com.bigdata.ha.halog

Interface IHALogReader

All Known Implementing Classes:

HALogReader

public interface IHALogReader

Interface for reading on an HA Log.

Field Summary

Fields

Modifier and Type	Field and Description
static String	HA_LOG_EXT The filename extension used for the HALog files.
static FileFilter	HALOG_FILTER A FileFilter that visits all files ending with the HA_LOG_EXT and the names of all direct child directories.

Method Summary

Methods

Modifier and Type	Method and Description
void	close() Closes the reader iff it is open.
void	computeDigest(MessageDigest digest) Compute the digest.
IRootBlockView	getClosingRootBlock() The IRootBlockView for the committed state AFTER the write set contained in the HA log file has been applied.
IRootBlockView	getOpeningRootBlock() The IRootBlockView for the committed state BEFORE the write set contained in the HA log file.
boolean	hasMoreBuffers() Checks whether we have reached the end of the file (blocking).
boolean	isEmpty() Return true if the root blocks in the log file have the same commit counter.
boolean	isLive() Return true iff this is was the live HALog at the moment when it was opened (if true, then this flag will remain true even if the writer moves onto another HALog file).
boolean	isOpen() Return true iff the reader is open.
IHAWriteMessage	processNextBuffer(ByteBuffer clientBuffer) Attempts to read the next IHAWriteMessage and then the expected buffer, that is read into the client buffer.

Field Detail

HA_LOG_EXT

static final String HA_LOG_EXT

The filename extension used for the HALog files.

See Also:

Constant Field Values

HALOG_FILTER

static final FileFilter HALOG_FILTER

A FileFilter that visits all files ending with the HA_LOG_EXT and the names of all direct child directories. This FileFilter may be used to establish recursive scans of the HALog directory.

Method Detail

close

void close()
           throws IOException

Closes the reader iff it is open.

Note: Closing a reader does not have a side-effect on any open reader or writer for the same file. Specifically, if two readers are open for the same file and one is closed, then other will remain open. Likewise, if a reader is open for the live HALog file, closing the writer will not close the reader and closing the reader will not close the writer.

Throws:

IOException

isEmpty

boolean isEmpty()

Return true if the root blocks in the log file have the same commit counter. Such log files are logically empty regardless of their length. FIXME The code disagress and will report [false] if the live log has the same root blocks but has not yet been closed.

isOpen

boolean isOpen()

Return true iff the reader is open.

getOpeningRootBlock

IRootBlockView getOpeningRootBlock()
                                   throws IOException

The IRootBlockView for the committed state BEFORE the write set contained in the HA log file.

Throws:

IOException

getClosingRootBlock

IRootBlockView getClosingRootBlock()
                                   throws IOException

The IRootBlockView for the committed state AFTER the write set contained in the HA log file has been applied.

Throws:

IOException

hasMoreBuffers

boolean hasMoreBuffers()
                       throws IOException

Checks whether we have reached the end of the file (blocking).

Note: This method will block if this is the live HALog. This allows a process to block until the next message is made available on the live HALog by the writer.

Note: This method is non-blocking if this is not the live HALog since the decision can be made deterministically by inspecting the #of messages available (in the closing root block) and the #of messages consumed by the reader.

Returns:

Return false if (a) the file is closed on entry to this method; (b) the live HALog is closed while waiting for more data to become available; or (c) the end of a historical HALog file has been reached. Return true iff more data can be read from the file.

Throws:

IOException - if there is an error reading from the backing file.

processNextBuffer

IHAWriteMessage processNextBuffer(ByteBuffer clientBuffer)
                                  throws IOException

Attempts to read the next IHAWriteMessage and then the expected buffer, that is read into the client buffer. The IHAWriteMessage is returned to the caller.

Note: The caller's buffer will be filled in IFF the data is on the HALog. For some IHABufferStrategy implementations, that data is not present in the HALog. The caller's buffer will not be modified and the caller is responsible for getting the data from the IHABufferStrategy (e.g., for the WORMStrategy).

Note: IF the buffer is filled, then the limit will be the #of bytes ready to be transmitted and the position will be zero.

Parameters:

clientBuffer - A buffer from the DirectBufferPool.INSTANCE.

Throws:

IOException

computeDigest

void computeDigest(MessageDigest digest)
                   throws DigestException,
                          IOException

Compute the digest. If the IHALogReader is backed by the live HALog, then only the digest of the then current extent will be computed.

Throws:

DigestException

IOException

isLive

boolean isLive()

Return true iff this is was the live HALog at the moment when it was opened (if true, then this flag will remain true even if the writer moves onto another HALog file).