Sekolah Tinggi Ilmu Statistik (STIS) 1 Dr. Said Mirza Pahlevi,
M.Eng.
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Lecture 3 2 Dr. Said Mirza Pahlevi, M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) Todays Lecture Concurrency
Control Meaning of serializability Conflict serializability View
serializability Locking Methods 3 Dr. Said Mirza Pahlevi, M.Eng.
Last time lecture
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Second Subtopic (from previous lecture) 4 Dr. Said Mirza
Pahlevi, M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) View Serializability
Conflict equivalent-----View equivalent Conflict
serializable-----View serializable 5 Dr. Said Mirza Pahlevi,
M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) Motivating example
Schedule Q T 1 T 2 T 3 Read(A) Write(A) 6 Dr. Said Mirza Pahlevi,
M.Eng. t1:t1: t2:t2: t3:t3: t4:t4:
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Sekolah Tinggi Ilmu Statistik (STIS) Motivating Example Same as
Q = r 1 (A) w 2 (A) w 1 (A) w 3 (A) P(Q): T1 T2 T3 Not conflict
serializable! 7 Dr. Said Mirza Pahlevi, M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) Compare Q to Serial
Schedule (S) QT 1 T 2 T 3 Read(A) Write(A) Write(A) Write(A) S T 1
T 2 T 3 Read(A) Write(A) Write(A) Write(A) T 1 reads same thing in
Q dan S T 2, T 3 read same thing (nothing?) After Q or S, DB is
left in same state So what is wrong with Q? T 1 reads same thing in
Q dan S T 2, T 3 read same thing (nothing?) After Q or S, DB is
left in same state So what is wrong with Q? 8 Dr. Said Mirza
Pahlevi, M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) View Equivalent Two
schedules S1 and S2 are view equivalent: 1.IF in S1: w j (A) r i
(A), THEN in S2: w j (A) r i (A) 2.IF in S1: r i (A) reads initial
DB value, THEN in S2: r i (A) also reads initial DB value 3.IF in
S1: T i does last write on A, THEN in S2: T i also does last write
on A Dr. Said Mirza Pahlevi, M.Eng. 9 means reads value produced
by
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Sekolah Tinggi Ilmu Statistik (STIS) View Serializability
Schedule is view serializable if it is view equivalent to a serial
schedule. Offers less stringent definition of schedule equivalence
than conflict serializability. Every conflict serializable schedule
is view serializable, although converse is not true. It can be
shown that any view serializable schedule that is not conflict
serializable contains one or more blind writes. 10 Dr. Said Mirza
Pahlevi, M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) View Serializable Test Dr.
Said Mirza Pahlevi, M.Eng. 11 add 1. Add final transaction T f that
reads all DB e.g.: S = ..W 1 (A).. W 2 (A) R f (A) last A
write
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Sekolah Tinggi Ilmu Statistik (STIS) add View Serializable Test
Dr. Said Mirza Pahlevi, M.Eng. 12 2. Add initial transaction T b
that writes all DB e.g.: S = w b (A) ... r 1 (A) w 2 (A)
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Sekolah Tinggi Ilmu Statistik (STIS) View Serializable Test Dr.
Said Mirza Pahlevi, M.Eng. 13 0 3. Create labeled precedence (LP)
graph of S: (a) If w i (A) r j (A) in S, add T i T j means reads
value produced by
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Sekolah Tinggi Ilmu Statistik (STIS) View Serializable Test Dr.
Said Mirza Pahlevi, M.Eng. 14 p p 0 0 (b) For each w i (A) r j (A)
do consider each w k (A): [T k T b ] (i) IF T i T b T j T f THEN
insert T k T i some new p T j T k (ii) IF T i =T b T j T f THEN
insert T j T k (iii) IF T i T b T j =T f THEN insert T k T i
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Sekolah Tinggi Ilmu Statistik (STIS) View Serializable Test Dr.
Said Mirza Pahlevi, M.Eng. 15 4. Check if LP(S) is acyclic; if so,
then S is view serializable - For each pair of p arcs (p 0), choose
one
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Sekolah Tinggi Ilmu Statistik (STIS) Example 1: View
Serializable (VS) Dr. Said Mirza Pahlevi, M.Eng. 16 Check if Q is
view serializable: Q = r 1 (A) w 2 (A) w 1 (A) w 3 (A) Q = w b (A)
r 1 (A) w 2 (A) w 1 (A) w 3 (A) r f (A) T3T3 T2T2 T1T1 TfTf TbTb
rule 3a 0 0 0 0 rule 3b (ii) 0 0 rule 3b (iii) Acyclic graph!! Q is
VS Acyclic graph!! Q is VS
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Sekolah Tinggi Ilmu Statistik (STIS) Z=w b (A) r 1 (A) w 2 (A)
r 3 (A) w 1 (A) w 3 (A) r f (A) T3T3 T2T2 T1T1 TfTf TbTb 0 0 0 0 0
Acyclic graph, so Z is VS (equivalent to T b T 1 T 2 T 3 T f )
Acyclic graph, so Z is VS (equivalent to T b T 1 T 2 T 3 T f ) do
not pick this one of 1 pair do not pick this one of 1 pair 17 Dr.
Said Mirza Pahlevi, M.Eng. 1 1 0 0 Example 2: View Serializable
(VS) rule 3b (i) rule 3b (ii) rule 3b (iii) rule 3a
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Sekolah Tinggi Ilmu Statistik (STIS) Recoverability
Serializability identifies schedules that maintain database
consistency, assuming no transaction fails. Could also examine
recoverability of transactions within schedule. If transaction
fails, atomicity requires effects of transaction to be undone
Durability states that once transaction commits, its changes cannot
be undone (without running another, compensating, transaction). 18
Dr. Said Mirza Pahlevi, M.Eng.
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Sekolah Tinggi Ilmu Statistik (STIS) If T9 Rollback Instead of
Commit Dr. Said Mirza Pahlevi, M.Eng. 19 We should undo T10 but T10
has committed the transaction (durability does not allow it). This
is a non- recoverable schedule
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Sekolah Tinggi Ilmu Statistik (STIS) Recoverable Schedule A
schedule where, for each pair of transactions T i and T j, if T j
reads a data item previously written by T i, then the commit
operation of T i precedes the commit operation of T j. If commit
operation of T j precedes the commit operation of T i then it is
unrecoverable, because of the durability. 20 Dr. Said Mirza
Pahlevi, M.Eng.
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Third Topic Dr. Said Mirza Pahlevi, M.Eng. 21
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Sekolah Tinggi Ilmu Statistik (STIS) Concurrency Control
Techniques Two basic concurrency control techniques: Locking,
Timestamping. Both are conservative approaches: delay transactions
in case they conflict with other transactions. Optimistic methods
assume conflict is rare and only check for conflicts at commit. Dr.
Said Mirza Pahlevi, M.Eng. 22
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Sekolah Tinggi Ilmu Statistik (STIS) Locking Transaction uses
locks to deny access to other transactions and so prevent incorrect
updates. Most widely used approach to ensure serializability.
Generally, a transaction must claim a shared (read) or exclusive
(write) lock on a data item before read or write. Lock prevents
another transaction from modifying item or even reading it, in the
case of a write lock. Dr. Said Mirza Pahlevi, M.Eng. 23
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Sekolah Tinggi Ilmu Statistik (STIS) Locking - Basic Rules 1.
If transaction has shared lock on item, it can read but not update
item. 2. If transaction has exclusive lock on item, it can both
read and update item. 3. Reads cannot conflict, so more than one
transaction can hold shared locks simultaneously on same item. 4.
Exclusive lock gives transaction exclusive access to that item. Dr.
Said Mirza Pahlevi, M.Eng. 24
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Sekolah Tinggi Ilmu Statistik (STIS) Locking - Basic Rules 5.
Some systems allow transaction to upgrade read lock to an exclusive
lock, or downgrade exclusive lock to a shared lock. But this basic
rules does not guarantee serializability of schedules by themselves
Dr. Said Mirza Pahlevi, M.Eng. 25
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Sekolah Tinggi Ilmu Statistik (STIS) Incorrect Locking Schedule
Dr. Said Mirza Pahlevi, M.Eng. 26 Valid schedule using basic rules:
S = {write_lock(T 9, bal x ), read(T 9, bal x ), write(T 9, bal x
), unlock(T 9, bal x ), write_lock(T 10, bal x ), read(T 10, bal x
), write(T 10, bal x ), unlock(T 10, bal x ), write_lock(T 10, bal
y ), read(T 10, bal y ), write(T 10, bal y ), unlock(T 10, bal y ),
commit(T 10 ), write_lock(T 9, bal y ), read(T 9, bal y ), write(T
9, bal y ), unlock(T 9, bal y ), commit(T 9 ) }
