org.mapdb

Class TxEngine

java.lang.Object

org.mapdb.TxEngine

All Implemented Interfaces:

java.io.Closeable, java.lang.AutoCloseable, Engine

public class TxEngine
extends java.lang.Object
implements Engine

Naive implementation of Snapshots on top of StorageEngine. On update it takes old value and stores it aside.

TODO merge snapshots down with Storage for best performance

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	TxEngine.Tx

Nested classes/interfaces inherited from interface org.mapdb.Engine

Engine.CloseOnJVMShutdown, Engine.ReadOnly, Engine.ReadOnlyWrapper

Field Summary

Fields inherited from interface org.mapdb.Engine

CLOSED_ENGINE, RECID_CLASS_CATALOG, RECID_FIRST, RECID_LAST_RESERVED, RECID_NAME_CATALOG, RECID_RECORD_CHECK

Method Summary

Modifier and Type	Method and Description
boolean	canRollback()
boolean	canSnapshot()
void	clearCache() clears any underlying cache
void	close() Close store/cache.
void	commit() Makes all changes made since the previous commit/rollback permanent.
void	compact()
<A> boolean	compareAndSwap(long recid, A expectedOldValue, A newValue, Serializer<A> serializer) Updates existing record in atomic (Compare And Swap) manner.
static Engine	createSnapshotFor(Engine engine)
<A> void	delete(long recid, Serializer<A> serializer) Remove existing record from store/cache
<A> A	get(long recid, Serializer<A> serializer) Get existing record.
Engine	getWrappedEngine() if this is wrapper return underlying engine, or null
boolean	isClosed() Checks whether Engine was closed.
boolean	isReadOnly() Check if you can write into this Engine.
long	preallocate() Preallocates recid for not yet created record.
<A> long	put(A value, Serializer<A> serializer) Insert new record.
void	rollback() Undoes all changes made in the current transaction.
Engine	snapshot() Returns read-only snapshot of data in Engine.
<A> void	update(long recid, A value, Serializer<A> serializer) Update existing record with new value.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

createSnapshotFor

public static Engine createSnapshotFor(Engine engine)

canSnapshot

public boolean canSnapshot()

Specified by:

canSnapshot in interface Engine

Returns:

true if engine can create read-only snapshots

snapshot

public Engine snapshot()

Description copied from interface: Engine

Returns read-only snapshot of data in Engine.

Specified by:

snapshot in interface Engine

getWrappedEngine

public Engine getWrappedEngine()

Description copied from interface: Engine

if this is wrapper return underlying engine, or null

Specified by:

getWrappedEngine in interface Engine

clearCache

public void clearCache()

Description copied from interface: Engine

clears any underlying cache

Specified by:

clearCache in interface Engine

compact

public void compact()

Specified by:

compact in interface Engine

preallocate

public long preallocate()

Description copied from interface: Engine

Preallocates recid for not yet created record. It does not insert any data into it.

Specified by:

preallocate in interface Engine

Returns:

new recid

put

public <A> long put(A value,
                    Serializer<A> serializer)

Description copied from interface: Engine

Insert new record.

Specified by:

put in interface Engine

Parameters:

value - records to be added

serializer - used to convert record into/from binary form

Returns:

recid (record identifier) under which record is stored.

get

public <A> A get(long recid,
                 Serializer<A> serializer)

Description copied from interface: Engine

Get existing record.

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it returns null or throws 'EndOfFileException'

Specified by:

get in interface Engine

Parameters:

recid - (record identifier) under which record was persisted

serializer - used to deserialize record from binary form

Returns:

record matching given recid, or null if record is not found under given recid.

update

public <A> void update(long recid,
                       A value,
                       Serializer<A> serializer)

Description copied from interface: Engine

Update existing record with new value.

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it throws 'EndOfFileException', but it may also corrupt store.

Specified by:

update in interface Engine

Parameters:

recid - (record identifier) under which record was persisted.

value - new record value to be stored

serializer - used to serialize record into binary form

compareAndSwap

public <A> boolean compareAndSwap(long recid,
                                  A expectedOldValue,
                                  A newValue,
                                  Serializer<A> serializer)

Description copied from interface: Engine

Updates existing record in atomic (Compare And Swap) manner. Value is modified only if old value matches expected value. There are three ways to match values, MapDB may use any of them:

1. Equality check oldValue==expectedOldValue when old value is found in instance cache
2. Deserializing oldValue using serializer and checking oldValue.equals(expectedOldValue)
3. Serializing expectedOldValue using serializer and comparing binary array with already serialized oldValue

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it throws 'EndOfFileException', but it may also corrupt store.

Specified by:

compareAndSwap in interface Engine

Parameters:

recid - (record identifier) under which record was persisted.

expectedOldValue - old value to be compared with existing record

newValue - to be written if values are matching

serializer - used to serialize record into binary form

Returns:

true if values matched and newValue was written

delete

public <A> void delete(long recid,
                       Serializer<A> serializer)

Description copied from interface: Engine

Remove existing record from store/cache

Recid must be a number returned by 'put' method. Behaviour for invalid recid (random number or already deleted record) is not defined, typically it throws 'EndOfFileException', but it may also corrupt store.

Specified by:

delete in interface Engine

Parameters:

recid - (record identifier) under which was record persisted

serializer - which may be used in some circumstances to deserialize and store old object

close

public void close()

Description copied from interface: Engine

Close store/cache. This method must be called before JVM exits to flush all caches and prevent store corruption. Also it releases resources used by MapDB (disk, memory..).

Engine can no longer be used after this method was called. If Engine is used after closing, it may throw any exception including NullPointerException

There is an configuration option DBMaker.Maker.closeOnJvmShutdown() which uses shutdown hook to automatically close Engine when JVM shutdowns.

Specified by:

close in interface java.io.Closeable

Specified by:

close in interface java.lang.AutoCloseable

Specified by:

close in interface Engine

isClosed

public boolean isClosed()

Description copied from interface: Engine

Checks whether Engine was closed.

Specified by:

isClosed in interface Engine

Returns:

true if engine was closed

commit

public void commit()

Description copied from interface: Engine

Makes all changes made since the previous commit/rollback permanent. In transactional mode (on by default) it means creating journal file and replaying it to storage. In other modes it may flush disk caches or do nothing at all (check your config options)

Specified by:

commit in interface Engine

rollback

public void rollback()

Description copied from interface: Engine

Undoes all changes made in the current transaction. If transactions are disabled it throws UnsupportedOperationException.

Specified by:

rollback in interface Engine

isReadOnly

public boolean isReadOnly()

Description copied from interface: Engine

Check if you can write into this Engine. It may be readonly in some cases (snapshot, read-only files).

Specified by:

isReadOnly in interface Engine

Returns:

true if engine is read-only

canRollback

public boolean canRollback()

Specified by:

canRollback in interface Engine

Returns:

true if engine supports rollback