Download 1117 Understanding InterBase Transactions

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
concurrency
Snapshot transaction isolation
read_committed
Read committed transaction
isolation
consistency
Consistency (table stability)
isolation
IBTransaction Params
Keywords
Access Mode
read
Read only access
write
Read/write access
IBTransaction Params
Keywords
Lock Resolution
wait
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 Level
shared
lock_write=TableName
shared
lock_read=TableName
Snapshot & read
committed can update.
All read
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 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 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 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?
Thank You
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You can contact me further at …
[email protected]