Transaction Management and Concurrency Control

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Chapter 9

• Transaction Management and Concurrency Control
• Database Systems Design, Implementation, and
  Management, Sixth Edition, Rob and Coronel

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In this chapter, you will learn

• What a database transaction is and what its
  properties are
• How database transactions are managed
• What concurrency control is and what role it
  plays in maintaining the databases integrity
• What locking methods are and how they work
• How database recovery management is used to
  maintain database integrity

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What is a Transaction?

• Any action that reads from and/or writes to a
  database may consist of
• Simple SELECT statement to generate a list of
  table contents
• A series of related UPDATE statements to change
  the values of attributes in various tables
• A series of INSERT statements to add rows to one
  or more tables
• A combination of SELECT, UPDATE, and INSERT
  statements

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What is a Transaction? (continued)

• A logical unit of work that must be either
  entirely completed or aborted
• Successful transaction changes the 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
• The equivalent of a single SQL statement in an
  application program or transaction

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The Relational Schema for the Ch09_SaleCo Database
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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 a
  devastating effect on database integrity
• Some DBMSs provide means by which user can define
  enforceable constraints based on business rules
• Other integrity rules are enforced automatically
  by the DBMS when table structures are properly
  defined, thereby letting the DBMS validate some
  transactions

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Tracing the Transaction in the Ch09_SaleCo
Database
Figure 9.2
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Transaction Properties

• Atomicity
• Requires that all operations (SQL requests) of a
  transaction be completed
• Durability
• Indicates permanence of databases consistent
  state

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Transaction Properties (continued)

• Serializability
• Ensures that the concurrent execution of several
  transactions yields consistent results
• Isolation
• Data used during execution of a transaction
  cannot be used by second transaction until first
  one is completed

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Transaction Management with SQL

• ANSI has defined standards that govern SQL
  database transactions
• Transaction support is provided by two SQL
  statements COMMIT and ROLLBACK
• ANSI standards require that, when a transaction
  sequence is initiated by a user or an application
  program,
• it must continue through all succeeding SQL
  statements until one of four events occurs

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The Transaction Log

• Stores
• A record for the beginning of transaction
• For each transaction component (SQL statement)
• Type of operation being performed (update,
  delete, insert)
• Names of objects affected by the transaction (the
  name of the table)
• Before and after values for updated fields
• Pointers to previous and next transaction log
  entries for the same transaction
• The ending (COMMIT) of the transaction

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A Transaction Log
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Concurrency Control

• Coordination of simultaneous transaction
  execution in a multiprocessing database system
• Objective is to ensure transaction
  serializability in a multiuser database
  environment

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Concurrency Control

• Important ? 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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Normal Execution of Two Transactions
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Lost Updates

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Correct Execution of Two Transactions
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An Uncommitted Data Problem
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Retrieval During Update
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Transaction Results Data Entry Correction
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Inconsistent Retrievals
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The Scheduler

• Special DBMS program establishes order of
  operations within which concurrent transactions
  are executed
• Interleaves the execution of database operations
  to ensure serializability and isolation of
  transactions

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The Scheduler (continued)

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

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Read/Write Conflict Scenarios Conflicting
Database Operations Matrix
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Concurrency Controlwith 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 the transactions

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Lock Granularity

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

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Lock Granularity (continued)

• Database-level lock
• Entire database is locked
• Table-level lock
• Entire table is locked
• Page-level lock
• Entire diskpage is locked

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Lock Granularity (continued)

• Row-level lock
• Allows concurrent transactions to access
  different rows of the same table, even if the
  rows are located on the same page
• Field-level lock
• Allows concurrent transactions to access the same
  row, as long as they require the use of different
  fields (attributes) within that row

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A Database-Level Locking Sequence
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An Example of a Table-Level Lock
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Example of a Page-Level Lock
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An Example of a Row-Level Lock
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Lock Types

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

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An Example of a Binary Lock
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Two-Phase Lockingto Ensure Serializability

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

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Two-Phase Lockingto Ensure Serializability
(continued)

• 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
  obtainedthat is, until the transaction is in its
  locked point

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Two-Phase Locking Protocol
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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
• Control through
• Prevention
• Detection
• Avoidance

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How a Deadlock Condition Is Created
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Concurrency Control with Time Stamping Methods

• Assigns a global unique time stamp to each
  transaction
• Produces an explicit order in which transactions
  are submitted to the 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 the younger is rolled
  back and rescheduled
• Wound/wait
• Older transaction rolls back the younger
  transaction and reschedules it

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Wait/Die and Wound/WaitConcurrency Control
Schemes
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Concurrency Controlwith Optimistic Methods

• Optimistic approach
• Based on the assumption that the 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 are read, validation, and write

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Database Recovery Management

• Database recovery
• Restores database from a given state, usually
  inconsistent, to a previously consistent state
• Based on the atomic transaction property
• All portions of the transaction must be treated
  as a single logical unit of work, in which all
  operations must be applied and completed to
  produce a consistent database
• If transaction operation cannot be completed,
  transaction must be aborted, and any changes to
  the database must be rolled back (undone)

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Transaction Recovery

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

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Transaction Recovery (continued)

• Write-through
• Database is immediately updated by transaction
  operations during the transactions execution,
  even before the transaction reaches its commit
  point

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A Transaction Log for Transaction Recovery
Examples
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Summary

• Transaction
• Sequence of database operations that access the
  database
• Represents real-world events
• Must be a logical unit of work
• No portion of the transaction can exist by itself
• Takes a database from one consistent state to
  another
• One in which all data integrity constraints are
  satisfied

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Summary (continued)

• SQL provides support for transactions through the
  use of two statements COMMIT and ROLLBACK
• Concurrency control coordinates the simultaneous
  execution of transactions
• Scheduler is responsible for establishing order
  in which concurrent transaction operations are
  executed

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Summary (continued)

• Lock guarantees unique access to a data item by a
  transaction
• Database recovery restores the database from a
  given state to a previous consistent state