Week 6 - Discussion Forum HSIN
Heathersimf
Database Systems: Design, Implementation, and
Management Tenth Edition
Chapter 10 Transaction Management and Concurrency Control
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
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Objectives (cont’d.)
– 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?
• 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
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Evaluating Transaction Results
• Not all transactions update 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
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Figure 9.2
Transaction Properties
• 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’d.)
• 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 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
• Transaction log stores: – 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
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Concurrency Control
• Coordination of simultaneous transaction execution in a multiprocessing database
• Objective is to ensure serializability of transactions in a multiuser environment
• Three main 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
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Uncommitted Data
• Uncommitted data phenomenon: – Two transactions are executed concurrently – First transaction rolled back after second already
accessed uncommitted data
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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
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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
– Pessimistic locking • Use of locks based on the assumption that conflict
between transactions is likely
– Lock manager • Responsible for assigning and policing the locks
used by transactions Database Systems, 10th Edition 22
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’d.)
• 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 (cont’d.)
• 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 • Requires use of different fields (attributes) within
the row
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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
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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
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Two-Phase Locking to Ensure Serializability (cont’d.)
• 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
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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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Deadlocks (cont’d.)
• 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
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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
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Database Recovery Management
• Restores database to previous consistent state • Based on atomic transaction property
– All portions of transaction are treated as single logical unit of work
– All operations are applied and completed to produce consistent database
• If transaction operation cannot be completed: – Transaction aborted
– Changes to database are rolled back
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Transaction Recovery
• Write-ahead-log protocol: ensures transaction logs are written before data is updated
• Redundant transaction logs: ensure physical disk failure will not impair ability to recover
• Buffers: temporary storage areas in primary memory
• Checkpoints: operations in which DBMS writes all its updated buffers to disk
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Transaction Recovery (cont’d.)
• Deferred-write technique – Only transaction log is updated
• Recovery process: identify last checkpoint – If transaction committed before checkpoint:
• Do nothing
– If transaction committed after checkpoint: • Use transaction log to redo the transaction
– If transaction had ROLLBACK operation: • Do nothing
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Transaction Recovery (cont’d.)
• Write-through technique – Database is immediately updated by transaction
operations during transaction’s execution
• Recovery process: identify last checkpoint – If transaction 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
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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 Database Systems, 10th Edition 47
Summary (cont’d.)
• 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’d.)
• 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 Database Systems, 10th Edition 49
Summary (cont’d.)
• 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
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- Database Systems: Design, Implementation, and Management Tenth Edition
- Objectives
- Objectives (cont’d.)
- What Is a Transaction?
- PowerPoint Presentation
- Evaluating Transaction Results
- Slide 7
- Transaction Properties
- Transaction Properties (cont’d.)
- Transaction Management with SQL
- The Transaction Log
- Slide 12
- Concurrency Control
- Lost Updates
- Slide 15
- Uncommitted Data
- Slide 17
- Inconsistent Retrievals
- Slide 19
- Slide 20
- The Scheduler
- Concurrency Control with Locking Methods
- Lock Granularity
- Lock Granularity (cont’d.)
- Lock Granularity (cont’d.)
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Lock Types
- Slide 31
- Two-Phase Locking to Ensure Serializability
- Two-Phase Locking to Ensure Serializability (cont’d.)
- Slide 34
- Deadlocks
- Deadlocks (cont’d.)
- Slide 37
- Concurrency Control with Time Stamping Methods
- Wait/Die and Wound/Wait Schemes
- Slide 40
- Concurrency Control with Optimistic Methods
- Database Recovery Management
- Transaction Recovery
- Transaction Recovery (cont’d.)
- Slide 45
- Slide 46
- Summary
- Summary (cont’d.)
- Slide 49
- Slide 50