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    TRANSACTION MANAGEMENTin DBMS

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    Our fresher job

    section is

    exclusively for

    fresh graduates!Find jobs for

    freshers in major

    Indian cities

    including

    Bangalore,

    Chennai,

    Hyderabad, Pune

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    Colleges

    Find details about

    any college in

    India or search for

    courses.

    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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  • 8/2/2019 Entire Schedules

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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

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