com.sleepycat.je.log

Class LogBuffer

java.lang.Object

com.sleepycat.je.log.LogBuffer

All Implemented Interfaces:

LogSource

public class LogBuffer
extends Object
implements LogSource

LogBuffers hold outgoing, newly written log entries. Space is allocated via the allocate() method that returns a LogBufferSegment object. The LogBuffer.writePinCount is incremented each time space is allocated. Once the caller copies data into the log buffer, the pin count is decremented via the free() method. Readers of a log buffer wait until the pin count is zero. The pin count is incremented under the readLatch. The pin count is decremented without holding the latch. Holding the readLatch will prevent the pin count from being incremented. Apart from the pin count, access to the buffer is protected by the readLatch and the LWL: - Write access requires holding both the LWL and the readLatch. - Read access requires holding either the LWL or the readLatch. Of course, for buffers outside the buffer pool, or in the process of being constructed, these rules do not apply and no latching is necessary. TODO: Although the above statement about latching reflects the current implementation, it would be better if we can remove the reliance on the LWL and protect all access to the buffer using the readLatch. To do this, the callers of getFirstLsn and hasRoom will have to acquire the readLatch.

See Also:

LogBufferPool

Method Summary

Modifier and Type	Method and Description
com.sleepycat.je.log.LogBufferSegment	allocate(int size) Allocate a segment out of the buffer.
void	free() Called with the buffer not latched.
ByteBuffer	getBytes(long fileOffset) Make a copy of this buffer (doesn't copy data, only buffer state) and position it to read the requested data.
ByteBuffer	getBytes(long fileOffset, int numBytes) Same as getBytes(long fileOffset) since buffer should always hold a whole entry.
ByteBuffer	getDataBuffer() Returns the buffer for read access (although some tests may write to the buffer).
long	getFirstLsn() Return first LSN held in this buffer.
int	getLogVersion() Entries in write buffers are always the current version.
void	latchForWrite() Acquires the readLatch, providing exclusive access to the buffer.
void	release() Releases the readLatch.
String	toString()
void	waitForZeroAndLatch() Acquire the buffer latched and with the buffer pin count equal to zero.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Method Detail

getFirstLsn

public long getFirstLsn()

Return first LSN held in this buffer. The LWL or readLatch must be held.

getDataBuffer

public ByteBuffer getDataBuffer()

Returns the buffer for read access (although some tests may write to the buffer). The LWL or readLatch must be held.

Returns:

the actual data buffer.

latchForWrite

public void latchForWrite()
                   throws DatabaseException

Acquires the readLatch, providing exclusive access to the buffer. When modifying the buffer, both the LWL and buffer latch must be held. Note that containsLsn() acquires the latch for reading. Call release() to release the latch. TODO: It would be possible to use a shared buffer latch to allow concurrent access by multiple readers. The access rules for would then be: - Write access requires holding both the LWL and the buffer latch EX. - Read access requires holding either the LWL or the buffer latch SH. Note that LogBufferPool.bumpCurrent calls latchForWrite, but it may actually only need read access.

Throws:

DatabaseException

release

public void release()

Releases the readLatch.

Specified by:

release in interface LogSource

See Also:

LogSource.release()

allocate

public com.sleepycat.je.log.LogBufferSegment allocate(int size)

Allocate a segment out of the buffer. The LWL and readLatch must be held.

Parameters:

size - of buffer to allocate

Returns:

null if not enough room, otherwise a LogBufferSegment for the data.

free

public void free()

Called with the buffer not latched.

waitForZeroAndLatch

public void waitForZeroAndLatch()

Acquire the buffer latched and with the buffer pin count equal to zero.

getBytes

public ByteBuffer getBytes(long fileOffset)

Make a copy of this buffer (doesn't copy data, only buffer state) and position it to read the requested data. The LWL or readLatch must be held.

Specified by:

getBytes in interface LogSource

See Also:

LogSource.getBytes(long)

getBytes

public ByteBuffer getBytes(long fileOffset,
                           int numBytes)

Same as getBytes(long fileOffset) since buffer should always hold a whole entry. The LWL or readLatch must be held.

Specified by:

getBytes in interface LogSource

See Also:

LogSource.getBytes(long)

getLogVersion

public int getLogVersion()

Entries in write buffers are always the current version.

Specified by:

getLogVersion in interface LogSource

toString

public String toString()

Overrides:

toString in class Object