Chapter 16: Concurrency Control

Chapter 16 : Concurrency Control
Database Management Systems (INFSCI 1022)

Textbook: Database System Concepts - 5th Edition, 2005
Lock-Based Protocols
 A lock is a mechanism to control concurrent access to a data item
 Data items can be locked in two modes :
1. exclusive (X) mode. Data item can be both read as well as
written. X-lock is requested using lock-X instruction.
2. shared (S) mode. Data item can only be read. S-lock is
requested using lock-S instruction.
 Lock requests are made to concurrency-control manager. Transaction can
proceed only after request is granted.
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Textbook: Database System Concepts - 5th Edition, 2005 INFSCI 1022
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Lock-Based Protocols (Cont.)
 Lock-compatibility matrix
 A transaction may be granted a lock on an item if the requested lock is

compatible with locks already held on the item by other transactions
 Any number of transactions can hold shared locks on an item,
 but if any transaction holds an exclusive on the item no other
transaction may hold any lock on the item.
 If a lock cannot be granted, the requesting transaction is made to wait till
all incompatible locks held by other transactions have been released.
The lock is then granted.
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Lock-Based Protocols (Cont.)

 Example of a transaction performing locking:
T2: lock-S(A);
read (A);
unlock(A);
lock-S(B);
read (B);
unlock(B);
display(A+B)
 Locking as above is not sufficient to guarantee serializability — if A and B
get updated in-between the read of A and B, the displayed sum would be
wrong.
 A locking protocol is a set of rules followed by all transactions while
requesting and releasing locks. Locking protocols restrict the set of
possible schedules.
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Pitfalls of Lock-Based Protocols
 Consider the partial schedule
 Neither T3 nor T4 can make progress — executing lock-S(B) causes T4

to wait for T3 to release its lock on B, while executing lock-X(A) causes
T3 to wait for T4 to release its lock on A.
 Such a situation is called a deadlock.
 To handle a deadlock one of T3 or T4 must be rolled back
and its locks released.
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Pitfalls of Lock-Based Protocols (Cont.)
 The potential for deadlock exists in most locking protocols. Deadlocks

are a necessary evil.
 Starvation is also possible if concurrency control manager is badly
designed. For example:
 A transaction may be waiting for an X-lock on an item, while a
sequence of other transactions request and are granted an S-lock
on the same item.
 The same transaction is repeatedly rolled back due to deadlocks.
 Concurrency control manager can be designed to prevent starvation.
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The Two-Phase Locking Protocol
 This is a protocol which ensures conflict-serializable schedules.
 Phase 1: Growing Phase
 transaction may obtain locks
 transaction may not release locks
 Phase 2: Shrinking Phase
 transaction may release locks
 transaction may not obtain locks
 The protocol assures serializability. It can be proved that the
transactions can be serialized in the order of their lock points (i.e.
the point where a transaction acquired its final lock).
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The Two-Phase Locking Protocol (Cont.)
 Two-phase locking does not ensure freedom from deadlocks

 Cascading roll-back is possible under two-phase locking. To avoid
this, follow a modified protocol called strict two-phase locking. Here
a transaction must hold all its exclusive locks till it commits/aborts.
 Rigorous two-phase locking is even stricter: here all locks are held
till commit/abort. In this protocol transactions can be serialized in the
order in which they commit.
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The Two-Phase Locking Protocol (Cont.)
 There can be conflict serializable schedules that cannot be obtained if

two-phase locking is used.
 However, in the absence of extra information (e.g., ordering of access
to data), two-phase locking is needed for conflict serializability in the
following sense:
Given a transaction Ti that does not follow two-phase locking, we can
find a transaction Tj that uses two-phase locking, and a schedule for Ti
and Tj that is not conflict serializable.
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Lock Conversions
 Two-phase locking with lock conversions:
– First Phase:
 can acquire a lock-S on item
 can acquire a lock-X on item
 can convert a lock-S to a lock-X (upgrade)
– Second Phase:
 can release a lock-S
 can release a lock-X
 can convert a lock-X to a lock-S (downgrade)
 This protocol assures serializability. But still relies on the programmer to
insert the various locking instructions.
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Multiple Granularity
 Allow data items to be of various sizes and define a hierarchy of data
granularities, where the small granularities are nested within larger
ones
 Can be represented graphically as a tree (but don't confuse with tree-
locking protocol)
 When a transaction locks a node in the tree explicitly, it implicitly locks
all the node's descendents in the same mode.
 Granularity of locking (level in tree where locking is done):
 fine granularity (lower in tree): high concurrency, high locking
overhead
 coarse granularity (higher in tree): low locking overhead, low
concurrency
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Example of Granularity Hierarchy
The levels, starting from the coarsest (top) level are

 database
 area
 file
 record
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6
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.
 intention locks allow a higher level node to be locked in S or X mode
without having to check all descendent nodes.
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Compatibility Matrix with

Intention Lock Modes
 The compatibility matrix for all lock modes is:
IS IX S S IX X
IS     ×
IX   × × ×
S  ×  × ×
S IX  × × × ×
X × × × × ×
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Multiple Granularity Locking Scheme
 Transaction Ti can lock a node Q, using the following rules:
1. The lock compatibility matrix must be observed.
2. The root of the tree must be locked first, and may be locked in any
mode.
3. A node Q can be locked by Ti in S or IS mode only if the parent of Q
is currently locked by Ti in either IX or IS mode.
4. A node Q can be locked by Ti in X, SIX, or IX mode only if the parent
of Q is currently locked by Ti in either IX or SIX mode.
5. Ti can lock a node only if it has not previously unlocked any node
(that is, Ti is two-phase).
Ti can unlock a node Q only if none of the children of Q are currently
6.
locked by Ti.
 Observe that locks are acquired in root-to-leaf order, whereas they are
released in leaf-to-root order.
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Deadlock Handling
 Consider the following two transactions:
T1: write (X) T2: write(Y)
write(Y) write(X)
 Schedule with deadlock
T1 T2
lock-X on X
write (X)
lock-X on Y
write (X)
wait for lock-X on X
wait for lock-X on Y
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8
Deadlock Handling
 System is deadlocked if there is a set of transactions such that every
transaction in the set is waiting for another transaction in the set.
 Deadlock prevention protocols ensure that the system will never
enter into a deadlock state. Some prevention strategies :
 Require that each transaction locks all its data items before it
begins execution (predeclaration).
 Impose partial ordering of all data items and require that a
transaction can lock data items only in the order specified by the
partial order (graph-based protocol).
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More Deadlock Prevention Strategies

 Following schemes use transaction timestamps for the sake of deadlock
prevention alone.
 wait-die scheme — non-preemptive
 older transaction may wait for younger one to release data item.
Younger transactions never wait for older ones; they are rolled back
instead.
 a transaction may die several times before acquiring needed data
item
 wound-wait scheme — preemptive
 older transaction wounds (forces rollback) of younger transaction
instead of waiting for it. Younger transactions may wait for older
ones.
 may be fewer rollbacks than wait-die scheme.
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Deadlock prevention (Cont.)
 Both in wait-die and in wound-wait schemes, a rolled back
transactions is restarted with its original timestamp. Older transactions
thus have precedence over newer ones, and starvation is hence
avoided.
 Timeout-Based Schemes :
 a transaction waits for a lock only for a specified amount of time.
After that, the wait times out and the transaction is rolled back.
 thus deadlocks are not possible
 simple to implement; but starvation is possible. Also difficult to
determine good value of the timeout interval.
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Deadlock Detection
 Deadlocks can be described as a wait-for graph, which consists of a
pair G = (V,E),
 V is a set of vertices (all the transactions in the system)
 E is a set of edges; each element is an ordered pair Ti →Tj.
 If Ti → Tj is in E, then there is a directed edge from Ti to Tj, implying
that Ti is waiting for Tj to release a data item.
 When Ti requests a data item currently being held by Tj, then the edge
Ti Tj is inserted in the wait-for graph. This edge is removed only when
Tj is no longer holding a data item needed by Ti.
 The system is in a deadlock state if and only if the wait-for graph has a
cycle. Must invoke a deadlock-detection algorithm periodically to look
for cycles.
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Deadlock Detection (Cont.)
Wait-for graph without a cycle Wait-for graph with a cycle
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Deadlock Recovery
 When deadlock is detected :
 Some transaction will have to rolled back (made a victim) to break
deadlock. Select that transaction as victim that will incur minimum
cost.
 Rollback -- determine how far to roll back transaction
 Total rollback: Abort the transaction and then restart it.
 More effective to roll back transaction only as far as necessary
to break deadlock.
 Starvation happens if same transaction is always chosen as
victim. Include the number of rollbacks in the cost factor to avoid
starvation
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End of Chapter
Database Management Systems (INFSCI 1022)

Textbook: Database System Concepts - 5th Edition, 2005
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Chapter 16 : Concurrency Control

Database Management Systems (INFSCI 1022)

 A transaction may be granted a lock on an item if the requested lock is

Lock-Based Protocols (Cont.)

 Neither T3 nor T4 can make progress — executing lock-S(B) causes T4

Pitfalls of Lock-Based Protocols (Cont.)

 The potential for deadlock exists in most locking protocols. Deadlocks

The Two-Phase Locking Protocol (Cont.)

 Two-phase locking does not ensure freedom from deadlocks

 There can be conflict serializable schedules that cannot be obtained if

Example of Granularity Hierarchy

The levels, starting from the coarsest (top) level are

Compatibility Matrix with

More Deadlock Prevention Strategies

Wait-for graph without a cycle Wait-for graph with a cycle

Database Management Systems (INFSCI 1022)

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