Transaction Management and Concurrency Control
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• Logical unit of work • Must be either entirely completed or aborted• No intermediate states are acceptable
What is a Transaction?
Figure 9.1
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• Examine current account balance
• Consistent state after transaction• No changes made to Database
Example Transaction
SELECT ACC_NUM, ACC_BALANCEFROM CHECKACCWHERE ACC_NUM = ‘0908110638’;
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• Register credit sale of 100 units of product X to customer Y for $500
• Consistent state only if both transactions are fully completed
• DBMS doesn’t guarantee transaction represents real-world event
Example Transaction
UPDATE PRODUCTSET PROD_QOH = PROD_QOH - 100WHERE PROD_CODE = ‘X’;UPDATE ACCT_RECEIVABLESET ACCT_BALANCE = ACCT_BALANCE + 500WHERE ACCT_NUM = ‘Y’;
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• Atomicity – All transaction operations must be completed– Incomplete transactions aborted
• Durability – Permanence of consistent database state
• Consistency / Serializability – Conducts transactions in serial order– Important in multi-user and distributed databases
• Isolation – Transaction data cannot be reused until its execution
complete
Transaction Properties (ACID)
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Example of Fund Transfer
• Transaction to transfer $50 from account A to account B:1. read(A)
2. A := A – 50
3. write(A)
4. read(B)
5. B := B + 50
6. write(B)
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Example of Fund Transfer
• Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.
• Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.
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Example of Fund Transfer
• Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures.
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Example of Fund Transfer
• Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database
(the sum A + B will be less than it should be).Can be ensured by running transactions serially, that is one after the other.
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Transaction state
• Active– the initial state; the transaction stays in this state
while it is executing• Partially committed
– after the final statement has been executed.• Failed
– after the discovery that normal execution can no longer proceed.
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Transaction state
• Aborted – after the transaction has been rolled back and the
database restored to its state prior to the start of the transaction. Two options after it has been aborted:
• restart the transaction – only if no internal logical error
• kill the transaction
• Committed, after successful completion.
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DBMS Transaction Subsystem
Transaction Manager
Scheduler/ Lock
Manager
Recovery Manager
Buffer Manager
File Manager
Access Manager
Systems Manager
Database and system
catalog
Database Manager
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DBMS Transaction Subsystem
• Trans. Mgr. – coordinates transactions on behalf of application
program. It communicates with scheduler.
• Scheduler – implements a strategy for concurrency control.
• Recovery manager – If any failure occurs, recovery manager handles it.
• Buffer manager – in charge of transferring data between disk storage and
main memory.
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DBMS Transaction Subsystem
• File manager – manipulates the underlying storage files and manages
the allocation of storage space on disk.
• Access manager – File manager does not directly manage the physical
input and output of data, rather it passes the requests on to the access manager.
• System manager – Appropriate access method is used to either read or
write data into the system manager.
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• Transaction support– COMMIT
– ROLLBACK
• User initiated transaction sequence must continue until: – COMMIT statement is reached
– ROLLBACK statement is reached
– End of a program reached
– Program reaches abnormal termination
Transaction Management with SQL
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• Tracks all transactions that update database• May be used by ROLLBACK command• May be used to recover from system failure• Log stores
– Record for beginning of transaction– Each SQL statement
• Operation• Names of objects• Before and after values for updated fields• Pointers to previous and next entries
– Commit Statement
Transaction Log
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Transaction Log Example
Table 9.1
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• Coordinates simultaneous transaction execution in multiprocessing database– Ensure serializability of transactions in multiuser
database environment
– Potential problems in multiuser environments• Lost updates• Uncommitted data• Inconsistent retrievals
Concurrency Control
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Lost Updates
Table 9.2
Table 9.3
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Uncommitted Data
Table 9.5
Table 9.4
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Inconsistent Retrievals
Table 9.6
Table 9.7
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Inconsistent Retrievals (con’t.)
Table 9.8
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• Establishes order of concurrent transaction execution
• Interleaves execution of database operations to ensure serializability
• Bases actions on concurrency control algorithms– Locking
– Time stamping
• Ensures efficient use of computer’s CPU
The Scheduler
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Read/Write Conflict Scenarios:Conflicting Database Operations Matrix
Table 9.9
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Concurrency Control with Locking Methods
• Lock guarantees current transaction exclusive use of data item
• Acquires lock prior to access• Lock released when transaction is
completed • DBMS automatically initiates and enforces
locking procedures• Managed by lock manager• Lock granularity indicates level of lock use
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Database-Level Locking Sequence
Figure 9.2
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Table-Level Lock Example
Figure 9.3
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Page-Level Lock Example
Figure 9.4
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Row-Level Lock Example
Figure 9.5
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Field Level Lock
• Access same row as long as they require different fields (attributes) within that row
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Lock Types
• Binary Locks• Shared/Exclusive Locks
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• Two states– Locked (1)
– Unlocked (0)
• Locked objects unavailable to other objects– Unlocked objects open to any transaction
– Transaction unlocks object when complete
Binary Locks
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Example of Binary Lock Table
Table 9.10
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Shared/Exclusive Locks
• Shared– Exists when concurrent transactions granted READ
access – Produces no conflict for read-only transactions– Issued when transaction wants to read and exclusive
lock not held on item
• Exclusive– Exists when access reserved for locking transaction– Used when potential for conflict exists– Issued when transaction wants to update unlocked
data
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Problems with Locking
• Transaction schedule may not be serializable– Managed through two-phase locking
• Schedule may create deadlocks– Managed by using deadlock detection and
prevention techniques
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Two-Phase Locking
• Growing phase• Shrinking phase• Governing 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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Two-Phase Locking Protocol
Figure 9.6
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Deadlocks
• Occurs when two transactions wait for each other to unlock data
• Called deadly embrace• Control techniques
– Deadlock prevention
– Deadlock detection
– Deadlock avoidance
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How Deadlock Conditions Created
Table 9.11
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Concurrency Control with Time Stamping Methods
• Assigns global unique time stamp to each transaction• Produces order for transaction submission• Properties
– Uniqueness– Monotonicity
• DBMS executes conflicting operations in time stamp order
• Each value requires two additional time stamps fields– Last time field read– Last update
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Concurrency Control with Optimistic Methods
• Assumes most database operations do not conflict
• Transaction executed without restrictions until committed
• Phases:– Read Phase
– Validation Phase
– Write Phase
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• Restores a database to previously consistent state
• Based on the atomic transaction property• Level of backup
– Full backup
– Differential
– Transaction log
Database Recovery Management
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• Software• Hardware• Programming Exemption• Transaction• External
Causes of Database Failure
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• Deferred-write and Deferred-update– Changes are written to the transaction log
– Database updated after transaction reaches commit point
• Write-through– Immediately updated by during execution
– Before the transaction reaches its commit point
– Transaction log also updated
– Transaction fails, database uses log information
to ROLLBACK
Transaction Recovery
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