2.1 - Data Con Currency

8/4/2019 2.1 - Data Con Currency

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2010 IBM Corporation

Information Management

Information Management Ecosystem PartnershipsIBM Canada Lab

Summer/Fall 2010

Data Concurrency


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2 2010 IBM Corporation


Agenda

Database Transactions

Concurrency

Concurrency Issues

Concurrency Control

Isolation LevelsLockingSpecifying Isolation Levels


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Transaction

sequence of one or more SQL operations, grouped togetheras a single unit that occurs in one atomic action

also known as a unit of work

After a transaction to the database is made it can be madepermanent (committed) or backed out (rolled back)

manual-commit: use COMMIT or ROLLBACK statementsauto-commit: database manager performs a commit

operation after every SQL statement

Initiation and termination of a transaction defines the point ofdata consistency of data with the database

committed data versus uncommitted data


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Committed Data

data consistent with the databasechanges can always become committed data manually using

a COMMIT statementcommitted data can only be reversed/removed with new

SQL statements (within a new transaction)

accessible to all users and applications

Uncommitted Datadata inconsistent with the database

changes that occur during the transaction before a COMMITstatement is executed

changes can be reversed with ROLLBACKinaccessible to other users and applications unless

Uncommitted Read is used


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CONNECT TO DB employees

INSERT INTO employee VALUES (100, 'JOHN')

INSERT INTO employee VALUES (200, 'MANDY')

COMMIT

DELETE FROM employee WHERE name='MANDY'

UPDATE employee SET empID=101 where name='JOHN'

ROLLBACK

UPDATE employee SET name='JACK' where empID=100

COMMIT

ROLLBACK

empID name

100 JOHN

200 MANDY

empID name

100 JOHN

200 MANDY

empID name

100 JACK

200 MANDY

Connection to adatabase defines

first initiation

No changesapplied due to

ROLLBACK


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Concurrency

Concurrency

Sharing of resources by multiple interactive users orapplication programs at thesame time

Having multiple interactive users can lead to:Lost Update

Uncommitted ReadNon-repeatable ReadPhantom Read

Need to be able to control the degree of concurrency:With proper amount ofdata stability

Without loss ofperformance


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Concurrency Issues

Lost Update

Occurs when two transactions read and then attempt toupdate the same data, the second update will overwrite thefirst update before it is committed

1) Two applications, A and B, both read the same row andcalculate new values for one of the columns based on thedata that these applications read

2) A updates the row

3) Then B also updates the row

4) A's update lost




Concurrency Issues

Uncommitted Read

Occurs when uncommitted data is read during a transactionAlso known as a Dirty Read

1) Application A updates a value2) Application B reads that value before it is committed

3) A backs out of that update3) A backs out of that update

4) Calculations performed by B are based on theuncommitted data




Concurrency Issues

Non-repeatable Read

Occurs when a transaction reads the same row of data twiceand returns different data values with each read

1) Application A reads a row before processing otherrequests

2) Application B modifies or deletes the row and commitsthe change

3) A attempts to read the original row again

4) A sees the modified row or discovers that the originalrow has been deleted




Concurrency Issues

Phantom Read

Occurs when a search based on some criterion returnsadditional rows afterconsecutive searches during atransaction

1) Application A executes a query that reads a set ofrows based on some search criterion

2) Application B inserts new data that would satisfyapplication A's query

3) Application A executes its query again, within thesame unit of work, and some additional phantom valuesare returned


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Concurrency Control

Isolation Levels

determine how data is locked or isolated from otherconcurrently executing processes while the data is beingaccessed

are in effect while the transaction is in progress

There are four levels of isolation in DB2:Repeatable readRead stabilityCursor stability

Currently Committed

Uncommitted read


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Locking

Isolation levels are enforced by locks

locks limit or prevent data access by concurrent users orapplication processes

Locking Attributesresource being locked is called object

objects which can be explicitly locked are databases, tablesand table spacesobjects which can be implicitly locked are rows, index keys,

and tablesimplicit locks are acquired by DB2 according to isolation

level and processing situationsobject being locked represents granularity of locklength of time a lock is held is called duration and is affected

by isolation level


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Types of Locks

Share (S)

concurrent transactions are limited to read-only operations

Update (U)concurrent transactions are limited to read-only operationsif the transactions have not declared that they might update

a row, the database managerassumes that transactioncurrently looking at a row might update it

Exclusive (X)concurrent transactions are prevented from accessing the

data in any way

does not apply to transactions with an isolation level ofUR

Database manager places exclusive locks on every row thatis inserted, updated, or deleted


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Deadlock

Deadlock

Occurs when 2 (or more) competing operations are waitingfor each other to free some resource, but neither does, thusthe operations will never finish. Eg:

Deadlock Detectordiscovers deadlock cycles

randomly selects one of the transactions involved to rollback and terminate

transaction chosen is then sent an SQL error code, and

every lock it had acquired is released

pp1 modifies row 1 on Table A it holds an X lock on itpp2 modifies row 5 on Table B It holds an X lock on itpp2 tries to modify row 1 on Table A but it can't since App1 has the lock. It goes into WAIpp1 tries to modify row 5 on Table B but it can't since App2 has the lock. It goes into WAIEADLOCK as both operations can not complete


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Repeatable Read

Highest level of isolation

No dirty reads, non-repeatable reads or phantom reads

Locks the entire table or view being scanned for a queryProvides minimum concurrency

When to use Repeatable Read:

Changes to the result set are unacceptableData stability is more important than performance


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Read Stability

Similar to Repeatable Read but not as strictNo dirty reads or non-repeatable readsPhantom reads can occur

Locks only the retrieved or modified rows in a table or view

When to use Read Stability:

Application needs to operate in a concurrent environmentQualifying rows must remain stable for the duration of the

unit of workOnly issue unique queries during a unit of work

If the same query is issued more than once during a unit of

work, the same result set should not be required


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Cursor Stability

Default isolation level

No dirty readsNon-repeatable reads and phantom reads can occur

Locks only the row currently referenced by the cursor

When to use Cursor Stability:

Want maximum concurrency while seeing only committeddata


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Currently Committed

Currently Committed is a variation on Cursor Stability

Avoids timeouts and deadlocksLog based:

No management overhead

Situation Result

Reader blocks Reader No

Reader blocks Writer Maybe

Writer blocks Reader Yes

Writer blocks Writer Yes

Situation Result

Reader blocks Reader No

Reader blocks Writer No

Writer blocks Reader No

Writer blocks Writer Yes

CursorStability

CurrentlyCommitted


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Up to DB2 9.5

Cursor Stabilityis the default isolation level

Now in DB2 9.7Currently Committed is the default forNEW databasesCurrently Committed is disabled for upgraded databases,

i.e., Cursor Stability semantics are used Applications that depend on the old behavior (writers

blocking readers) will need to update their logic or disablethe Currently Committed semantics

Currently Committed


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Transaction A Transaction Bupdate T1 set col1 = ? where col2 =2

update T2 set col1 = ? where col2 = ?

select * from T2 where col2 >= ?

select * from T1 where col5 = ? and col2= ?

DEADLOCK!!

Waiting because is

reading uncommitted data

Waiting because is

reading uncommitted data

Example Cursor Stability Semantics


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No deadlocks, no timeouts in this scenario!

Example Currently Committed Semantics

Transaction A Transaction Bupdate T1 set col1 = ? where col2 =2

update T2 set col1 = ? where col2 = ?

select * from T2 where col2 >= ?

select * from T1 where col5 = ? and col2= ?

commit

commit

No lockingReads last committed version

of the data

No lockingReads last committed version

of the data


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Currently Committed How to use it?

cur_commit DB config parameter

ON: default for new DBs created in DB2 9.7 CC semanticsin place

DISABLED: default value for existing DBs old CS

semantics in place

PRECOMPILE/BINDCONCURRENTACCESSRESOLUTION: Specifies theconcurrent access resolution to use for statements in thepackage.

USE CURRENTLY COMMITTEDWAIT FOR OUTCOME


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Uncommitted Read

Lowest level of isolation

Dirty reads, non-repeatable reads and phantom reads canoccur

Locks only rows being modified in a transaction involvingDROP or ALTER TABLE

Provides maximum concurrency When to use Uncommitted Read:

Querying read-onlytablesUsing only SELECT statementsRetrieving uncommitted data is acceptable

Uncommitted Read with Read-Write tablesUR behaves like CS with updateable cursors


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Isolation Levels

Summary

Application Type High data stability required High data stability notrequired

Read-write transactions Read Stability (RS) Cursor Stability (CS)

Read-only transactions Repeatable Read (RR) orRead Stability (RS)

Uncommited Read (UR)

Isolation Level Dirty Read Non-repeatableRead

PhantomRead

Repeatable Read (RR) - - -

Read Stability (RS) - - Possible

Cursor Stability (CS) - Possible PossibleUncommitted read (UR) Possible Possible Possible

DEFAULT


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Specifying Isolation Levels

Precompile / Bind

ISOLATION option of PREP or BIND commandCan determine isolation level of a package by executing the

following query

Statement LevelUse the WITH {RR, RS, CS, UR} clauseThe WITH UR option applies only to read-only operations

ensure that a result table is read-only by specifying FORREAD ONLY in the SQL statementOverrides the isolation level specified for the package

SELECT * FROM tb1 WITH RR

SELECT ISOLATION FROM syscat.packages

WHERE pkgname = 'pkgname'

AND pkgschema = 'pkgschema'


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Specifying Isolation Levels

Dynamic SQL within the current session

SET CURRENT ISOLATIONFor all subsequent dynamic SQL statements within the same

session

JDBC or SQLJ at run timeSQLJ profile customizer (db2sqljcustomize command)

CLI or ODBC at run timeCHANGE ISOLATION LEVEL command specified during the

program preparation process

CHANGE ISOLATION LEVELTO RR


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2010 IBM Corporation


Information Management Ecosystem PartnershipsIBM Canada Lab

Summer/Fall 2010Questions?

E-mail: [email protected]: DB2 Academic Workshop

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2.1 - Data Con Currency