
Tuples
Intention Lock Modes
 In addition to S and X lock modes, there are three additional lock
modes with multiple granularity:
 intention-shared (IS): indicates explicit locking at a lower level of
the tree but only with shared locks.
 intention-exclusive (IX): indicates explicit locking at a lower level
with exclusive or shared locks
 shared and intention-exclusive (SIX): the subtree rooted by that
node is locked explicitly in shared mode and explicit locking is being
done at a lower level with exclusive-mode locks.
MGL: multiple granularity locking
Before locking an item, Xact
must set “intention locks”
on all its ancestors.
 For unlock, go from specific
to general (i.e., bottom-up).
 SIX mode: Like S & IX at
the same time.

--
IS IX S
X





IS 



IX 





--
S
X

Parent
locked in
IS
IX
S
SIX
X
Child can be
locked in
IS, S
IS, S, IX, X, SIX
[S, IS] not necessary
X, IX, [SIX]
none
P
C
Example
T1(IS) , T2(IX)
R1
t1
t2
T1(S)
t3
t4
T2(X)
Multiple Granularity Locking Scheme
 Transaction Ti can lock a node Q, using the following rules:
(1) Follow multiple granularity comp function
(2) Lock root of tree first, any mode
(3) Node Q can be locked by Ti in S or IS only if
parent(Q) can be locked by Ti in IX or IS
(4) Node Q can be locked by Ti in X,SIX,IX only
if parent(Q) locked by Ti in IX,SIX
(5) Ti uses 2PL
(6) Ti can unlock node Q only if none of Q’s
children are locked by Ti
 Observe that locks are acquired in root-to-leaf order,
whereas they are released in leaf-to-root order.
Examples
T1(IX)
T1(IS)
R
R
T1(IX)
t2
t1
t3
T1(S)
t4
t3
t2
t1
t4
T1(X)
f2.1
f2.2
f4.2
f4.2
f2.1
f4.2
f2.2
T1(SIX)
Can T2 access object f2.2 in X mode?
What locks will T2 get?
R
T1(IX)
t2
t1
t3
t4
T1(X)
f2.1
f2.2
f4.2
f4.2
f4.2
Examples
 T1 scans R, and updates a few tuples:
 T1 gets an SIX lock on R, then repeatedly gets an S lock on tuples of R,
and occasionally upgrades to X on the tuples that needs to update.
 T2 uses an index to read only part of R:
 T2 gets an IS lock on R, and repeatedly gets an S lock on tuples of R.
 T3 reads all of R:
 T3 gets an S lock on R.
 OR, T3 could behave like T2; can
use lock escalation to decide which.
--
IS IX S
X





IS 



IX 





--
S
X

Optimistic CC
 Locking is a conservative approach in which conflicts are
prevented. Disadvantages:
 Lock management overhead.
 Deadlock detection/resolution.
 Lock contention for heavily used objects.
 If conflicts are rare, we might be able to gain concurrency by not
locking, and instead checking for conflicts before Xacts commit.
Validation-Based Protocol
 Execution of transaction Ti is done in three phases.
1. Read and execution phase: Ti reads all values and makes
copies to local variables (private workspace.) Ti writes only to
temporary local variables. No locking.
2. Validation phase: Transaction Ti performs a ``validation test''
to determine if local variables can be written without violating
serializability.
3. Write phase: If Ti is validated, the updates are applied to the
database; otherwise, Ti is rolled back.
 optimistic concurrency control: transaction executes fully in
the hope that all will go well during validation
Validation-Based Protocol (Cont.)
 Each transaction Ti has 3 timestamps
 Start(Ti) : the time when Ti started its execution
 Validation(Ti): the time when Ti entered its validation phase

Finish(Ti) : the time when Ti finished its write phase
 Serializability order is based on Validation(Ti).
Key idea: validation is atomic!
Validation-Based Protocol
To implement validation, system keeps the following sets:
 FIN = transactions that have finished phase 3 (and are all done)
 VAL = transactions that have successfully finished phase 2
(validation)
 For each transaction the Read and Write Sets
Example of what validation must prevent:
RS(T1)={B}
WS(T1)={B,D}
T1
start
T2
start

RS(T2)={A,B} =

WS(T2)={C}
T1
T2
validated
validated
time
T2 validation will fail!
Example of what validation must allow:
RS(T1)={B}
WS(T1)={B,D}
T1
start
T2
start

RS(T2)={A,B}
=
WS(T2)={C}
T1
T2
validated
validated
T1
finish
phase 3
T2
start
time
Another thing validation must prevent:
RS(T1)={A}
RS(T2)={A,B}
WS(T1)={D,E} WS(T2)={C,D}
T1
validated
T2
validated
finish
2
BAD: w2(D) w1T(D)
finish
T1
time
Another thing validation must allow:
RS(T1)={A}
RS(T2)={A,B}
WS(T1)={D,E} WS(T2)={C,D}
T1
T2
validated
validated
finish
finish
T1
T1
time
Validation rules for Tj:
(1) When Tj starts phase 1:
IGNORE(Tj)  FIN
(2) at Tj Validation:
if check (Tj) then
[ VAL  VAL U {Tj};
do write phase;
FIN  FIN U {Tj} ]
All transactions that either validated
or finished after the start of Tj
Check (Tj):
For Ti  VAL - IGNORE (Tj) DO
IF [ WS(Ti)  RS(Tj)   OR
Ti  FIN ] THEN RETURN false;
RETURN true;
Is this check too restrictive ?
Improving Check(Tj)
For Ti  VAL - IGNORE (Tj) DO
IF [ WS(Ti)

RS(Tj)   OR
(Ti  FIN
AND WS(Ti)  WS(Tj)  )]
THEN RETURN false;
RETURN true;
Example:
U: RS(U)={B}
WS(U)={D}
T: RS(T)={A,B}
WS(T)={A,C}
start
validate
finish
W: RS(W)={A,D}
WS(W)={A,C}
V: RS(V)={B}
WS(V)={D,E}
U,T,V successful; W abort and roll back
Timestamp-Based Protocols
 Idea:

Decide in advance ordering of xctions

Ensure concurrent schedule serializes to serial order decided
 Timestamps
1. TS(Ti) is time Ti entered the system
2. Data item timestamps:
1. W-TS(Q): Largest timestamp of any xction that wrote Q
2. R-TS(Q): Largest timestamp of any xction that read Q
 Timestamps -> serializability order
Timestamp CC
 Idea: If action pi of Xact Ti conflicts with action qj of
Xact Tj, and TS(Ti) < TS(Tj), then pi must occur
before qj. Otherwise, restart violating Xact.
When Xact T wants to read Object O
 If TS(T) < W-TS(O), this violates timestamp order of T w.r.t. writer
of O.
 So, abort T and restart it with a new, larger TS. (If restarted with
same TS, T will fail again!)
 If TS(T) > W-TS(O):
 Allow T to read O.
 Reset R-TS(O) to max(R-TS(O), TS(T))
 Change to R-TS(O) on reads must be written to disk (log)! This
and restarts represent overheads.
U writes O
T reads O
T start
U start
When Xact T wants to Write Object O
1) If TS(T) < R-TS(Q), then the value of Q that T is producing was
needed previously, and the system assumed that that value would
never be produced. write rejected, T is rolled back and restarts.
2) If TS(T) < W-TS(Q), then T is attempting to write an obsolete value
of Q. Hence, this write operation is rejected, and T is rolled back.
3) Otherwise, the write operation is executed, and W-TS(Q) is set to
TS(T).
U reads Q
T writes Q
Another approach in 2)
is to ignore the write
and continue!!
Thomas Write Rule
T start
U start
Timestamp CC and Recoverability

Unfortunately, unrecoverable
schedules are allowed:
T1
W(A)
T2
R(A)
W(B)
Commit
 Timestamp CC can be modified to allow only recoverable schedules:
 Buffer all writes until writer commits (but update WTS(O) when the
write is allowed.)
 Block readers T (where TS(T) > WTS(O)) until writer of O commits.
 Similar to writers holding X locks until commit, but still not quite 2PL.
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