com.amazon.carbonado

Enum IsolationLevel

java.lang.Object

java.lang.Enum<IsolationLevel>

com.amazon.carbonado.IsolationLevel

All Implemented Interfaces:

java.io.Serializable, java.lang.Comparable<IsolationLevel>

public enum IsolationLevel
extends java.lang.Enum<IsolationLevel>

Describes a transaction isolation level. Transaction levels, in order from lowest to highest are:

• READ_UNCOMMITTED
• READ_COMMITTED
• REPEATABLE_READ
• SNAPSHOT
• SERIALIZABLE

A transaction's isolation level is usually READ_COMMITTED or REPEATABLE_READ by default. Forcing a lower level, like READ_COMMITTED, is useful when performing a long cursor iteration. It releases locks during iteration rather than holding on to them until the transaction exits.

SNAPSHOT isolation is special in that it uses multiversion concurrency control (MVCC). A commit may fail with an OptimisticLockException. Few repositories are expected to support this level, however.

Author:

Brian S O'Neill

See Also:

Repository.enterTransaction(IsolationLevel), Transaction

Enum Constant Summary

Enum Constants

Enum Constant and Description
NONE Indicates that no actual transaction is in progress.
READ_COMMITTED Indicates that dirty reads are prevented.
READ_UNCOMMITTED Indicates that dirty reads, non-repeatable reads and phantom reads can occur.
REPEATABLE_READ Indicates that dirty reads and non-repeatable reads are prevented.
SERIALIZABLE Indicates that dirty reads, non-repeatable reads and phantom reads are prevented.
SNAPSHOT Indicates that dirty reads, non-repeatable reads and phantom reads are prevented.

Method Summary

Methods

Modifier and Type	Method and Description
IsolationLevel	highestCommon(IsolationLevel level) Returns the highest common isolation level between this and the one given.
boolean	isAtLeast(IsolationLevel level) Returns true if this isolation level is at least as high as the one given.
boolean	isAtMost(IsolationLevel level) Returns true if this isolation level is no higher than the one given.
IsolationLevel	lowestCommon(IsolationLevel level) Returns the lowest common isolation level between this and the one given.
static IsolationLevel	valueOf(java.lang.String name) Returns the enum constant of this type with the specified name.
static IsolationLevel[]	values() Returns an array containing the constants of this enum type, in the order they are declared.

Methods inherited from class java.lang.Enum

clone, compareTo, equals, finalize, getDeclaringClass, hashCode, name, ordinal, toString, valueOf

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Enum Constant Detail

NONE

public static final IsolationLevel NONE

Indicates that no actual transaction is in progress. If this level is specified when entering a transaction, it uses auto-commit mode.

READ_UNCOMMITTED

public static final IsolationLevel READ_UNCOMMITTED

Indicates that dirty reads, non-repeatable reads and phantom reads can occur. This level allows modifications by one transaction to be read by another transaction before any changes have been committed (a "dirty read"). If any of the changes are rolled back, the second transaction will have retrieved an invalid modification.

This level is also known as degree 1 isolation.

READ_COMMITTED

public static final IsolationLevel READ_COMMITTED

Indicates that dirty reads are prevented. Non-repeatable reads and phantom reads can occur. This level only prohibits a transaction from reading modifications with uncommitted changes in it.

This level is also known as degree 2 isolation.

REPEATABLE_READ

public static final IsolationLevel REPEATABLE_READ

Indicates that dirty reads and non-repeatable reads are prevented. Phantom reads can occur. This level prohibits a transaction from reading uncommitted changes, and it also prohibits the situation where one transaction reads a record, a second transaction alters the record, and the first transaction rereads the record, getting different values the second time (a "non-repeatable read").

SNAPSHOT

public static final IsolationLevel SNAPSHOT

Indicates that dirty reads, non-repeatable reads and phantom reads are prevented. Commits can still fail however, as snapshot isolation avoids using locks.

SERIALIZABLE

public static final IsolationLevel SERIALIZABLE

Indicates that dirty reads, non-repeatable reads and phantom reads are prevented. Phantoms are records returned as a result of a search, but which were not seen by the same transaction when the identical search criteria was previously used. For example, another transaction may have inserted records which match the original search.

This level is also known as degree 3 isolation.

Method Detail

values

public static IsolationLevel[] values()

Returns an array containing the constants of this enum type, in the order they are declared. This method may be used to iterate over the constants as follows:

for (IsolationLevel c : IsolationLevel.values())
    System.out.println(c);

Returns:

an array containing the constants of this enum type, in the order they are declared

valueOf

public static IsolationLevel valueOf(java.lang.String name)

Returns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)

Parameters:

name - the name of the enum constant to be returned.

Returns:

the enum constant with the specified name

Throws:

java.lang.IllegalArgumentException - if this enum type has no constant with the specified name

java.lang.NullPointerException - if the argument is null

isAtLeast

public boolean isAtLeast(IsolationLevel level)

Returns true if this isolation level is at least as high as the one given.

isAtMost

public boolean isAtMost(IsolationLevel level)

Returns true if this isolation level is no higher than the one given.

lowestCommon

public IsolationLevel lowestCommon(IsolationLevel level)

Returns the lowest common isolation level between this and the one given.

highestCommon

public IsolationLevel highestCommon(IsolationLevel level)

Returns the highest common isolation level between this and the one given.