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    Chameli Devi Group of Institutions: Cs502 Dbms Unit-4 Deadlock in DBMS

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    This document discusses deadlocks in database management systems (DBMS). It provides an example of a deadlock between two transactions, Transaction 1 and Transaction 2, that are each waiting for resources held by the other. The document then discusses different approaches to handling deadlocks, including deadlock avoidance, detection using a wait-for graph, and prevention techniques like the wait-die and wound-wait schemes which assign timestamps to transactions.

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    Chameli Devi Group of Institutions: Cs502 Dbms Unit-4 Deadlock in DBMS

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    This document discusses deadlocks in database management systems (DBMS). It provides an example of a deadlock between two transactions, Transaction 1 and Transaction 2, that are each waiting for resources held by the other. The document then discusses different approaches to handling deadlocks, including deadlock avoidance, detection using a wait-for graph, and prevention techniques like the wait-die and wound-wait schemes which assign timestamps to transactions.

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    Deadlock in DBMS

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    This document discusses deadlocks in database management systems (DBMS). It provides an example of a deadlock between two transactions, Transaction 1 and Transaction 2, that are each waiting for resources held by the other. The document then discusses different approaches to handling deadlocks, including deadlock avoidance, detection using a wait-for graph, and prevention techniques like the wait-die and wound-wait schemes which assign timestamps to transactions.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    122 views19 pages

    Chameli Devi Group of Institutions: Cs502 Dbms Unit-4 Deadlock in DBMS

    Uploaded by

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    This document discusses deadlocks in database management systems (DBMS). It provides an example of a deadlock between two transactions, Transaction 1 and Transaction 2, that are each waiting for resources held by the other. The document then discusses different approaches to handling deadlocks, including deadlock avoidance, detection using a wait-for graph, and prevention techniques like the wait-die and wound-wait schemes which assign timestamps to transactions.

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    Download as pptx, pdf, or txt
    of 19

    CHAMELI DEVI GROUP OF

    INSTITUTIONS

    CS502 DBMS
    Unit- 4
    Deadlock in DBMS

    Presented By:
    Miss. Renu Jaiswal
    Assistant Professor
    CSE Department
    Introduction
    • A deadlock is a condition where two or more
    transactions are waiting indefinitely for one
    another to give up locks.
    • Deadlock is said to be one of the most feared
    complications in DBMS as no task ever gets
    finished and is in waiting state forever.

    Ms. Renu Jaiswal 2


    Example
    • For example: In the student table, transaction
    T1 holds a lock on some rows and needs to
    update some rows in the grade table.
    Simultaneously, transaction T2 holds locks on
    some rows in the grade table and needs to
    update the rows in the Student table held by
    Transaction T1.

    Ms. Renu Jaiswal 3


    • Now, the main problem arises. Now Transaction
    T1 is waiting for T2 to release its lock.
    • Similarly, transaction T2 is waiting for T1 to
    release its lock.
    • All activities come to a halt state and remain at a
    standstill.
    • It will remain in a standstill until the DBMS detects
    the deadlock and aborts one of the transactions.

    Ms. Renu Jaiswal 4


    Deadlock

    Ms. Renu Jaiswal 5


    Deadlock Avoidance

    • When a database is stuck in a deadlock state,


    then it is better to avoid the database rather
    than aborting or restating the database.
    • This is a waste of time and resource.

    Ms. Renu Jaiswal 6


    Deadlock Avoidance
    • Deadlock avoidance mechanism is used to
    detect any deadlock situation in advance.
    • A method like "wait for graph" is used for
    detecting the deadlock situation but this
    method is suitable only for the smaller
    database. For the larger database, deadlock
    prevention method can be used.

    Ms. Renu Jaiswal 7


    Deadlock Detection

    • In a database, when a transaction waits


    indefinitely to obtain a lock, then the DBMS
    should detect whether the transaction is
    involved in a deadlock or not.
    • The lock manager maintains a Wait for the
    graph to detect the deadlock cycle in the
    database.

    Ms. Renu Jaiswal 8


    Wait for Graph

    • This is the suitable method for deadlock


    detection.
    • In this method, a graph is created based on
    the transaction and their lock.
    • If the created graph has a cycle or closed loop,
    then there is a deadlock.

    Ms. Renu Jaiswal 9


    Wait for Graph
    • The wait for the graph is maintained by the
    system for every transaction which is waiting
    for some data held by the others. The system
    keeps checking the graph if there is any cycle
    in the graph.

    Ms. Renu Jaiswal 10


    The wait for a graph for the above scenario is
    shown below:

    Ms. Renu Jaiswal 11


    Deadlock Prevention

    • Deadlock prevention method is suitable for a


    large database.
    • If the resources are allocated in such a way
    that deadlock never occurs.
    • Then the deadlock can be prevented.

    Ms. Renu Jaiswal 12


    Deadlock Prevention
    • The Database management system analyzes
    the operations of the transaction whether
    they can create a deadlock situation or not.
    • If they do, then the DBMS never allowed that
    transaction to be executed.

    Ms. Renu Jaiswal 13


    Wait-Die scheme

    • In this scheme, if a transaction requests for a


    resource which is already held with a
    conflicting lock by another transaction then
    the DBMS simply checks the timestamp of
    both transactions.
    • It allows the older transaction to wait until the
    resource is available for execution.

    Ms. Renu Jaiswal 14


    Wait-Die scheme
    • Let's assume there are two transactions Ti and Tj and let
    TS(T) is a timestamp of any transaction T. If T2 holds a lock
    by some other transaction and T1 is requesting for
    resources held by T2 then the following actions are
    performed by DBMS:
    • Check if TS(Ti) < TS(Tj) - If Ti is the older transaction and Tj
    has held some resource, then Ti is allowed to wait until the
    data-item is available for execution. That means if the
    older transaction is waiting for a resource which is locked
    by the younger transaction, then the older transaction is
    allowed to wait for resource until it is available.

    Ms. Renu Jaiswal 15


    Wait-Die scheme
    • Check if TS(Ti) < TS(Tj) - If Ti is older
    transaction and has held some resource and if
    Tj is waiting for it, then Tj is killed and
    restarted later with the random delay but with
    the same timestamp.

    Ms. Renu Jaiswal 16


    Wound wait scheme

    • In wound wait scheme, if the older transaction


    requests for a resource which is held by the
    younger transaction.
    • Then older transaction forces younger one to
    kill the transaction and release the resource.
    • After the minute delay, the younger
    transaction is restarted but with the same
    timestamp.

    Ms. Renu Jaiswal 17


    Wound wait scheme
    • If the older transaction has held a resource
    which is requested by the Younger
    transaction, then the younger transaction is
    asked to wait until older releases it.

    Ms. Renu Jaiswal 18


    THANK YOU

    Ms. Renu Jaiswal 19

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