Database Systems
( 資料庫系統 )
December 27, 2004
Chapter 17
Announcements
• Assignment #10 (Crash Recovery) will be (is) out on the
course homepage
• We will cover chapter 17.1 ~ 17.4 only.
• Final exam period 1.10.2005 ~ 1.15.2005
Cool & Practical Ubicomp Project
Concurrency Control
Conflict Serializable Schedules
• Two schedules are
conflict equivalent
if:
– Involve the same (write/read) actions of the same transactions – Every pair of conflicting actions is ordered the same way (conflict
ing actions are actions on the same data object and at least one of the action is a write.)
• Schedule S is
conflict serializable
if S is conflict equivale
nt to a
serial schedule
– A serial schedule is a schedule with no interleaving actions from different transactions.
– A serializable schedule is a schedule that produces identical res ult as some serial schedule.
– A conflict serializable schedule is serializable.
Example
• The right schedule is
serializable, but not conflict
serializable:
– Why serializable? Same result as T1,T2,T3.
– Why not conflict serializable? Writes of T1 and T2 (conflicting actions) are ordered differently than the serial schedule of
T1,T2,T3. T1 T2 T3 R(A) W(A) Commit W(A) Commit W(A) Commit
Precedence Graph
• How do we know a given
schedule is conflict serializable?
• Capture the conflicting actions on
a
precedence graph
& check for
cycle in the graph.
– One node per transaction
– Edge from Ti to Tj if an action of Ti
precedes and conflicts (R/W, W/R, W/R) with one of Tj’s actions.
• Schedule is conflict serializable if
and only if its precedence graph
is acyclic.
T1 T2 T3 R(A) W(A) Commit W(A) Commit W(A) Commit T1 T2 1 2 1 2Review: Strict 2PL
• Strict Two-phase Locking (Strict 2PL) Protocol
:
– If a transaction T wants to read an object, it requests a shared lock. Denote as S(O). If a transaction T wants to write an object, it requests an exclusive lock. Denote as X(O).
– Locks are released only when transaction is completed (aborted).
– If a transaction holds an X lock on an object, no other
transaction can get a lock (S or X) on that object.
• Strict 2PL allows only schedules whose precedence
graph is acyclic!
– Strict 2PL allows only conflict serializable schedules.
– Why? Say, T1 & T2 have conflicting actions. If T1 obtains the X lock on the data object first, T2 must wait until T1 is done. T2’s potential conflicting actions cannot precede T1’s conflicting actions.
Two-Phase Locking (2PL)
• 2PL Protocol is the same as Strict 2PL, except
– A transaction can release locks before the end (unlike Strict
2PL), i.e., after it is done with reading & writing the objects.
– However, a transaction can not request additional locks after it
releases any locks.
– 2PL has lock growing phase and shrinking phase.
•
2PL also produces conflict serializable schedules.
•What is the benefit of 2PL over strict 2PL?
– Smaller lock holding time -> better concurrency
•
What is the benefit of strict 2PL over 2PL?
Non-recoverable
Schedule
• Schedule allowed by 2PL
may not be recoverable in
aborts:
– Say T1 aborts and we need to undo T1.
– But T2 has read a value for A that should never been there.
– But T2 has committed! (may not be able to undo committed actions). T1 T2 X(A) R(A) W(A) Release X(A) X(A) R(A) W(A) X(B) R(B) Release X(A) W(B) Release X(B) Commit Abort
View Serializability
• Conflict serializable is sufficient but not nec essary for serializability. (too strict)
– More general sufficient condition is view seri alizability.
• Schedules S1 and S2 are view equivalent if :
1. If Ti reads initial value of A in S1, then Ti als
o reads initial value of A in S2.
2. If Ti reads value of A written by Tj in S1, the
n Ti also reads value of A written by Tj in S2.
3. If Ti writes final value of A in S1, then Ti also
writes final value of A in S2.
• A view serializable schedule is view equival
T1 T2 T3 R(A) W(A) W(A) W(A) T1 T2 T3 R(A) W(A) W(A) 1 3 3 1
Lock Management
• Lock and unlock requests are handled by the
lock
manager
.
• Each lock manager maintains a
lock table
of
l
ock table
entries.
• Each lock table entry keeps info about:
– The data object (page, record) being locked
– Number of transactions currently holding a lock (>1 if shared
mode)
– Type of lock held (shared or exclusive) – Lock request queue
Lock Management Implementation
• If a shared lock is requested:
– Check if the request queue is empty. Check if the lock is in exclusive/share mode. If (yes,share), grant the lock & update the lock table entry.
• When a transaction aborts or commits, it releases all
lock.
– Update the lock table entry. Check for lock request queue, …
• Locking and unlocking have to be atomic operations:
– E.g., cannot have concurrent operations on the same lock table entry.
• Latches: locks for reading and writing pages to disks
(ensure they are atomic)
Lock Conversions
• Lock upgrade: transaction that holds
a shared lock can be upgraded to
hold an exclusive lock.
– Get shared lock on each row in a table. – When a row meets the condition, get an
exclusive lock.
• Alternative approach is lock
downgrade.
– Get exclusive lock on each row in a table.
– When a row does not meet the
condition, downgrade to shared lock.
UPDATE Sailors S SET S.age=10
WHERE S.name=“Joe” AND S.rating=8
Deadlocks
• Deadlock: Cycle of transactions waiting for
locks to be released by each other.
• Two ways of dealing with deadlocks:
– Deadlock detection – Deadlock prevention
• Deadline Detection:
– Create a waits-for graph:
• Nodes are transactions
• There is an edge from Ti to Tj if Ti is waiting for Tj to
release a lock
– Periodically check for cycles in the waits-for graph,
• cycle = Deadlock
• Resolve a deadlock by aborting a transaction on a cycle.
T1 T2 S(A) R(A) S(B) R(B) X(B): W(B) X(A): W(A)
Deadlock Detection
T1
T2
T4
T3
T1
T2
T4
T3
T1 T2 T3 T4 S(A) R(A) X(B) W(B) S(B) S(C) R(C) X(C) X(B) X(A)Deadlock Prevention
• Basic ideas:
– Assign priorities to transactions based on timestamps when they start up. (lower timestamps = higher priority)
– Lower priorities cannot wait for higher-priority transactions.
• Assume Ti wants a lock that Tj holds. Two policies are
possible to prevent deadlocks:
– Wait-Die: If Ti has higher priority, Ti waits for Tj; otherwise Ti aborts (lower-priority T never waits for higher-priority T)
– Wound-wait: If Ti has higher priority, Tj aborts; otherwise Ti waits. (higher-priority T never waits for lower-priority T)
• Why these two policies prevent deadlocks?
Wait-Die Policy
• Lower-priority T never waits for higher-priority T • If Ti has higher priority, Ti waits for Tj; • Otherwise Ti aborts T1 T2 T3 T4 S(A) R(A) X(B) W(B) S(B) S(C) R(C) X(C) X(B) // abort X(A) // abort
Wound-Wait Policy
• Higher-priority Tnever waits for l ower-priority T • If Ti has higher p riority, Tj aborts; • Otherwise Ti wai ts. T1 T2 T3 T4 S(A) R(A) X(B) W(B) S(B) // Abort T2 Abort
Deadlock Prevention (2)
• If a transaction re-starts, make sure it has its original tim
estamp.
– It will not be forever aborted due to low priority.
• Conservative 2PL
: ensure no deadlock (no blocking) duri
ng transaction
– Each transaction begins by getting all locks it will ever need – What is the tradeoff?