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Data ConcurrencyData Concurrency

David Konopnicki 1997David Konopnicki 1997

Revised by Mordo Shalom 2004Revised by Mordo Shalom 2004

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AgendaAgenda

Data concurrency, integrity and consistencyData concurrency, integrity and consistency The automatic locking mechanism in The automatic locking mechanism in

ORACLEORACLE Explicit (manual) lockingExplicit (manual) locking ORACLE lock management servicesORACLE lock management services

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What are concurrency and consistencyWhat are concurrency and consistency

In single user database, a user can modify In single user database, a user can modify data without concern of other users data without concern of other users modifying or accessing the same data.modifying or accessing the same data.

In multi-user database, several users In multi-user database, several users accessing the same data items is a problem.accessing the same data items is a problem.

Transaction: a set of user actions (e.g. SQL Transaction: a set of user actions (e.g. SQL queries) that must be executed together in queries) that must be executed together in some level of isolation. some level of isolation.

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Transactions in ORACLETransactions in ORACLE

Transactions are started automatically.Transactions are started automatically. A transaction is ended by:A transaction is ended by:

Commit (explicitly or by disconnect)Commit (explicitly or by disconnect) Rollback Rollback (explicity or by abort) (explicity or by abort) A DDL statement which begins a new transactionA DDL statement which begins a new transaction

A new transaction is started automatically with the A new transaction is started automatically with the next statement. next statement.

Autonomous transactions: Autonomous transactions: Transactions nested in timeTransactions nested in time The inner transaction may commit while the outer abortsThe inner transaction may commit while the outer aborts

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Error HandlingError Handling

Statement level rollback: Effects of an Statement level rollback: Effects of an erroneous statement are allways rolled back.erroneous statement are allways rolled back.

Resumable Errors (Out of Space, etc.), Resumable Errors (Out of Space, etc.), Resumable Statements .Resumable Statements .

Errors returned to the program may be Errors returned to the program may be handled by it, then the transaction continues, handled by it, then the transaction continues, otherwise it abortsotherwise it aborts

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Commit (Regular Transactions)Commit (Regular Transactions) Before Commit, changes were made to:Before Commit, changes were made to:

The database buffers The database buffers Rollback segmentsRollback segments Redo Log buffersRedo Log buffers

During Commit:During Commit: The SCN number of the transaction is written to the The SCN number of the transaction is written to the

rollback segments.rollback segments. Redo log buffers are writen to disc.Redo log buffers are writen to disc. The SCN is written to the redo log. (COMMIT)The SCN is written to the redo log. (COMMIT) Locks are releasedLocks are released

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Commit (Discrete Transactions)Commit (Discrete Transactions) Short, Nondistributed transactionsShort, Nondistributed transactions Declared as such by Declared as such by

BEGIN_DISCRETE_TRANSACTIONBEGIN_DISCRETE_TRANSACTION Before Commit, changes were made to:Before Commit, changes were made to:

Special Redo Log buffersSpecial Redo Log buffers During Commit:During Commit:

Redo log buffers are writen to disc.Redo log buffers are writen to disc. The SCN is written to the redo log. (COMMIT)The SCN is written to the redo log. (COMMIT) Database buffers are updatedDatabase buffers are updated Locks are releasedLocks are released

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Types of RollbackTypes of Rollback Statement levelStatement level Rollback to savepointRollback to savepoint Rollback on user requestRollback on user request Rollback because abnormal process Rollback because abnormal process

terminationtermination Multiple Rollbacks because abnormal instance Multiple Rollbacks because abnormal instance

terminationtermination Rollback of incomplete transactions during Rollback of incomplete transactions during

recovery.recovery.

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RollbackRollback Before Rollback, changes were made to:Before Rollback, changes were made to:

The database buffers The database buffers Rollback segmentsRollback segments Redo Log buffersRedo Log buffers

During Rollback:During Rollback: The rollback segments are applied to the database The rollback segments are applied to the database

buffers.buffers. Locks are releasedLocks are released

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2PC – Two Phase Commit2PC – Two Phase Commit In distributed transactions.In distributed transactions. Transparent to user.Transparent to user. First phase: All the servers are put in pre-First phase: All the servers are put in pre-

commit status (some of them may abort).commit status (some of them may abort). Second phase: If all the servers are in pre-Second phase: If all the servers are in pre-

commit, all of them commit otherwise all of commit, all of them commit otherwise all of them roll back.them roll back.

RECO process resumes in-doubt 2PC’s RECO process resumes in-doubt 2PC’s (network failures etc..)(network failures etc..)

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DefinitionsDefinitions

Data concurrency: Coordination access to Data concurrency: Coordination access to data by several users.data by several users.

Data consistency: A user sees a consistent Data consistency: A user sees a consistent view of the data i.e. all data committed by view of the data i.e. all data committed by other transactions as of that time and all the other transactions as of that time and all the changes made by the user up to that time.changes made by the user up to that time.

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General concurrency issuesGeneral concurrency issues

Some of the problems:Some of the problems: Inconsistent reads (in one query)Inconsistent reads (in one query) Non-repeatable reads (phantoms)Non-repeatable reads (phantoms) Dirty readsDirty reads Lost updatesLost updates Destructive DDL operationsDestructive DDL operations

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General locking conceptsGeneral locking concepts

Locks Locks are used to prevent destructive are used to prevent destructive interactions between users accessing the interactions between users accessing the same resources.same resources.

Resources Resources are:are: User objects (tables and rows)User objects (tables and rows) System objectsSystem objects

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Restrictiveness of locksRestrictiveness of locks

Exclusive locks: prohibit the sharing of a Exclusive locks: prohibit the sharing of a resourceresource

Share locks: allow sharingShare locks: allow sharing

Share locks allow a higher degree of data Share locks allow a higher degree of data concurrency.concurrency.

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DeadlocksDeadlocks

Transaction 1 Time Transaction 2Update EMPset sal = sal * 1.1whereempno=1000;

1 Update EMPset sal = sal * 1.1whereempno=2000;

Update EMPset sal=sal*1.1whereempno=2000;

2 Update EMPset sal = sal * 1.1whereempno=1000;

DEADLOCK 3 ERROR

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Solving DeadlocksSolving Deadlocks

Local deadlocks are resolved using a wait-Local deadlocks are resolved using a wait-for graph.for graph.

In distributed ORACLE, timeout is used.In distributed ORACLE, timeout is used. Deadlock is solved by rolling-back a Deadlock is solved by rolling-back a

statement in one of the transactions statement in one of the transactions involved in the deadlock.involved in the deadlock.

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Lock EscalationLock Escalation

Locks on rows -> locks on table.Locks on rows -> locks on table. Causes a lot of deadlocksCauses a lot of deadlocks Therefore: ORACLE never escalates locks.Therefore: ORACLE never escalates locks.

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Lock conversionLock conversion

Re-request on an already locked item.Re-request on an already locked item. For a less restrictive lock: No problem.For a less restrictive lock: No problem. For a more restrictive lock: May wait.For a more restrictive lock: May wait.

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Transactions and Data ConcurrencyTransactions and Data Concurrency

All locks acquired by statements within a All locks acquired by statements within a transaction are hold for the duration of the transaction are hold for the duration of the transaction.transaction.

That changes made by statements of a That changes made by statements of a transaction only become visible to other transaction only become visible to other transactions after the first one is committed.transactions after the first one is committed.

The locks acquired by a transaction are The locks acquired by a transaction are released when it is committed or rolled-back.released when it is committed or rolled-back.

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Multi-version consistency modelMulti-version consistency model

ORACLE provides two levels of read-ORACLE provides two levels of read-consistency:consistency: Statement level read consistency that is always Statement level read consistency that is always

enforced.enforced. Transaction level read consistency.Transaction level read consistency.

read-only transaction.read-only transaction. set manually using exclusive locks.set manually using exclusive locks.

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Rollback Segment (for read consistency)Rollback Segment (for read consistency)

Current SCNCurrent SCN

10231023

10241024

10231023

10241024

10231023

10241024

10231023

10231023

10231023

RollbackRollbackSegmentsSegments

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ISO-SQL92 Isolation LevelsISO-SQL92 Isolation Levels

Isolation Level

Dirty Read

Nonrepeatable Read

Phantom Read

Read Uncommitted

Y Y Y

Read Committed

N Y Y

Repeatable Read

N N Y

Serializable N N N

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Oracle 9i Isolation LevelsOracle 9i Isolation Levels

Isolation Level Nonrepeatable and Phantom Reads

Read Committed Y

Serializable/ Read Only

N

Serializable Transactions may getSerializable Transactions may get::

Can not serialize access … errorCan not serialize access … error..

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How ORACLE locks dataHow ORACLE locks data

Readers do not wait for readers.Readers do not wait for readers. Writers do not wait for readers.Writers do not wait for readers. Writers only waits for writers if they Writers only waits for writers if they

attempt to update the same rows at the same attempt to update the same rows at the same time.time.

ORACLE automatically locks data at the ORACLE automatically locks data at the lowest level of restrictiveness.lowest level of restrictiveness.

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Two types of locksTwo types of locks

DML: protects data in tables.DML: protects data in tables. Row locks.Row locks. Table locks.Table locks.

DDL: protects the schema.DDL: protects the schema.

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Row locksRow locks

Inserted when a row is modified by Inserted when a row is modified by INSERT, UPDATE, DELETE and INSERT, UPDATE, DELETE and SELECT ... FOR UPDATE.SELECT ... FOR UPDATE.

Acquiring a row lock necessitates the Acquiring a row lock necessitates the acquisition of a table lock (DDL).acquisition of a table lock (DDL).

A row lock is alwaysA row lock is always exclusive ( exclusive (remember, remember, only writers wait for each other). only writers wait for each other).

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Table locks - DMLTable locks - DML

DML statement Row locks? Mode of table lock

Select ... From ... No None

Insert ... Into ... Yes RX

Update Table ... Yes RX

Delete From Table ... Yes RX

Select From Table ... For Update of ...

Yes RS

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Locks CompatibilityLocks Compatibility

RS RX S SRX X

RS Y Y Y Y N

RX Y Y N N N

S Y N Y N N

SRX Y N N N N

X N N N N N