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TRANSACTION MANAGEMENTin DBMS
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A sequence of many actions which areconsidered to be one atomic unit of
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Special actions: commit, abortFor now, assume reads and writes are
on tuples;
A transaction is a unit of work which
completes as a unit or fails as a unit.The ACID Properties
Atomicity: All actions in thetransaction happen, or none happen.
Consistency: If each transaction is
consistent, and the DB starts consistent,
it ends up consistent.
Isolation: Execution of one transaction
is isolated from that of other
transactions.
Durability: If a transaction commits,
its effects persist.
A transaction is a collection of
operations involving data items in a
database. Read, insert, delete, and
update are example operations. There
are four important properties of
transactions that a DBMS must ensure
to maintain data in the face of
concurrent access and system failures:Atomicity: Users should be able to
regard the execution of each transaction
as atomic: either all actions are
executed or none are executed. Users
should not have to worry about the
effect of incomplete transactions
(example, when a system crash occurs).
Consistency: Each transaction, run by
itself with no concurrent execution ofother transactions, must preserve the
consistency of the database. This
property is called consistency. Ensuring
this property of the transaction is the
responsibility of the user.
Isolation: Users should be able to
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understand the transaction without
considering the effect of other
concurrently executing transactions,
even if the DBMS interleaves the
actions of several transactions.Transactions are isolated, or protected,
from the effects of concurrently
scheduling other transactions.
Durability: Once the DBMS informs
the user that a transaction has been
successfully completed, its effects
should persist even if the system
crashes before all its changes are
reflected on disk.
Passing the ACID Test
Concurrency ControlGuaranteesConsistency and Isolation, given
Atomicity.
Logging and RecoveryGuarantees
Atomicity and Durability.
Atomicity
All-or-nothing, no partial results. An
event either happens and is committed
or fails and is rolled back.e.g. in a money transfer, debit one
account, credit the other. Either both
debiting and crediting operations
succeed, or neither of them do.
Transaction failure is called Abort
Commit and abort are irrevocableactions. There is no undo for these
actions.
An Abort undoes operations that havealready been executed
For database operations, restore the
datas previous value from before thetransaction (Rollback-it); a Rollback
command will undo all actions taken
since the last commit for that user.
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But some real world operations arenot undoable.
Examples - transfer money, print ticket,
fire missile
Consistency Every transaction should maintain DBconsistency
Referential integrity - e.g. each orderreferences an existing customer number
and existing part numbers
The books balance (debits = credits,assets = liabilities)
Consistency preservation is a propertyof a transaction, not of the database
mechanisms for controlling it (unlike
the A, I, and D of ACID)
If each transaction maintains
consistency, then a serial execution of
transactions does also
A database state consists of thecomplete set of data values in the
database
A database state is consistent if the
database obeys all the integrityconstraint
A transaction brings the database fromone consistent state to another
consistent state
Consistent state diagram
IsolationIntuitively, the effect of a set of
transactions should be the same as if
they ran independently. Formally, an interleaved execution of
transactions is serializable if its effect is
equivalent to a serial one.
Implies a user view where the systemruns each users transaction stand-alone.
Of course, transactions in fact run with
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lots of concurrency, to use device
parallelismthis will be covered later.
Transactions can use common data(shared data)
They can use the same data processingmechanisms (time sharing)
Durability
When a transaction commits, itsresults will survive failures (e.g. of the
application, OS, DB system even of
the disk).
Makes it possible for a transaction to
be a legal contract.
Implementation is usually via a log
DB system writes all transaction
updates to a log file
to commit, it adds a record
commit(Ti) to the log
When the commit record is on disk,
the transaction is committed.
system waits for disk ack before
acknowledging to user
Active: transaction is started and is
issuing reads and writes to the database Partially committed: operations aredone and values are ready to be written
to the database
Committed: writing to the database is
permitted and successfully completed
Abort: the transaction or the systemdetects a fatal error
Terminated: transaction leaves the
system A transaction reaches its commit point
when all operations accessing the
database are completed and the result
has been recorded in the log. It then
writes a [commit, ] and terminates
When a system failure occurs, search
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the log file for entries [start, ]
and if there are no logged entries
[commit, ]then undo all operations that
have logged entries [write, , X,
old_value, new_value]Why is Concurrency Control Needed?
Several problems occur whenconcurrent transactions execute in an
uncontrolled manner
A schedule of concurrent transactions
is a particular sequence of interleaving
of their read or write operations
In general a transaction, has a set ofdata items it accesses (read set), and a
set of data items it modifies (write set)
Lost Update Problem - Successfully
completed update is overridden by
another user.
What should the final Order Value be?
Which Update has been lost?
Loss of T2s update avoided by
preventing T1 from reading balx until
after update.
Uncommitted Dependency Problem /Temporary Update Problem
Occurs when one transaction can seeintermediate results of another
transaction before it has committed.
What should the final Order Value be?
Where is the temporary update?Uncommitted Dependency Problem
Problem avoided by preventing T3
from reading balx until after T4commits or aborts.
Incorrect Summary Problem
What should the total Order Value be?
Which order was accumulated beforeupdate, and which after?
Schedules - actions of transactions as
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seen by the DBMS
Schedule: An interleaving of actions
from a set of transactions, where the
actions of any one transaction are in the
original order. When two or more transactions arerunning concurrently, the steps of the
transactions would normally be
interleaved. The interleaved execution
of transactions is decided by the
database scheduler which receives a
stream of user requests that arise from
the active transactions. A particular
sequencing (usually interleaved) of the
actions of a set of transactions is called
a schedule.
Represents some actual sequence of
database actions.
Example: R 1 (A), W 1 (A), R 2 (B),
W 2 (B), R 1 (C), W 1 (C)
T1 T2
R(A)
W(A)
R(B)W(B)
R(C)
W(C)
In a complete schedule, each
transaction ends in commit or abort.
Initial State + Schedule Final State
Serial schedule is a schedule in which
all the operations of one transaction are
completed before another transactioncan begin (that is, there is no
interleaving).
One sensible isolated, consistentschedule:
Run transactions one at a time, in aseries.
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NOTE: Different serial schedules can
have different final states; all are OK
-- DBMS makes no guarantees about
the order in which concurrently
submitted transactions are executed. Serializable Schedule
A schedule whose effect on the DBstate is the same as that of some serialschedule
All serial schedules are serializable.
But the reverse may not be true
Let T be a set of n transactions . If the
n transactions are executed serially (call
this execution S), we assume they
terminate properly and leave the
database in a consistent state. A
concurrent execution of the n
transactions in T (call this execution C)
is called serializable if the execution is
computationally equivalent to a serial
execution. There may be more than one
such serial execution. That is, the
concurrent execution C always
produces exactly the same effect on thedatabase as some serial execution S
does. (Note that S is some serial
execution of T, not necessarily the
order). A serial schedule is always
correct since we assume transactions do
not depend on each other and
furthermore, we assume, that each
transaction when run in isolation
transforms a consistent database into anew consistent state and therefore a set
of transactions executed one at a time
(i.e. serially) must also be correct.
Final state is what some serial
schedule would have produced.
Aborted Xacts are not part of
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schedule; ignore them for now (they are
made to `disappear by using logging).
In general, serializability is a good
propertyBut difficult to achieve due
to lack of effective algorithms. Serializability
Objective of a concurrency controlprotocol is to schedule transactions in
such a way as to avoid any interference.
Could run transactions serially, but
this limits degree of concurrency or
parallelism in system.
Serializability identifies thoseexecutions of transactions guaranteed to
ensure consistency.
Schedule
Sequence of reads/writes by set of
concurrent transactions.
Serial Schedule
Schedule where operations of each
transaction are executed consecutively
without any interleaved operations from
other transactions.
Nonserial Schedule Schedule where operations from set ofconcurrent transactions are interleaved.
Objective of serializability is to findnonserial schedules that allow
transactions to execute concurrently
without interfering with one another.
In other words, want to find nonserial
schedules that are equivalent to some
serial schedule. Such a schedule iscalled serializable.
In serializability, ordering of read/writes
is important:
(a) If two transactions only read a data
item, they do not conflict and order is
not important.
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(b) If two transactions either read or
write completely separate data items,
they do not conflict and order is not
important.
(c) If one transaction writes a data itemand another reads or writes same data
item, order of execution is important.
Serializability Violations
Two transactions T1 and T2 are said
to conflict if some action t1 of T1 and
an action t2 of T2 access the same
object and at least one of the actions is a
write. The conflict is called a RW-
conflict if the write set of one
transaction intersects with the read set
of another. A WW-conflict occurs if the
conflict is between two writes.
Result is not equal to any serialexecution!
W- R conflict: T2 reads something T1wrote previously (dirty read).
RW Conflicts (Unrepeatable Read)
T2 overwrites what T1 read.
If T1 reads it again, it will seesomething new!
Example when this would happen? The
increment/ decrement example.
Again, not equivalent to a serial
execution.
WW Conflicts (Overwriting
Uncommitted Data)
T2 overwrites what T1 wrote.
Example: 2 transactions to update itemsto be kept equal.
Usually occurs in conjunction with
other anomalies.
Aborted Transactions: All actions of
aborted transactions are to be undone
as if aborted transactions never
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happened.
Two Issues:
How does one undo the effects of a
transaction? Well cover this in logging/
recoveryWhat if another transaction sees theseeffects?? Must undo that transaction as
well!
Cascading Aborts
T1 T2
R(A)
W(A)
R(A)
W(A)
abort
Abort of T1 requires abort of T2!Cascading Abort
What about WW conflicts & aborts?
T2 overwrites a value that T1 writes.
T1 aborts: its remembered valuesare restored.
Lose T2s write! We will see how tosolve this, too.
Recoverable ScheduleA schedule where, for each pair of
transactions Ti and Tj, if Tj reads a data
item previously written by Ti, then the
commit operation of Ti precedes the
commit operation of Tj.
Unrecoverable Schedule
T1 T2
R(A)
W(A)R(A)
W(A)
commit
abort
Recoverable Schedule
T1 T2
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R(A)
W(A)
R(A)
W(A)
commitcommit
Abort of T1 in first figure requires abort
of T2!
But T2 has already committed and
hence cannot undo its actions! This is
unrecoverable schedule. A recoverable
schedule is one in which this cannot
happen.
i. e. a transaction commits only after
all the transactions it depends on (i. e.
it reads from or overwrites) commit.
An ACA (avoids cascading abort)
schedule is one in which cascading
abort cannot arise.
A transaction only reads/ writes datafrom committed transactions.
Recoverable implies ACA (but notvice- versa!). Real systems typically
ensure that only recoverable schedulesarise (through locking).
Concurrency Control Techniques
Two basic concurrency control
techniques:
Locking,
Timestamping. Both are conservative approaches:
delay transactions in case they conflict
with other transactions. Optimisticmethods assume conflict is rare and
only check for conflicts at commit.
LockingThe concept of locking data items is one
of the main techniques for controlling
the concurrent execution of
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transactions.
A lock is a variable associated with a
data item in the database.
Generally there is a lock for each data
item in the database. A lock describes the status of the dataitem with respect to possible operations
that can be applied to that item
used for synchronising the access by
concurrent transactions to the database
items.
A transaction locks an object before
using it
When an object is locked by another
transaction, the requesting transaction
must wait
Binary locks have two possible states: 1.locked (lock_item (X) operation)and
2.unlocked (unlock (X) operation
Multiple-mode locks allow concurrent
access to the same item by several
transactions. Three possible states:
1.read locked or shared locked (othertransactions are allowed to read the
item)
2.write locked or exclusive locked (asingle transaction exclusively holds the
lock on the item) and
3.unlocked. Locks are held in a lock table.
upgrade lock: read lock to write lock
downgrade lock: write lock to readlock
TYPES OF LOCKS
SHARED LOCKS
EXCLUSIVE LOCKSTransaction uses locks to deny access to
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other transactions and so prevent
incorrect updates.
Most widely used approach to ensureserializability.
Generally, a transaction must claim ashared (read) or exclusive (write) lock
on a data item before read or write.
Lock prevents another transactionfrom modifying item or even reading it,
in the case of a write lock.
Locking - Basic Rules If transaction has shared lock on item,
can read but not update item.
If transaction has exclusive lock on
item, can both read and update item.
Reads cannot conflict, so more thanone transaction can hold shared locks
simultaneously on same item.
Exclusive lock gives transactionexclusive access to that item.
Some systems allow transaction to
upgrade read lock to an exclusive lock,
or downgrade exclusive lock to a shared
lock. Two-Phase Locking (2PL)
Strict 2PL:
If T wants to read an object, first
obtains an S lock.
If T wants to modify an object, first
obtains X lock.
Hold all locks until end of transaction.
Guarantees serializability, and
recoverable schedule, too! also avoidsWW problems!
2PL:
Slight variant of strict 2PL
transactions can release locks before
the end (commit or abort)
? But after releasing any lock it can
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acquire no new locksGuaranteesserializability
A two-phase locking ( 2PL) scheme is a
locking scheme in which a transaction
cannot request a new lock afterreleasing a lock. Two phase locking
therefore involves two phases:
Growing Phase ( Locking Phase) -When locks are acquired and none
released.
Shrinking Phase ( Unlocking Phase) -When locks are released and none
acquired.
The attraction of the two-phase
algorithm derives from a theorem which
proves that the two-phase locking
algorithm always leads to serializable
schedules. This is a sufficient condition
for serializability although it is not
necessary.
Strict two-phase locking ( Strict 2PL) is
the most widely used locking protocol,
and has following two rules:
If a transaction wants to read(respectively, modify) an object, it first
requests a shared (respectively,
exclusive) lock on the object.
All locks held by a transaction are
released when the transaction is
completed
In effect the locking protocol allows
only safe interleavings of transactions.
Q) Three transactions A, B and C arrivein the time sequence A, then B and then
C. The transactions are run concurrently
on the database. Can we predict what
the result would be if 2PL is used?
No, we cannot do that since we arenot able to predict which serial schedule
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the 2PL schedule is going to be
equivalent to. The 2PL schedule could
be equivalent to any of the following six
serial schedules: ABC, ACB, BAC,
BCA, CAB, CBA. Two-Phase Locking (2PL)Transaction follows 2PL protocol if all
locking operations precede first unlock
operation in the transaction.
Two phases for transaction:
Growing phase - acquires all locks butcannot release any locks.
Shrinking phase - releases locks butcannot acquire any new locks.
Preventing Lost Update Problem using
2PL Eg in slides
Preventing Uncommitted DependencyProblem using 2PL
Locking Granularity
A database item which can be locked
could be
a database record
a field value of a database record
the whole database Trade-offs
?coarse granularity - the larger the data
item size, the lower the degree of
concurrency
?fine granularity - the smaller the data
item size, the more locks to be managed
and stored, and the more lock/unlock
operations needed.
DeadlockAn impasse that may result when two
(or more) transactions are each waiting
for locks held by the other to be
released. Eg in slides
Conditions For Deadlock
Mutual Exclusion
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Hold And Wait Non Preemption
Circular Wait
Recovery
Occurs in case of transaction failures. Database (DB) is restored to the mostrecent consistent state just before the
time of failure.
To do this, the DB system needs
information about changes applied by
various transactions. It is the system
log.
Contents of System Log:
[start_transaction, T]: Indicates that
transaction T has started execution.
[write_item, T, X, old_value,new_value]: Indicates that transaction T
has changed the value of DB item X
from old_value to new_value.
[read_item, T, X]: Indicates thattransaction T has read the value of DB
item X.
[commit, T]: Indicates that transaction
T has completed successfully, andaffirms that its effect can be committed
(recorded permenantly) to the database.
[abort, T]: Indicates that transaction Thas been aborted.
Deadlock
Only one way to break deadlock: abortone or more of the transactions.
Deadlock should be transparent to
user, so DBMS should restarttransaction(s).
Three general techniques for handlingdeadlock:
Timeouts.
Deadlock prevention.
Deadlock detection and recovery.
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Timeouts
Transaction that requests lock will
only wait for a system-defined period of
time.
If lock has not been granted withinthis period, lock request times out.
In this case, DBMS assumestransaction may be deadlocked, even
though it may not be, and it aborts and
automatically restarts the transaction.
Deadlock Prevention
DBMS looks ahead to see if
transaction would cause deadlock and
never allows deadlock to occur.
Could order transactions using
transaction timestamps:
Wait-Die - only an older transaction
can wait for younger one, otherwise
transaction is aborted (dies) and
restarted with same timestamp.
Wound-Wait - only a younger
transaction can wait for an older one. If
older transaction requests lock held by
younger one, younger one is aborted(wounded).
Deadlock Detection and Recovery
DBMS allows deadlock to occur but
recognizes it and breaks it.
Usually handled by construction of
wait-for graph (WFG) showing
transaction dependencies:
Create a node for each transaction.
Create edge Ti -> Tj, if Ti waiting tolock item locked by Tj.
Deadlock exists if and only if WFG
contains cycle.
WFG is created at regular intervals. Recovery Outline
Restore to most recent consistent
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state just before time of failure
Use data in the log file
Catastrophic Failure
Restore database from backup
Replay transactions from log file Database becomes inconsistent (non-catastrophic errors)
Undo or Redo last transactions untilconsistent state is restored
Recovery Algorithms for Non-
catastrophic Errors:
Deferred Update (NO-UNDO/REDO):
Data written to buffersNot physically updated until after
commit point reached and logs have
been updated
No undo is even necessary
Redo might be necessary ontransactions that have been logged but
not physically updated
Known as the NO-UNDO/REDO
algorithm
Immediate Update (UNDO/REDO):
Database being updated as transactionoccurs
However, log always force writtenfirst
Partially completed transactions will
have to be undone
Committed transactions might have tobe redone
Known as the UNDO/REDO
algorithmVariation on the scheme:
Data is physically updated beforecommit
Only requires UNDO
Known as the UNDO/NO-REDOalgorithm
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Logging Record REDO and UNDO
information, for every update, in a log.
Sequential writes to log (put it on a
separate disk).Minimal info (diff) written to log, somultiple updates fit in a single log page.
Log: An ordered list of REDO/ UNDO
actions
Log record contains:
and additional control infoCaching of Disk Blocks:
Disk blocks typically cached to mainmemory
Changes made to cache block which
is then written back at some later time
Many DBMSs even handle the low-
level I/O
DBMS Caching:
All database accesses check to see ifrequired item is in the cache first. If not,
item is loaded into cache
Dirty bit: Determines if cache block
has been updated and needs to bewritten back to disk
Pin/Unpin bit: Is it OK to write blockback to disk yet?
In-place updating: Block is written
back out to same location. Overwrite
original
Shadowing: Block is written to new
location
? Old copy is kept? Before Image (BFIM) & After Image
(AFIM)
The Write- Ahead Logging Protocol:
Must force the log record for an
update before the corresponding data
page gets to disk.
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Must write all log records for atransaction before commit
The rule that all transactions follow in
the WAL protocol is "Write the log
before you write the data. When atransaction wants to update a record, it
pins the page containing the record in
the main-memory buffer pool, modifies
the page in memory, generates an
undo/redo record, forces the undo/redo
record to the log, and unpins the page in
the buffer pool. At some later time, the
page replacement algorithm or a
checkpoint will write the page back to
the database.
WAL protocol:
#1 guarantees Atomicity.
#2 guarantees Durability.
Each log record has a unique LogSequence Number (LSN). LSNs always
increasing.
Each data page contains a pageLSN.The LSN of the most recent log record
for an update to that page. System keeps track of flushedLSN.The max LSN flushed so far.
WAL: Before a page is written,pageLSN
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all updated DB buffers written out to
disk
Any committed transaction occurringbefore checkpoint in log can be
considered permanent (wont have to beredone after crash)
Actions
suspend execution of all transactions
force-write all modified buffers to
disk
write checkpoint entry in log andforce write log
resume transactions Checkpointing: Periodically, the
DBMS creates a checkpoint, in order to
minimize the time taken to recover in
the event of a system crash. It quiesces
the system (makes all currently
executing transactions pause), writes all
dirty buffers to disk, and then allows
transactions to resume normal
processing.
The problem with this simple
checkpoint is that it makes the dataunavailable for too long, possibly
several minutes, while the checkpoint is
being done. There is another technique
called Fuzzy checkpoint, which does
not have this problem, because it does
not quiesce the system. Instead, for each
buffer, the fuzzy checkpoint procedure
latches the buffer (gets an exclusive
semaphore on it), writes it to disk if it isdirty, and then unlatches the buffer. In
addition, fuzzy checkpoint writes the
IDs of the currently active transactionsto the log. Fuzzy checkpoint just locks
buffers one at a time and releases them.
How does the system recovers from a
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crash: The crash recovery algorithm
reads the most recent checkpoint
information from the log, which yields a
set of transaction IDs that were active
at the time of the checkpoint. Then itscans the log forward from the
checkpoint, reapplying every undo/redo
record to the database (this is called
REDO ALL). During the forward pass,
it analyzes the log to determine which
transactions did not commit or abort
before the crash. These transactions are
called the "losers." Then, the recovery
algorithm scans the log in reverse,
undoing log records for all the losers
(this is called UNDO LOSERS).
Additional Crash Issues: Whathappens if system crashes during
Analysis? During REDO? How do you
limit the amount of work in REDO ?
Flush asynchronously in the
background.
Watch hot spots!
How do you limit the amount of workin UNDO ?
Avoid long- running transactions.
Responses
Author:sri phani
kumari 19 May 2008
Member
Level:
Silver Poin
ts : 2
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