Chapter 15 : Concurrency Control

Database System Concepts, 6th Ed.

©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
 Chapter 15: Concurrency Control
„ Lock-Based Protocols
„ Timestamp-Based Protocols
„ Validation-Based Protocols
„ Multiple Granularity
„ Multiversion Schemes
„ Insert and Delete Operations
„ Concurrency in Index Structures

Database System Concepts - 6th Edition 15.2 ©Silberschatz, Korth and Sudarshan
 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.

Database System Concepts - 6th Edition 15.3 ©Silberschatz, Korth and Sudarshan
 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,
z 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.

Database System Concepts - 6th Edition 15.4 ©Silberschatz, Korth and Sudarshan
 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.

Database System Concepts - 6th Edition 15.5 ©Silberschatz, Korth and Sudarshan
 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.
z To handle a deadlock one of T3 or T4 must be rolled back
and its locks released.

Database System Concepts - 6th Edition 15.6 ©Silberschatz, Korth and Sudarshan
 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:
z 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.
z The same transaction is repeatedly rolled back due to deadlocks.
„ Concurrency control manager can be designed to prevent starvation.

Database System Concepts - 6th Edition 15.7 ©Silberschatz, Korth and Sudarshan
 The Two-Phase Locking Protocol
„ This is a protocol which ensures conflict-serializable schedules.
„ Phase 1: Growing Phase
z transaction may obtain locks
z transaction may not release locks
„ Phase 2: Shrinking Phase
z transaction may release locks
z 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).

Database System Concepts - 6th Edition 15.8 ©Silberschatz, Korth and Sudarshan
 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.

Database System Concepts - 6th Edition 15.9 ©Silberschatz, Korth and Sudarshan
 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.

Database System Concepts - 6th Edition 15.10 ©Silberschatz, Korth and Sudarshan
 Lock Conversions
„ Two-phase locking with lock conversions:
– First Phase:
z can acquire a lock-S on item
z can acquire a lock-X on item
z can convert a lock-S to a lock-X (upgrade)
– Second Phase:
z can release a lock-S
z can release a lock-X
z 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.

Database System Concepts - 6th Edition 15.11 ©Silberschatz, Korth and Sudarshan
 Automatic Acquisition of Locks
„ A transaction Ti issues the standard read/write instruction, without
explicit locking calls.
„ The operation read(D) is processed as:
if Ti has a lock on D
then
read(D)
else begin
if necessary wait until no other
transaction has a lock-X on D
grant Ti a lock-S on D;
read(D)
end

Database System Concepts - 6th Edition 15.12 ©Silberschatz, Korth and Sudarshan
 Automatic Acquisition of Locks (Cont.)
„ write(D) is processed as:
if Ti has a lock-X on D
then
write(D)
else begin
if necessary wait until no other trans. has any lock on D,
if Ti has a lock-S on D
then
upgrade lock on D to lock-X
else
grant Ti a lock-X on D
write(D)
end;
„ All locks are released after commit or abort

Database System Concepts - 6th Edition 15.13 ©Silberschatz, Korth and Sudarshan
 Implementation of Locking
„ A lock manager can be implemented as a separate process to which
transactions send lock and unlock requests.
„ The lock manager replies to a lock request by sending a lock grant
messages (or a message asking the transaction to roll back, in case of
a deadlock).
„ The requesting transaction waits until its request is answered.
„ The lock manager maintains a data-structure called a lock table to
record granted locks and pending requests.
„ The lock table is usually implemented as an in-memory hash table
indexed on the name of the data item being locked.

Database System Concepts - 6th Edition 15.14 ©Silberschatz, Korth and Sudarshan
 Lock Table
„ Black rectangles indicate granted locks,
white ones indicate waiting requests
„ Lock table also records the type of lock
granted or requested
„ New request is added to the end of the
queue of requests for the data item, and
granted if it is compatible with all earlier
locks
„ Unlock requests result in the request
being deleted, and later requests are
checked to see if they can now be
granted
„ If transaction aborts, all waiting or
granted requests of the transaction are
deleted
z lock manager may keep a list of
locks held by each transaction, to
implement this efficiently

Database System Concepts - 6th Edition 15.15 ©Silberschatz, Korth and Sudarshan
 Graph-Based Protocols
„ Graph-based protocols are an alternative to two-phase locking.
„ Impose a partial ordering → on the set D = {d1, d2 ,..., dh} of all data
items.
z If di → dj then any transaction accessing both di and dj must
access di before accessing dj.
z Implies that the set D may now be viewed as a directed acyclic
graph, called a database graph.
„ The tree-protocol is a simple kind of graph protocol.

Database System Concepts - 6th Edition 15.16 ©Silberschatz, Korth and Sudarshan
 Tree Protocol

1. Only exclusive locks are allowed.

2. The first lock by Ti may be on any data item. Subsequently, a data Q
can be locked by Ti only if the parent of Q is currently locked by Ti.
3. Data items may be unlocked at any time.
4. A data item that has been locked and unlocked by Ti cannot
subsequently be relocked by Ti .

Database System Concepts - 6th Edition 15.17 ©Silberschatz, Korth and Sudarshan
 Graph-Based Protocols (Cont.)
„ The tree protocol ensures conflict serializability as well as freedom from
deadlock.
„ Unlocking may occur earlier in the tree-locking protocol than in the two-
phase locking protocol.
z shorter waiting times, and increase in concurrency
z protocol is deadlock-free, no rollbacks are required
„ Drawbacks
z Protocol does not guarantee recoverability or cascade freedom
 Need to introduce commit dependencies to ensure recoverability
z Transactions may have to lock data items that they do not access.
 increased locking overhead, and additional waiting time
 potential decrease in concurrency
„ Schedules not possible under two-phase locking are possible under tree
protocol, and vice versa.

Database System Concepts - 6th Edition 15.18 ©Silberschatz, Korth and Sudarshan
 Deadlock Handling
„ Consider the following two transactions:
T1: write (X) T2: write(Y)
write(Y) write(X)
„ Schedule with deadlock

Database System Concepts - 6th Edition 15.19 ©Silberschatz, Korth and Sudarshan
 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:
z Require that each transaction locks all its data items before it
begins execution (predeclaration).
z 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).

Database System Concepts - 6th Edition 15.20 ©Silberschatz, Korth and Sudarshan
 More Deadlock Prevention Strategies
„ Following schemes use transaction timestamps for the sake of deadlock
prevention alone.
„ wait-die scheme — non-preemptive
z older transaction may wait for younger one to release data item.
Younger transactions never wait for older ones; they are rolled back
instead.
z a transaction may die several times before acquiring needed data
item
„ wound-wait scheme — preemptive
z older transaction wounds (forces rollback) of younger transaction
instead of waiting for it. Younger transactions may wait for older
ones.
z may be fewer rollbacks than wait-die scheme

Database System Concepts - 6th Edition 15.21 ©Silberschatz, Korth and Sudarshan
 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:
z 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.
z thus deadlocks are not possible
z simple to implement; but starvation is possible. Also difficult to
determine good value of the timeout interval.

Database System Concepts - 6th Edition 15.22 ©Silberschatz, Korth and Sudarshan
 Deadlock Detection
„ Deadlocks can be described as a wait-for graph, which consists of a
pair G = (V,E),
z V is a set of vertices (all the transactions in the system)
z 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.

Database System Concepts - 6th Edition 15.23 ©Silberschatz, Korth and Sudarshan
 Deadlock Detection (Cont.)

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

Database System Concepts - 6th Edition 15.24 ©Silberschatz, Korth and Sudarshan
 Deadlock Recovery
„ When deadlock is detected:
z Some transaction will have to rolled back (made a victim) to break
deadlock. Select that transaction as victim that will incur minimum
cost.
z 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.
z Starvation happens if same transaction is always chosen as
victim. Include the number of rollbacks in the cost factor to avoid
starvation

Database System Concepts - 6th Edition 15.25 ©Silberschatz, Korth and Sudarshan
 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):
z fine granularity (lower in tree): high concurrency, high locking
overhead
z coarse granularity (higher in tree): low locking overhead, low
concurrency

Database System Concepts - 6th Edition 15.26 ©Silberschatz, Korth and Sudarshan
 Example of Granularity Hierarchy

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

z database
z area
z file
z record

Database System Concepts - 6th Edition 15.27 ©Silberschatz, Korth and Sudarshan
 Intention Lock Modes
„ In addition to S and X lock modes, there are three additional lock
modes with multiple granularity:
z intention-shared (IS): indicates explicit locking at a lower level of
the tree but only with shared locks.
z intention-exclusive (IX): indicates explicit locking at a lower level
with exclusive or shared locks
z 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.

Database System Concepts - 6th Edition 15.28 ©Silberschatz, Korth and Sudarshan
 Compatibility Matrix with Intention Lock Modes

„ The compatibility matrix for all lock modes is:

Database System Concepts - 6th Edition 15.29 ©Silberschatz, Korth and Sudarshan
 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).
6. Ti can unlock a node Q only if none of the children of Q are
currently locked by Ti.
„ Observe that locks are acquired in root-to-leaf order, whereas they
are released in leaf-to-root order.

Database System Concepts - 6th Edition 15.30 ©Silberschatz, Korth and Sudarshan
 Timestamp-Based Protocols
„ Each transaction is issued a timestamp when it enters the system. If
an old transaction Ti has time-stamp TS(Ti), a new transaction Tj is
assigned time-stamp TS(Tj) such that TS(Ti) <TS(Tj).
„ The protocol manages concurrent execution such that the time-
stamps determine the serializability order.
„ In order to assure such behavior, the protocol maintains for each
data Q two timestamp values:
z W-timestamp(Q) is the largest time-stamp of any transaction
that executed write(Q) successfully.
z R-timestamp(Q) is the largest time-stamp of any transaction
that executed read(Q) successfully.

Database System Concepts - 6th Edition 15.31 ©Silberschatz, Korth and Sudarshan
 Timestamp-Based Protocols (Cont.)
„ The timestamp ordering protocol ensures that any conflicting read and
write operations are executed in timestamp order.
„ Suppose a transaction Ti issues a read(Q):
1. If TS(Ti) ≤ W-timestamp(Q), then Ti needs to read a value of Q
that was already overwritten.
 Hence, the read operation is rejected, and Ti is rolled back.
2. If TS(Ti)≥ W-timestamp(Q), then the read operation is executed,
and R-timestamp(Q) is set to max(R-timestamp(Q), TS(Ti)).

Database System Concepts - 6th Edition 15.32 ©Silberschatz, Korth and Sudarshan
 Timestamp-Based Protocols (Cont.)
„ Suppose that transaction Ti issues write(Q).
1. If TS(Ti) < R-timestamp(Q), then the value of Q that Ti is
producing was needed previously, and the system assumed that
that value would never be produced.
 Hence, the write operation is rejected, and Ti is rolled back.
2. If TS(Ti) < W-timestamp(Q), then Ti is attempting to write an
obsolete value of Q.
 Hence, this write operation is rejected, and Ti is rolled back.
3. Otherwise, the write operation is executed, and W-timestamp(Q)
is set to TS(Ti).

Database System Concepts - 6th Edition 15.33 ©Silberschatz, Korth and Sudarshan
 Example Use of the Protocol
A partial schedule for several data items for transactions with
timestamps 1, 2, 3, 4, 5

Database System Concepts - 6th Edition 15.34 ©Silberschatz, Korth and Sudarshan
 Correctness of Timestamp-Ordering Protocol

„ The timestamp-ordering protocol guarantees serializability since all

the arcs in the precedence graph are of the form:

Thus, there will be no cycles in the precedence graph.

„ Timestamp protocol ensures freedom from deadlock as no
transaction ever waits.
„ But the schedule may not be cascade-free, and may not even be
recoverable.

Database System Concepts - 6th Edition 15.35 ©Silberschatz, Korth and Sudarshan
 Recoverability and Cascade Freedom
„ Problem with timestamp-ordering protocol:
z Suppose Ti aborts, but Tj has read a data item written by Ti
z Then Tj must abort; if Tj had been allowed to commit earlier, the
schedule is not recoverable.
z Further, any transaction that has read a data item written by Tj
must abort
z This can lead to cascading rollback --- that is, a chain of
rollbacks
„ Solution 1:
z A transaction is structured such that its writes are all performed
at the end of its processing
z All writes of a transaction form an atomic action; no transaction
may execute while a transaction is being written
z A transaction that aborts is restarted with a new timestamp
„ Solution 2: Limited form of locking: wait for data to be committed
before reading it
„ Solution 3: Use commit dependencies to ensure recoverability

Database System Concepts - 6th Edition 15.36 ©Silberschatz, Korth and Sudarshan
 Thomas’ Write Rule
„ Modified version of the timestamp-ordering protocol in which obsolete
write operations may be ignored under certain circumstances.
„ When Ti attempts to write data item Q, if TS(Ti) < W-timestamp(Q),
then Ti is attempting to write an obsolete value of {Q}.
z Rather than rolling back Ti as the timestamp ordering protocol
would have done, this {write} operation can be ignored.
„ Otherwise this protocol is the same as the timestamp ordering
protocol.
„ Thomas' Write Rule allows greater potential concurrency.

z Allows some view-serializable schedules that are not conflict-

serializable.

Database System Concepts - 6th Edition 15.37 ©Silberschatz, Korth and Sudarshan
 View Serializability
„ Let S and S´ be two schedules with the same set of transactions. S
and S´ are view equivalent if the following three conditions are met,
for each data item Q,
1. If in schedule S, transaction Ti reads the initial value of Q, then in
schedule S´ also transaction Ti must read the initial value of Q.
2. If in schedule S transaction Ti executes read(Q), and that value
was produced by transaction Tj (if any), then in schedule S´ also
transaction Ti must read the value of Q that was produced by the
same write(Q) operation of transaction Tj .
3. The transaction (if any) that performs the final write(Q) operation
in schedule S must also perform the final write(Q) operation in
schedule S´.
As can be seen, view equivalence is also based purely on reads and
writes alone.

Database System Concepts - 6th Edition 15.38 ©Silberschatz, Korth and Sudarshan
 View Serializability (Cont.)
„ A schedule S is view serializable if it is view equivalent to a serial
schedule.
„ Every conflict serializable schedule is also view serializable.
„ Below is a schedule which is view-serializable but not conflict
serializable.

„ What serial schedule is above equivalent to?

„ Every view serializable schedule that is not conflict serializable has
blind writes.

Database System Concepts - 6th Edition 15.39 ©Silberschatz, Korth and Sudarshan
 Other Notions of Serializability
„ The schedule below produces same outcome as the serial schedule
< T1, T5 >, yet is not conflict equivalent or view equivalent to it.

„ Determining such equivalence requires analysis of operations other

than read and write.

Database System Concepts - 6th Edition 15.40 ©Silberschatz, Korth and Sudarshan
 Test for View Serializability
„ The precedence graph test for conflict serializability cannot be used
directly to test for view serializability.
z Extension to test for view serializability has cost exponential in the
size of the precedence graph.
„ The problem of checking if a schedule is view serializable falls in the
class of NP-complete problems.
z Thus, the existence of an efficient algorithm is extremely unlikely.
„ However, practical algorithms that just check some sufficient
conditions for view serializability can still be used.

Database System Concepts - 6th Edition 15.41 ©Silberschatz, Korth and Sudarshan
 Validation-Based Protocol
„ Execution of transaction Ti is done in three phases.
1. Read and execution phase: Transaction Ti writes only to
temporary local variables
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.
„ The three phases of concurrently executing transactions can be
interleaved, but each transaction must go through the three phases in
that order.
z Assume for simplicity that the validation and write phase occur
together, atomically and serially
 i.e., only one transaction executes validation/write at a time.
„ Also called as optimistic concurrency control since transaction
executes fully in the hope that all will go well during validation

Database System Concepts - 6th Edition 15.42 ©Silberschatz, Korth and Sudarshan
 Validation-Based Protocol (Cont.)
„ Each transaction Ti has 3 timestamps:
z Start(Ti) : the time when Ti started its execution
z Validation(Ti): the time when Ti entered its validation phase
z Finish(Ti) : the time when Ti finished its write phase
„ Serializability order is determined by timestamp given at validation
time, to increase concurrency.
z Thus TS(Ti) is given the value of Validation(Ti).
„ This protocol is useful and gives greater degree of concurrency if
probability of conflicts is low.
z because the serializability order is not pre-decided, and
z relatively few transactions will have to be rolled back.

Database System Concepts - 6th Edition 15.43 ©Silberschatz, Korth and Sudarshan
 Validation Test for Transaction Tj
„ If for all Ti with TS (Ti) < TS (Tj) either one of the following condition
holds:
z finish(Ti) < start(Tj)
z start(Tj) < finish(Ti) < validation(Tj) and the set of data items
written by Ti does not intersect with the set of data items read by
Tj.
then validation succeeds and Tj can be committed. Otherwise,
validation fails and Tj is aborted.
„ Justification: Either the first condition is satisfied, and there is no
overlapped execution, or the second condition is satisfied and
„ the writes of Tj do not affect reads of Ti since they occur after Ti
has finished its reads.
„ the writes of Ti do not affect reads of Tj since Tj does not read
any item written by Ti.

Database System Concepts - 6th Edition 15.44 ©Silberschatz, Korth and Sudarshan
 Schedule Produced by Validation
„ Example of schedule produced using validation

Database System Concepts - 6th Edition 15.45 ©Silberschatz, Korth and Sudarshan
 Multiversion Schemes
„ Multiversion schemes keep old versions of data item to increase
concurrency.
z Multiversion Timestamp Ordering
z Multiversion Two-Phase Locking
„ Each successful write results in the creation of a new version of the
data item written.
„ Use timestamps to label versions.
„ When a read(Q) operation is issued, select an appropriate version of
Q based on the timestamp of the transaction, and return the value of
the selected version.
„ reads never have to wait as an appropriate version is returned
immediately.

Database System Concepts - 6th Edition 15.46 ©Silberschatz, Korth and Sudarshan
 Multiversion Timestamp Ordering
„ Each data item Q has a sequence of versions <Q1, Q2,...., Qm>. Each
version Qk contains three data fields:
z Content -- the value of version Qk.
z W-timestamp(Qk) -- timestamp of the transaction that created
(wrote) version Qk
z R-timestamp(Qk) -- largest timestamp of a transaction that
successfully read version Qk
„ when a transaction Ti creates a new version Qk of Q, Qk's W-
timestamp and R-timestamp are initialized to TS(Ti).
„ R-timestamp of Qk is updated whenever a transaction Tj reads Qk, and
TS(Tj) > R-timestamp(Qk).

Database System Concepts - 6th Edition 15.47 ©Silberschatz, Korth and Sudarshan
 Multiversion Timestamp Ordering (Cont)
„ Suppose that transaction Ti issues a read(Q) or write(Q) operation. Let
Qk denote the version of Q whose write timestamp is the largest write
timestamp less than or equal to TS(Ti).
1. If transaction Ti issues a read(Q), then the value returned is the
content of version Qk.
2. If transaction Ti issues a write(Q)
1. if TS(Ti) < R-timestamp(Qk), then transaction Ti is rolled back.
2. if TS(Ti) = W-timestamp(Qk), the contents of Qk are overwritten
3. else a new version of Q is created.
„ Observe that
z Reads always succeed.
z A write by Ti is rejected if some other transaction Tj that (in the
serialization order defined by the timestamp values) should read
Ti's write, has already read a version created by a transaction older
than Ti.
„ Protocol guarantees serializability.

Database System Concepts - 6th Edition 15.48 ©Silberschatz, Korth and Sudarshan
 Multiversion Two-Phase Locking
„ Differentiates between read-only transactions and update transactions
„ Update transactions acquire read and write locks, and hold all locks up
to the end of the transaction. That is, update transactions follow rigorous
two-phase locking.
z Each successful write results in the creation of a new version of the
data item written.
z each version of a data item has a single timestamp whose value is
obtained from a counter ts-counter that is incremented during
commit processing.
„ Read-only transactions are assigned a timestamp by reading the
current value of ts-counter before they start execution; they follow the
multiversion timestamp-ordering protocol for performing reads.

Database System Concepts - 6th Edition 15.49 ©Silberschatz, Korth and Sudarshan
 Multiversion Two-Phase Locking (Cont.)
„ When an update transaction wants to read a data item:
z it obtains a shared lock on it, and reads the latest version.
„ When it wants to write an item
z it obtains X lock on; it then creates a new version of the item and
sets this version's timestamp to ∞.
„ When update transaction Ti completes, commit processing occurs:
z Ti sets timestamp on the versions it has created to ts-counter + 1
z Ti increments ts-counter by 1
„ Read-only transactions that start after Ti increments ts-counter will see
the values updated by Ti.
„ Read-only transactions that start before Ti increments the
ts-counter will see the value before the updates by Ti.
„ Only serializable schedules are produced.

Database System Concepts - 6th Edition 15.50 ©Silberschatz, Korth and Sudarshan
 MVCC: Implementation Issues
„ Creation of multiple versions increases storage overhead
z Extra tuples
z Extra space in each tuple for storing version information
„ Versions can, however, be garbage collected
z E.g., if Q has two versions Q5 and Q9, and the oldest active
transaction has timestamp > 9, than Q5 will never be required
again

Database System Concepts - 6th Edition 15.51 ©Silberschatz, Korth and Sudarshan
 Snapshot Isolation
„ Motivation: Decision support queries that read large amounts of data
have concurrency conflicts with OLTP transactions that update a few
rows
z Poor performance results
„ Solution 1: Give logical “snapshot” of database state to read only
transactions, read-write transactions use normal locking
z Multiversion 2-phase locking
z Works well, but how does system know a transaction is read only?
„ Solution 2: Give snapshot of database state to every transaction,
updates alone use 2-phase locking to guard against concurrent
updates
z Problem: variety of anomalies such as lost update can result
z Partial solution: snapshot isolation level (next slide)
 Proposed by Berenson et al, SIGMOD 1995
 Variants implemented in many database systems
– E.g., Oracle, PostgreSQL, SQL Server 2005

Database System Concepts - 6th Edition 15.52 ©Silberschatz, Korth and Sudarshan
 Snapshot Isolation
„ A transaction T1 executing with Snapshot T1 T2 T3
Isolation
W(Y := 1)
z takes snapshot of committed data at
start Commit
z always reads/modifies data in its own Start
snapshot R(X) Æ 0
z updates of concurrent transactions are R(Y)Æ 1
not visible to T1
z writes of T1 complete when it commits W(X:=2)
z First-committer-wins rule: W(Z:=3)
 Commits only if no other concurrent Commit
transaction has already written data R(Z) Æ 0
that T1 intends to write.
R(Y) Æ 1
W(X:=3)
Concurrent updates not visible
Own updates are visible Commit-Req
Not first-committer of X Abort
Serialization error, T2 is rolled back

Database System Concepts - 6th Edition 15.53 ©Silberschatz, Korth and Sudarshan
 Benefits of SI
„ Reading is never blocked
z and also doesn’t block other txns activities
„ Performance similar to Read Committed
„ Avoids the usual anomalies
z No dirty read
z No lost update
z No non-repeatable read
z Predicate based selects are repeatable (no phantoms)
„ Problems with SI
z SI does not always give serializable executions
 Serializable: among two concurrent txns, one sees the effects
of the other
 In SI: neither sees the effects of the other
z Result: Integrity constraints can be violated

Database System Concepts - 6th Edition 15.54 ©Silberschatz, Korth and Sudarshan
 Snapshot Isolation
„ E.g., of problem with SI
z T1: x:=y
z T2: y:= x
z Initially x = 3 and y = 17
 Serial execution: x = ??, y = ??
 if both transactions start at the same time, with snapshot
isolation: x = ?? , y = ??
„ Called skew write
„ Skew also occurs with inserts
z E.g.,:
 Find max order number among all orders
 Create a new order with order number = previous max + 1

Database System Concepts - 6th Edition 15.55 ©Silberschatz, Korth and Sudarshan
 Snapshot Isolation Anomalies
„ SI breaks serializability when txns modify different items, each based
on a previous state of the item the other modified
z Not very common in practice
 E.g., the TPC-C benchmark runs correctly under SI
 when txns conflict due to modifying different data, there is
usually also a shared item they both modify too (like a total
quantity) so SI will abort one of them
z But does occur
 Application developers should be careful about write skew
„ SI can also cause a read-only transaction anomaly, where read-only
transaction may see an inconsistent state even if updaters are
serializable
z We omit details

Database System Concepts - 6th Edition 15.56 ©Silberschatz, Korth and Sudarshan
 SI In Oracle and PostgreSQL
„ Warning: SI used when isolation level is set to serializable, by Oracle and
PostgreSQL
z PostgreSQL’s implementation of SI described in Section 26.4.1.3
z Oracle implements “first updater wins” rule (variant of “first committer
wins”)
 concurrent writer check is done at time of write, not at commit time
 Allows transactions to be rolled back earlier
z Neither supports true serializable execution
„ Can sidestep for specific queries by using select .. for update in Oracle
and PostgreSQL
z Locks the data which is read, preventing concurrent updates
z E.g.,
1. select max(orderno) from orders for update
2. read value into local variable maxorder
3. insert into orders (maxorder+1, …)

Database System Concepts - 6th Edition 15.57 ©Silberschatz, Korth and Sudarshan
 Insert and Delete Operations
„ If two-phase locking is used :
z A delete operation may be performed only if the transaction
deleting the tuple has an exclusive lock on the tuple to be deleted.
z A transaction that inserts a new tuple into the database is given an
X-mode lock on the tuple
„ Insertions and deletions can lead to the phantom phenomenon.
z A transaction that scans a relation
 (e.g., find sum of balances of all accounts in Perryridge)
and a transaction that inserts a tuple in the relation
 (e.g., insert a new account at Perryridge)
(conceptually) conflict in spite of not accessing any tuple in
common.
z If only tuple locks are used, non-serializable schedules can result
 E.g., the scan transaction does not see the new account, but
reads some other tuple written by the update transaction

Database System Concepts - 6th Edition 15.58 ©Silberschatz, Korth and Sudarshan
 Insert and Delete Operations (Cont.)
„ The transaction scanning the relation is reading information that
indicates what tuples the relation contains, while a transaction
inserting a tuple updates the same information.
z The information should be locked.
„ One solution:
z Associate a data item with the relation, to represent the
information about what tuples the relation contains.
z Transactions scanning the relation acquire a shared lock in the
data item.
z Transactions inserting or deleting a tuple acquire an exclusive
lock on the data item. (Note: locks on the data item do not conflict
with locks on individual tuples.)
„ Above protocol provides very low concurrency for insertions/
deletions.
„ Index locking protocols provide higher concurrency while preventing
the phantom phenomenon, by requiring locks
on certain index buckets.

Database System Concepts - 6th Edition 15.59 ©Silberschatz, Korth and Sudarshan
 Weak Levels of Consistency
„ Degree-two consistency: differs from two-phase locking in that S-locks
may be released at any time, and locks may be acquired at any time
z X-locks must be held till end of transaction
z Serializability is not guaranteed, programmer must ensure that no
erroneous database state will occur]
„ Cursor stability:
z For reads, each tuple is locked, read, and lock is immediately
released
z X-locks are held till end of transaction
z Special case of degree-two consistency

Database System Concepts - 6th Edition 15.60 ©Silberschatz, Korth and Sudarshan
 Weak Levels of Consistency in SQL
„ SQL allows non-serializable executions
z Serializable: is the default
z Repeatable read: allows only committed records to be read, and
repeating a read should return the same value (so read locks should
be retained)
 However, the phantom phenomenon need not be prevented
– T1 may see some records inserted by T2, but may not see
others inserted by T2
z Read committed: same as degree two consistency, but most
systems implement it as cursor-stability
Read uncommitted: allows even uncommitted data to be read
z
„ In many database systems, read committed is the default consistency
level
z has to be explicitly changed to serializable when required
 set isolation level serializable

Database System Concepts - 6th Edition 15.61 ©Silberschatz, Korth and Sudarshan
 Index Locking Protocol
„ Index locking protocol:
z Every relation must have at least one index.
z A transaction can access tuples only after finding them through
one or more indices on the relation
z A transaction Ti that performs a lookup must lock all the index
leaf nodes that it accesses, in S-mode
 Even if the leaf node does not contain any tuple satisfying
the index lookup (e.g., for a range query, no tuple in a leaf is
in the range)
z A transaction Ti that inserts, updates or deletes a tuple ti in a
relation r
 must update all indices to r
 must obtain exclusive locks on all index leaf nodes affected
by the insert/update/delete
z The rules of the two-phase locking protocol must be observed
„ Guarantees that phantom phenomenon won’t occur

Database System Concepts - 6th Edition 15.62 ©Silberschatz, Korth and Sudarshan
 Concurrency in Index Structures
„ Indices are unlike other database items in that their only job is to help in
accessing data.
„ Index-structures are typically accessed very often, much more than
other database items.
z Treating index-structures like other database items, e.g., by 2-phase
locking of index nodes can lead to low concurrency.
„ There are several index concurrency protocols where locks on internal
nodes are released early, and not in a two-phase fashion.
z It is acceptable to have nonserializable concurrent access to an
index as long as the accuracy of the index is maintained.
 In particular, the exact values read in an internal node of a
B+-tree are irrelevant so long as we land up in the correct leaf
node.

Database System Concepts - 6th Edition 15.63 ©Silberschatz, Korth and Sudarshan
 Concurrency in Index Structures (Cont.)
„ Example of index concurrency protocol:
„ Use crabbing instead of two-phase locking on the nodes of the B+-tree, as
follows. During search/insertion/deletion:
z First lock the root node in shared mode.
z After locking all required children of a node in shared mode, release the lock
on the node.
z During insertion/deletion, upgrade leaf node locks to exclusive mode.
z When splitting or coalescing requires changes to a parent, lock the parent in
exclusive mode.
„ Above protocol can cause excessive deadlocks
z Searches coming down the tree deadlock with updates going up the tree
z Can abort and restart search, without affecting transaction
„ Better protocols are available; see Section 16.9 for one such protocol, the B-link
tree protocol
z Intuition: release lock on parent before acquiring lock on child
 And deal with changes that may have happened between lock release
and acquire

Database System Concepts - 6th Edition 15.64 ©Silberschatz, Korth and Sudarshan
 Next-Key Locking
„ Index-locking protocol to prevent phantoms required locking entire leaf
z Can result in poor concurrency if there are many inserts
„ Alternative: for an index lookup
z Lock all values that satisfy index lookup (match lookup value, or
fall in lookup range)
z Also lock next key value in index
z Lock mode: S for lookups, X for insert/delete/update
„ Ensures that range queries will conflict with inserts/deletes/updates
z Regardless of which happens first, as long as both are concurrent

Database System Concepts - 6th Edition 15.65 ©Silberschatz, Korth and Sudarshan
 End of Chapter

Thanks to Alan Fekete and Sudhir Jorwekar for Snapshot

Isolation examples

Database System Concepts, 6th Ed.

©Silberschatz, Korth and Sudarshan
See www.db-book.com for conditions on re-use
 Snapshot Read
„ Concurrent updates invisible to snapshot read

Database System Concepts - 6th Edition 15.67 ©Silberschatz, Korth and Sudarshan
 Snapshot Write: First Committer Wins

z Variant: “First-updater-wins”
 Check for concurrent updates when write occurs
 (Oracle uses this plus some extra features)
 Differs only in when abort occurs, otherwise equivalent
Database System Concepts - 6th Edition 15.68 ©Silberschatz, Korth and Sudarshan
 Figure 15.01

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 Figure in-15.1

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