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1096Understanding InterBase
Transactions
Bill Todd
The Database Group, Inc.
What is a Transaction
Atomicity
Consistency
Isolation
Durability
Atomicity
All changes to all tables made within the transaction must succeed or fail as a single atomic unit
ConsistencyDatabase will always be left in a logically
consistent state
On restart after a crash all active transactions are automatically rolled back
The database can never contain changes made by a transaction that did not commit
Isolation
Your transaction cannot see changes made by other transactions that have not committed
Durability
After a transaction commits the changes are a permanent.
The changes cannot be lost or undone.
Transaction Isolation Level
Your transaction’s isolation level controls when it will see changes made by other transactions
SQL Standard Isolation Levels
Read uncommitted (dirty read)
Read committed
Repeatable read
Serializable
Read Uncommitted
Your transaction can see changes made by other transactions that have not committed
Most databases do not support read uncommitted
Read Committed
Your transaction can see changes made by other committed transactions
Repeatable ReadIf you execute the same SELECT more than once
within your transaction you will get the same value for each row returned the first time in each subsequent result set
You will also get any new records that were inserted after the prior execution of the SELECT
Serializable
If you execute the same SELECT more than once within your transaction you will get the same result set each time
InterBase Isolation Levels
Snapshot = ANSI serializable
Read committed
How Versioning WorksAll data access takes place within a transaction
Each transaction is assigned a unique number
Each row version has the number of the transaction that created it
Transaction Inventory Pages
TIP tracks the state of all interesting transactions
Transaction can have four states– Active– Committed– Rolled back– Limbo
Starting a Snapshot Transaction
Transaction gets a copy of the TIP
TIP copy used to determine the state of other transactions at the moment the snapshot transaction started
Starting a Read Committed Transaction
Gets pointer to the TIP cache (TPC)
TPC pointer used to determine the current state of other transactions
Updating a RowCheck the TIP to see if there are active
transactions with a lower transaction number
If yes, create a new row version
If no, update the existing row
Reading a Row - Snapshot
Most recent version
Committed at time the reading snapshot transaction started
When a snapshot transaction reads it ignores all row versions committed after it started
Tran 90 Reads Row 123
Transaction Options
Access Mode
Lock resolution
Table reservation
Access ModeTransactions can be read only or read/write
Default is read/write
Read only transactions have lower overhead
Read only read committed transactions do not stop garbage collection
Setting Access Mode With IBX
For a read only transaction add the word “read” without the quotes to the IBTransaction Params property
For a read/write transaction add “write” to the Params property
Lock ResolutionWait
– A transaction with the wait option that tries to update a row your transaction has locked will wait until your transaction ends
NoWait– The transaction will raise an exception when
it tries to update a locked row
Table Reservation
Allows your transaction to lock tables when it starts
This guarantees your transaction access to all of the tables it needs
Table ReservationsShared, lock_read
Any transaction can update. Any transaction can read.
Shared, lock_write
Snapshot or read committed can update. All can read
Protected, lock_read
No updates. Any transaction can read
Protected, lock_write
No updates. Snapshot & read committed can read
IBTransaction.Params
Snapshot transaction reserving EMPLOYEE for protected, lock_read
Table name is case sensitive
concurrency nowait protected lock_read=EMPLOYEE
IBTransaction.Params
A read committed transaction that reserves EMPLOYEE for shared, lock_read
read_committed nowait shared lock_read=EMPLOYEE
Reserving Multiple Tables
concurrency nowait protected lock_read=EMPLOYEE shared lock_read=SALARY_HISTORY
IBTransaction Params Keywords
Isolation Levelconcurrency Snapshot transaction isolation
read_committed Read committed transaction isolation
consistency Consistency (table stability) isolation
IBTransaction Params Keywords
Access Moderead Read only access
write Read/write access
IBTransaction Params Keywords
Lock Resolutionwait Wait if the record to be updated is locked by
another transaction
nowait Return an error if the record to be updated is locked by another transaction
Keywords Isolation Levelshared
lock_write=TableName
Snapshot & read committed can update. All read
shared
lock_read=TableName
Any can update.
Any can read.
protected
lock_write=TableName
No update. Snapshot & read committed read
protected
lock_read=TableName
No update. Any can read
Ending a TransactionCommit – changes the transaction’s state on the
TIP from active to committed
Rollback – can degrade perfomance– If < 100,000 changes IB undoes changes and
commits– If 100,000 changes or more IB changes state
on the TIP from active to rolled back
Next Transaction
The number that will be assigned to the next transaction that starts
OITOldest Interesting Transaction
The oldest transaction whose state is not committed– Oldest Active Transaction– Oldest rolled back transaction– Oldest limbo transaction
Normally OIT = OAT and they advance when the OAT commits
What Makes the OIT Stick
Rollback with > 100,000 changes
Rollback of transactions that were active when the server crashed
Transaction stuck in limbo
OST
Oldest Snapshot Transaction
The lowest number that appears in the Oldest_Snapshot of any active transaction
How Oldest_Snapshot Field Is Set
Read only read committed – Oldest_Snapshot is not assigned
Read/write read committed – Oldest_Snapshot = Transaction #
Snapshot – Oldest_Snapshot = the oldest active read/write transaction
When the OST MovesWhen a new transaction starts
When a commit retaining occurs
When a sweep is run
Note: commit retaining on a snapshot – Commits existing transaction– Starts new transaction whose
Oldest_Snapshot is same as the original transaction
Garbage CollectionRemoves row versions whose transaction # is less
than the OIT
Occurs automatically when a row is accessed
Occurs for all rows when a sweep is run
Sweep IntervalDefault = OAT – OIT > 20,000
Automatic sweep will only happen if the OIT gets stuck
If the OIT is stuck due to a rollback a sweep will unstick it
You can change the sweep interval using IBConsole or gfix
Sweep Interval in IBConsole
Sweep Interval Using gfixgfix -h 10000 -user sysdba -password masterkey employee.gdb
Fixing Limbo Transactions
If the OIT is stuck because a transaction is in limbo you must fix it before sweeping
You can fix limbo transactions automatically with gfix
gfix -two_phase -user sysdba -password masterkey employee.gdb
Possible Problems
OIT gets stuck
OAT gets stuck
OIT Gets StuckIf OIT is stuck garbage collection stops
Number of row versions increases
Performance suffers– Retrieving a row with many versions takes
longer– The TIP gets larger– Database size increases
Stuck OIT Rare In InterBase 7.x
Rollback a transaction with over 100,000 changes
Rollback on restart after a server crash
Crash during two_phase commit leaves a transaction in limbo
OAT Gets StuckOAT gets stuck if a transaction is left active
If OAT is stuck OIT is also stuck
Not all active transactions stick the OAT in InterBase 7.1 SP1 and later
What Sticks the OAT?Read only read committed transactions can
remain active indefinitely without sticking the OAT
Read/write read committed transactions can remain active indefinitely if you call Commit Retaining after updating the database
Snapshot transactions always stick the OAT
Savepoints
Savepoint is a named point in a transaction that you can rollback to without rolling back the transaction
Creating a Savepoint
SAVEPOINT MY_SAVEPOINT
Creates a savepoint named MY_SAVEPOINT
Releasing a Savepoint
RELEASE SAVEPOINT MY_SAVEPOINT
Releases the named savepoint and frees Its resources
Rollback to a Savepoint
ROLLBACK TO SAVEPOINT MY_SAVEPOINT
Rolls back to MY_SAVEPOINT and continues the transaction
Transactions with isql
Easy way to test transaction behavior
Supports all transaction options
Can open multiple sessions to simulate multiple users
Use performance monitor to watch transactions
See chapter 10 of the Operations Guide for information on isql
isql Command Line Options
Starting isql
isql -u sysdba -p masterkey employee.gdb
Transactions in isqlIsql starts a transaction when it connects to a
database
Use COMMIT to end current transaction
Use SET TRANSACTION to start a transaction
SET TRANSACTION options
READ WRITE or READ ONLY Transaction access mode
WAIT or NOWAIT Lock resolution mode
RESERVING <tablename> FOR [SHARED or PROTECTED] [READ or WRITE]
Table reservations
SET TRANSACTION Isolation Level
ISOLATION LEVEL
SNAPSHOT [TABLE STABILITY] or
READ COMMITTED RECORD_VERSION or
READ COMMITTED NO RECORD_VERSION
Example
SET TRANSACTION READ ONLY NOWAIT ISOLATION LEVEL READ COMMITTED;
Monitoring Transactions
A Transaction’s Attachment
Transaction Summary Info
Questions?
