1096 Understanding 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

Consistency Database 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 Read If 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 Works All 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 Row Check 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 Mode Transactions 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 Resolution Wait –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 Reservations Shared, 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 Level concurrencySnapshot transaction isolation read_committedRead committed transaction isolation consistencyConsistency (table stability) isolation

IBTransaction Params Keywords Access Mode readRead only access writeRead/write access

IBTransaction Params Keywords Lock Resolution waitWait if the record to be updated is locked by another transaction nowaitReturn an error if the record to be updated is locked by another transaction

Keywords Isolation Level shared 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 Transaction Commit – 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

OIT Oldest 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 Moves When 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 Collection Removes 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 Interval Default = 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 gfix gfix -h 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 Stuck If 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 Stuck OAT 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 isql Isql 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 ONLYTransaction access mode WAIT or NOWAITLock resolution mode RESERVING 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

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