2. What is a Transaction?
• Logical unit of work
• Must be either entirely completed or aborted
• No intermediate states are acceptable
Figure 9.1
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3. Example Transaction
• Examine current account balance
SELECT ACC_NUM, ACC_BALANCE
FROM CHECKACC
WHERE ACC_NUM = ‘0908110638’;
• Consistent state after transaction
• No changes made to Database
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4. Example Transaction
• Register credit sale of 100 units of product X to
customer Y for $500
UPDATE PRODUCT
SET PROD_QOH = PROD_QOH - 100
WHERE PROD_CODE = ‘X’;
UPDATE ACCT_RECEIVABLE
SET ACCT_BALANCE = ACCT_BALANCE + 500
WHERE ACCT_NUM = ‘Y’;
• Consistent state only if both transactions are fully
completed
• DBMS doesn’t guarantee transaction represents
real-world event
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5. Transaction Properties (ACID)
• 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
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6. 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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7. 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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8. 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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9. 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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10. 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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11. 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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12. DBMS Transaction Subsystem
Transaction Scheduler/
Database Lock
Manager Manager
Manager
Buffer Recovery
Manager Manager
Access File
Manager Manager
Systems Database
Manager and system
catalog 12
13. 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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14. 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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15. Transaction Management with SQL
• 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
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16. Transaction Log
• 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
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23. The Scheduler
• 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
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25. 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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32. Binary Locks
• Two states
– Locked (1)
– Unlocked (0)
• Locked objects unavailable to other objects
– Unlocked objects open to any transaction
– Transaction unlocks object when complete
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34. 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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35. 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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36. 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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38. 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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40. 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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41. 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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42. Database Recovery Management
• Restores a database to previously consistent
state
• Based on the atomic transaction property
• Level of backup
– Full backup
– Differential
– Transaction log
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44. Transaction Recovery
• 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
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