Atomicity indicates the permanence of the database's consistent state

Database Fundamentals- Content Analysis

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