Database Fundamentals- Content Analysis

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Database Principles: Fundamentals of Design, Implementations and Management

Lecture11- CHAPTER 12: Transaction Management and Concurrency Control

Presented by Rabia Cherouk

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Objectives

  • In this chapter, you will learn:
  • About database transactions and their properties
  • What concurrency control is and what role it plays in maintaining the database’s integrity
  • What locking methods are and how they work
  • How stamping methods are used for concurrency control
  • How optimistic methods are used for concurrency control
  • How database recovery management is used to maintain database integrity

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What is a Transaction?

  • A transaction is a logical unit of work that must be either entirely completed or aborted
  • Successful transaction changes database from one consistent state to another
  • One in which all data integrity constraints are satisfied
  • Most real-world database transactions are formed by two or more database requests
  • Equivalent of a single SQL statement in an application program or transaction

Same as Fig. 12.1 in your book

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Same as Fig. 12.1 in your book

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Evaluating Transaction Results

  • Not all transactions update the database
  • SQL code represents a transaction because database was accessed
  • Improper or incomplete transactions can have devastating effect on database integrity
  • Some DBMSs provide means by which user can define enforceable constraints
  • Other integrity rules are enforced automatically by the DBMS

Same as Fig. 12.2 in your book

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Figure 9.2

Same as Fig. 12.2 in your book

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Transaction Properties

All transactions must display atomicity, consistency, durability and serializability (ACIDS).

  • Atomicity
  • All operations of a transaction must be completed
  • Consistency
  • Permanence of database’s consistent state
  • Isolation
  • Data used during transaction cannot be used by second transaction until the first is completed

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Transaction Properties (cont..)

  • Durability
  • Once transactions are committed, they cannot be undone
  • Serializability
  • Concurrent execution of several transactions yields consistent results
  • Multiuser databases are subject to multiple concurrent transactions

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Transaction Management with SQL

  • ANSI (American National Standard Institute) has defined standards that govern SQL database transactions
  • Transaction support is provided by two SQL statements: COMMIT and ROLLBACK
  • Transaction sequence must continue until:
  • COMMIT statement is reached
  • ROLLBACK statement is reached
  • End of program is reached
  • Program is abnormally terminated

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The Transaction Log

  • A DBMS uses a Transaction log to store:
  • A record for the beginning of transaction
  • For each transaction component:
  • Type of operation being performed (update, delete, insert)
  • Names of objects affected by transaction
  • “Before” and “after” values for updated fields
  • Pointers to previous and next transaction log entries for the same transaction
  • Ending (COMMIT) of the transaction

Table 12.1 in your book

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The Transaction Log

Table 12.1 in your book

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Concurrency Control

  • Is the coordination of simultaneous transaction execution in a multiprocessing database
  • Objective is to ensure serializability of transactions in a multiuser environment
  • Simultaneous execution of transactions over a shared database can create several data integrity and consistency problems
  • Lost updates
  • Uncommitted data
  • Inconsistent retrievals

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Lost Updates

  • Lost update problem:
  • Two concurrent transactions update same data element
  • One of the updates is lost
  • Overwritten by the other transaction

Lost Updates

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Lost Updates (cont..)

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Uncommitted Data

  • Uncommitted data phenomenon:
  • Two transactions executed concurrently
  • First transaction rolled back after second already accessed uncommitted data

Uncommitted Data

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Uncommitted Data (cont..)

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Inconsistent Retrievals

  • Inconsistent retrievals:
  • First transaction accesses data
  • Second transaction alters the data
  • First transaction accesses the data again
  • Transaction might read some data before they are changed and other data after changed
  • Yields inconsistent results

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The Scheduler

  • Special DBMS program
  • Purpose is to establish order of operations within which concurrent transactions are executed
  • Interleaves execution of database operations:
  • Ensures serializability
  • Ensures isolation
  • Serializable schedule
  • Interleaved execution of transactions yields same results as serial execution

The Scheduler (cont..)

  • Bases its actions on concurrency control algorithms
  • Ensures computer’s central processing unit (CPU) is used efficiently
  • Facilitates data isolation to ensure that two transactions do not update same data element at same time

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Database Recovery Management

Database recovery

  • Restores database from given state, usually inconsistent, to previously consistent state
  • Based on atomic transaction property
  • All portions of transaction treated as single logical unit of work
  • All operations applied and completed to produce consistent database

If transaction operation cannot be completed, transaction must be aborted, and any changes to database must be rolled back (undone)

Transaction Recovery

  • Makes use of deferred-write and write-through techniques
  • Deferred write
  • Transaction operations do not immediately update physical database
  • Only transaction log is updated
  • Database is physically updated only after transaction reaches its commit point using transaction log information

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Transaction Recovery (cont..)

  • Write-through technique
  • Database is immediately updated by transaction operations during transaction’s execution, even before transaction reaches its commit point
  • Recovery process
  • Identify last checkpoint
  • If transaction was committed before checkpoint
  • Do nothing
  • If transaction committed after last checkpoint
  • DBMS redoes the transaction using “after” values
  • If transaction had ROLLBACK or was left active
  • Do nothing because no updates were made

Transaction Recovery (cont..)

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Summary

  • Transaction: sequence of database operations that access database
  • Logical unit of work
  • No portion of transaction can exist by itself
  • Five main properties: atomicity, consistency, isolation, durability, and serializability
  • COMMIT saves changes to disk
  • ROLLBACK restores previous database state
  • SQL transactions are formed by several SQL statements or database requests

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Summary (cont..)

  • Transaction log keeps track of all transactions that modify database
  • Concurrency control coordinates simultaneous execution of transactions
  • Scheduler establishes order in which concurrent transaction operations are executed
  • Lock guarantees unique access to a data item by transaction
  • Two types of locks: binary locks and shared/exclusive locks

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Summary (cont..)

  • Serializability of schedules is guaranteed through the use of two-phase locking
  • Deadlock: when two or more transactions wait indefinitely for each other to release lock
  • Three deadlock control techniques: prevention, detection, and avoidance
  • Time stamping methods assign unique time stamp to each transaction
  • Schedules execution of conflicting transactions in time stamp order

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Summary (cont..)

  • Optimistic methods assume the majority of database transactions do not conflict
  • Transactions are executed concurrently, using private copies of the data
  • Database recovery restores database from given state to previous consistent state



CHAPTER 12: Transaction Management and Concurrency Control

  • ADDITIONAL SLIDES pages 635 to 644 in your

Book..

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Two-Phase Locking to Ensure Serializability (cont..)

  • Governed by the following rules:
  • Two transactions cannot have conflicting locks
  • No unlock operation can precede a lock operation in the same transaction
  • No data are affected until all locks are obtained—that is, until transaction is in its locked point

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Concurrency Control
with Locking Methods

  • Lock
  • Guarantees exclusive use of a data item to a current transaction
  • Required to prevent another transaction from reading inconsistent data
  • Lock manager
  • Responsible for assigning and policing the locks used by transactions

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Lock Granularity

  • Indicates level of lock use
  • Locking can take place at following levels:
  • Database
  • Table
  • Page
  • Row
  • Field (attribute)

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Lock Granularity (cont..)

  • Database-level lock
  • Entire database is locked
  • Table-level lock
  • Entire table is locked
  • Page-level lock
  • Entire diskpage is locked
  • Row-level lock
  • Allows concurrent transactions to access different rows of same table
  • Even if rows are located on same page
  • Field-level lock
  • Allows concurrent transactions to access same row as long as they
  • Require the use of different fields (attributes) within the row

Fig 12.3 in your book

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Fig 12.3 in your book

Fig 12.4 in your book

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Fig 12.4 in your book

Fig. 12.5 in your book

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Lock Granularity (cont..)

Fig. 12.5 in your book

Fig. 12.6 in your book

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Lock Granularity (cont..)

Fig. 12.6 in your book

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Lock Types

  • Binary lock
  • Two states: locked (1) or unlocked (0)
  • Exclusive lock
  • Access is specifically reserved for transaction that locked object
  • Must be used when potential for conflict exists
  • Shared lock
  • Concurrent transactions are granted read access on basis of a common lock

Table 12.10 in your book

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Table 12.10 in your book

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Two-Phase Locking to Ensure Serializability

  • Defines how transactions acquire and relinquish locks
  • Guarantees serializability, but does not prevent deadlocks
  • Growing phase
  • Transaction acquires all required locks without unlocking any data
  • Shrinking phase
  • Transaction releases all locks and cannot obtain any new lock

Deadlocks (cont..)

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Deadlocks

  • Condition that occurs when two transactions wait for each other to unlock data
  • Possible only if one of the transactions wants to obtain an exclusive lock on a data item
  • No deadlock condition can exist among shared locks

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Table 12.11 in your book

Deadlocks (cont..)

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Deadlocks (cont..)

  • Three techniques to control deadlock:
  • Prevention
  • Detection
  • Avoidance
  • Choice of deadlock control method depends on database environment
  • Low probability of deadlock, detection recommended
  • High probability, prevention recommended

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Concurrency Control
with Time Stamping Methods

  • Assigns global unique time stamp to each transaction
  • Produces explicit order in which transactions are submitted to DBMS
  • Uniqueness
  • Ensures that no equal time stamp values can exist
  • Monotonicity
  • Ensures that time stamp values always increase

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Wait/Die and Wound/Wait Schemes

  • Wait/die
  • Older transaction waits and younger is rolled back and rescheduled
  • Wound/wait
  • Older transaction rolls back younger transaction and reschedules it

Wait/Die and Wound/Wait Schemes (cont..)

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Concurrency Control
with Optimistic Methods

  • Optimistic approach
  • Based on assumption that majority of database operations do not conflict
  • Does not require locking or time stamping techniques
  • Transaction is executed without restrictions until it is committed
  • Phases: read, validation, and write