Class LogAccessFile


  • public class LogAccessFile
    extends java.lang.Object
    Wraps a RandomAccessFile file to provide buffering on log writes. Only supports the write calls required for the log! MT - unsafe. Caller of this class must provide synchronization. The one exception is with the log file access, LogAccessFile will touch the log only inside synchronized block protected by the semaphore, which is defined by the creator of this object. Write to the log buffers are allowed when there are free buffers even when dirty buffers are being written(flushed) to the disk by a different thread. Only one flush writes to log file at a time, other wait for it to finish. Except for flushLogAccessFile , SyncAccessLogFile other function callers must provide syncronization that will allow only one of them to write to the buffers. Log Buffers are used in circular fashion, each buffer moves through following stages: freeBuffers --> dirtyBuffers --> freeBuffers. Movement of buffers from one stage to another stage is synchronized using the object(this) of this class. A Checksum log record that has the checksum value for the data that is being written to the disk is generated and written before the actual data. Except for the large log records that does not fit into a single buffer, checksum is calcualted for a group of log records that are in the buffer when buffers is switched. Checksum log record is written into the reserved space in the beginning buffer. In case of a large log record that does not fit into a buffer, the checksum is written to the byte[] allocated for the big log record. Checksum log records helps in identifying the incomplete log disk writes during recovery. This is done by recalculating the checksum value for the data on the disk and comparing it to the the value stored in the checksum log record.
    • Field Detail

      • LOG_RECORD_FIXED_OVERHEAD_SIZE

        private static final int LOG_RECORD_FIXED_OVERHEAD_SIZE
        The fixed size of a log record is 16 bytes: int length : 4 bytes long instant : 8 bytes int trailing length : 4 bytes
        See Also:
        Constant Field Values
      • LOG_RECORD_HEADER_SIZE

        private static final int LOG_RECORD_HEADER_SIZE
        See Also:
        Constant Field Values
      • LOG_RECORD_TRAILER_SIZE

        private static final int LOG_RECORD_TRAILER_SIZE
        See Also:
        Constant Field Values
      • LOG_NUMBER_LOG_BUFFERS

        private static final int LOG_NUMBER_LOG_BUFFERS
        See Also:
        Constant Field Values
      • flushInProgress

        private boolean flushInProgress
      • logFileSemaphore

        private final java.lang.Object logFileSemaphore
      • mon_numWritesToLog

        static int mon_numWritesToLog
      • mon_numBytesToLog

        static int mon_numBytesToLog
      • inReplicationMasterMode

        boolean inReplicationMasterMode
      • inReplicationSlaveMode

        boolean inReplicationSlaveMode
      • checksumInstant

        private long checksumInstant
      • checksumLength

        private int checksumLength
      • checksumLogRecordSize

        private int checksumLogRecordSize
      • writeChecksum

        private boolean writeChecksum
      • checksumLogRecord

        private LogRecord checksumLogRecord
      • databaseEncrypted

        private boolean databaseEncrypted
    • Method Detail

      • writeLogRecord

        public void writeLogRecord​(int length,
                                   long instant,
                                   byte[] data,
                                   int data_offset,
                                   byte[] optional_data,
                                   int optional_data_offset,
                                   int optional_data_length)
                            throws StandardException,
                                   java.io.IOException
        Write a single log record to the stream.

        For performance pass all parameters rather into a specialized routine rather than maintaining the writeInt, writeLong, and write interfaces that this class provides as a standard OutputStream. It will make it harder to use other OutputStream implementations, but makes for less function calls and allows optimizations knowing when to switch buffers.

        This routine handles all log records which are smaller than one log buffer. If a log record is bigger than a log buffer it calls writeUnbufferedLogRecord().

        The log record written will always look the same as if the following code had been executed: writeInt(length) writeLong(instant) write(data, data_offset, (length - optional_data_length) ) if (optional_data_length != 0) write(optional_data, optional_data_offset, optional_data_length) writeInt(length)

        Parameters:
        length - (data + optional_data) length bytes to write
        instant - the log address of this log record.
        data - "from" array to copy "data" portion of rec
        data_offset - offset in "data" to start copying from.
        optional_data - "from" array to copy "optional data" from
        optional_data_offset - offset in "optional_data" to start copy from
        optional_data_length - length of optional data to copy.
        Throws:
        StandardException - Standard exception policy.
        java.io.IOException
      • appendLogRecordToBuffer

        private int appendLogRecordToBuffer​(byte[] buff,
                                            int pos,
                                            int length,
                                            long instant,
                                            byte[] data,
                                            int data_offset,
                                            byte[] optional_data,
                                            int optional_data_offset,
                                            int optional_data_length)
        Append a log record to a byte[]. Typically, the byte[] will be currentBuffer, but if a log record that is too big to fit in a buffer is added, buff will be a newly allocated byte[].
        Parameters:
        buff - The byte[] the log record is appended to
        pos - The position in buff where the method will start to append to
        length - (data + optional_data) length bytes to write
        instant - the log address of this log record.
        data - "from" array to copy "data" portion of rec
        data_offset - offset in "data" to start copying from.
        optional_data - "from" array to copy "optional data" from
        optional_data_offset - offset in "optional_data" to start copy from
        optional_data_length - length of optional data to copy.
        See Also:
        writeLogRecord(int, long, byte[], int, byte[], int, int)
      • writeInt

        private final int writeInt​(int i,
                                   byte[] b,
                                   int p)
      • writeLong

        private final int writeLong​(long l,
                                    byte[] b,
                                    int p)
      • writeInt

        public void writeInt​(int i)
      • writeLong

        public void writeLong​(long l)
      • write

        public void write​(int b)
      • write

        public void write​(byte[] b,
                          int off,
                          int len)
      • flushDirtyBuffers

        protected void flushDirtyBuffers()
                                  throws java.io.IOException
        Write data from all dirty buffers into the log file.

        A call for clients of LogAccessFile to insure that all privately buffered data has been writen to the file - so that reads on the file using one of the various scan classes will see all the data which has been writen to this point.

        Note that this routine only "writes" the data to the file, this does not mean that the data has been synced to disk unless file was opened in WRITE SYNC mode(rws/rwd). The only way to insure that is by calling is to call syncLogAccessFile() after this call in Non-WRITE sync mode(rw)

        MT-Safe : parallel thereads can call this function, only one threads does the flush and the other threads waits for the one that is doing the flush to finish. Currently there are two possible threads that can call this function in parallel 1) A Thread that is doing the commit 2) A Thread that is writing to the log and log buffers are full or a log records does not fit in a buffer. (Log Buffers full(switchLogBuffer() or a log record size that is greater than logbuffer size has to be writtern through writeToLog call directlty) Note: writeToLog() is not synchronized on the semaphore that is used to do buffer management to allow writes to the free buffers when flush is in progress.

        Throws:
        java.io.IOException
      • switchLogBuffer

        public void switchLogBuffer()
                             throws java.io.IOException,
                                    StandardException
        Appends the current Buffer to the dirty Buffer list and assigns a free buffer to be the currrent active buffer . Flushing of the buffer to disk is delayed if there is a free buffer available. dirty buffers will be flushed to the disk when flushDirtyBuffers() is invoked by a commit call or when no more free buffers are available.
        Throws:
        java.io.IOException
        StandardException
      • syncLogAccessFile

        public void syncLogAccessFile()
                               throws java.io.IOException,
                                      StandardException
        Guarantee all writes up to the last call to flushLogAccessFile on disk.

        A call for clients of LogAccessFile to insure that all data written up to the last call to flushLogAccessFile() are written to disk. This call will not return until those writes have hit disk.

        Note that this routine may block waiting for I/O to complete so callers should limit the number of resource held locked while this operation is called. It is expected that the caller Note that this routine only "writes" the data to the file, this does not mean that the data has been synced to disk. The only way to insure that is to first call switchLogBuffer() and then follow by a call of sync().

        Throws:
        java.io.IOException
        StandardException
      • corrupt

        public void corrupt()
                     throws java.io.IOException
        The database is being marked corrupted, get rid of file pointer without writing out anything more.
        Throws:
        java.io.IOException
      • setReplicationMasterRole

        protected void setReplicationMasterRole​(MasterFactory masterFac)
        Make this LogAccessFile pass chunks of log records (byte[]) to the MasterFactory when the chunks are written to disk.
        Parameters:
        masterFac - The MasterFactory service responsible for controlling the master side replication behaviour.
      • stopReplicationMasterRole

        protected void stopReplicationMasterRole()
        Stop this LogAccessFile from passing chunks of log records to the MasterFactory.
      • setReplicationSlaveRole

        protected void setReplicationSlaveRole()
        Method to put this LogAccessFile object in replication slave mode, effectively disabling checksum writes. Because checksums are received from the replication master, the slave can not be allowed to add it's own checksums - that would invalidate the checksums and would stop the database from recovering. Replication slave mode must therefore be set before LogAccessFile decides whether to write it's own checksums, and this method is therefore indirectly called from the constructor of this class by calling LogFactory.checkForReplication If replication slave mode for the database is stopped after this object has been created, checksums cannot be reenabled without creating a new instance of this class. That is conveniently handled as LogToFile.recover completes (which automatically happens once replication slave mode is no longer active)
        See Also:
        LogToFile.checkForReplication(org.apache.derby.impl.store.raw.log.LogAccessFile)
      • writeToLog

        private void writeToLog​(byte[] b,
                                int off,
                                int len,
                                long highestInstant)
                         throws java.io.IOException
        Throws:
        java.io.IOException
      • reserveSpaceForChecksum

        protected long reserveSpaceForChecksum​(int length,
                                               long logFileNumber,
                                               long currentPosition)
                                        throws StandardException,
                                               java.io.IOException
        reserve the space for the checksum log record in the log file.
        Parameters:
        length - the length of the log record to be written
        logFileNumber - current log file number
        currentPosition - current position in the log file.
        Returns:
        the space that is needed to write a checksum log record.
        Throws:
        StandardException
        java.io.IOException
      • writeChecksumLogRecord

        private void writeChecksumLogRecord​(byte[] buffer)
                                     throws java.io.IOException,
                                            StandardException
        Generate the checkum log record and write it into the log buffer. The checksum applies to all bytes from this checksum log record to the next one.
        Parameters:
        buffer - The byte[] the checksum is written to. The checksum is always written at the beginning of buffer.
        Throws:
        java.io.IOException
        StandardException
      • getChecksumLogRecordSize

        public int getChecksumLogRecordSize()
        Return the length of a checksum record