ReplicationLogBuffer (Apache Derby V10.14 Internals)

java.lang.Object
- org.apache.derby.impl.store.replication.buffer.ReplicationLogBuffer

```
public class ReplicationLogBuffer
extends java.lang.Object
```
Used for the replication master role only. When a Derby instance has the replication master role for a database 'x', all log records that are written to the local log file are also appended to this log buffer. The replication master service will consume chunks of log from this buffer and send it to the Derby instance with the slave role for 'x'. ReplicationLogBuffer consists of a number of LogBufferElements. Elements that are not in use are in the freeBuffers list, while elements that contains dirty log are in dirtyBuffers. Chunks of log records are appended to the buffer element in currentDirtyBuffer. Hence, the life cycle of buffer elements is: freeBuffers -> currentDirtyBuffer -> dirtyBuffers -> freeBuffers To append chunks of log records to the buffer, use appendLog(...) To consume chunks of log records, use next() followed by getData(), getLastInstant() and getSize(). These get-methods throw NoSuchElementException if next() returned false, meaning that there were no dirty log at the time next() was called. Threads: ReplicationLogBuffer is threadsafe. It can be used by a logger (LogToFile) and a log consumer (LogShipping service) concurrently without further synchronization. Important: If methods in this class calls methods outside this package (e.g. MasterFactory#workToDo), make sure that deadlocks are not introduced. If possible, a call to any method in another package should be done without holding latches in this class.

Field Summary

Fields
Modifier and Type	Field	Description
`private LogBufferElement`	`currentDirtyBuffer`
`static int`	`DEFAULT_NUMBER_LOG_BUFFERS`
`private int`	`defaultBufferSize`
`private java.util.LinkedList<LogBufferElement>`	`dirtyBuffers`
`private java.util.LinkedList<LogBufferElement>`	`freeBuffers`
`private java.lang.Object`	`listLatch`
`private MasterFactory`	`mf`
`private byte[]`	`outBufferData`
`private long`	`outBufferLastInstant`
`private int`	`outBufferStored`
`private java.lang.Object`	`outputLatch`
`private boolean`	`validOutBuffer`

Constructor Summary

Constructors
Constructor	Description
`ReplicationLogBuffer(int bufferSize, MasterFactory mf)`	Class constructor specifies the number of buffer elements and the master controller that creates this replication log buffer.

Method Summary

All Methods Instance Methods Concrete Methods
Modifier and Type	Method	Description
`void`	`appendLog(long greatestInstant, byte[] log, int logOffset, int logLength)`	Append a chunk of log records to the log buffer.
`byte[]`	`getData()`	Returns a byte[] containing a chunk of serialized log records.
`int`	`getFillInformation()`	Used to calculate the Fill Information.
`long`	`getLastInstant()`	Can be used so that only the necessary log records are sent when a flush(LogInstant flush_to_this) is called in the log factory.
`int`	`getSize()`
`boolean`	`next()`	Sets the output data to that of the next (oldest) buffer element in dirtyBuffers so that getData(), getLastInstant() and getSize() return values from the next oldest chunk of log.
`private void`	`switchDirtyBuffer()`	Appends the currentDirtyBuffer to dirtyBuffers, and makes a fresh buffer element from freeBuffers the currentDirtyBuffer.
`boolean`	`validData()`	Method to determine whether or not the buffer had any log records the last time next() was called.

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

- Field Detail
  - DEFAULT_NUMBER_LOG_BUFFERS
```
public static final int DEFAULT_NUMBER_LOG_BUFFERS
```
    See Also:
    
    Constant Field Values
  - dirtyBuffers
```
private final java.util.LinkedList<LogBufferElement> dirtyBuffers
```
  - freeBuffers
```
private final java.util.LinkedList<LogBufferElement> freeBuffers
```
  - currentDirtyBuffer
```
private LogBufferElement currentDirtyBuffer
```
  - validOutBuffer
```
private boolean validOutBuffer
```
  - outBufferData
```
private byte[] outBufferData
```
  - outBufferStored
```
private int outBufferStored
```
  - outBufferLastInstant
```
private long outBufferLastInstant
```
  - listLatch
```
private final java.lang.Object listLatch
```
  - outputLatch
```
private final java.lang.Object outputLatch
```
  - defaultBufferSize
```
private int defaultBufferSize
```
  - mf
```
private final MasterFactory mf
```
- Constructor Detail
  - ReplicationLogBuffer
```
public ReplicationLogBuffer(int bufferSize,
                            MasterFactory mf)
```
    Class constructor specifies the number of buffer elements and the master controller that creates this replication log buffer.
    
    Parameters:
    
    bufferSize - the default number of buffer elements
    
    mf - Used to notify the master controller that a log buffer element is full and work needs to be done.
- Method Detail
  - appendLog
```
public void appendLog(long greatestInstant,
                      byte[] log,
                      int logOffset,
                      int logLength)
               throws LogBufferFullException
```
    Append a chunk of log records to the log buffer.
    
    Parameters:
    
    greatestInstant - the instant of the log record that was added last to this chunk of log
    
    log - the chunk of log records
    
    logOffset - offset in log to start copy from
    
    logLength - number of bytes to copy, starting from logOffset
    
    Throws:
    
    LogBufferFullException - - thrown if there is not enough free space in the buffer to store the chunk of log.
  - next
```
public boolean next()
```
    Sets the output data to that of the next (oldest) buffer element in dirtyBuffers so that getData(), getLastInstant() and getSize() return values from the next oldest chunk of log. Used by the log consumer (the LogShipping service) to move to the next chunk of log in the buffer.
    
    Returns:
    
    true if there is log in the buffer, resulting in valid data for the get-methods
  - getData
```
public byte[] getData()
               throws java.util.NoSuchElementException
```
    Returns a byte[] containing a chunk of serialized log records. Always returns the log that was oldest at the time next() was called last time. Use next() to move to the next chunk of log records.
    
    Returns:
    
    A copy of the current byte[], which is a chunk of log
    
    Throws:
    
    java.util.NoSuchElementException - if there was no log in the buffer the last time next() was called.
  - validData
```
public boolean validData()
```
    Method to determine whether or not the buffer had any log records the last time next() was called.
    
    Returns:
    
    true if the buffer contained log records the last time next() was called. False if not, or if next() has not been called yet.
  - getSize
```
public int getSize()
            throws java.util.NoSuchElementException
```
    Returns:
    
    The number of bytes returned by getData
    
    Throws:
    
    java.util.NoSuchElementException - if there was no log in the buffer the last time next() was called.
  - getLastInstant
```
public long getLastInstant()
                    throws java.util.NoSuchElementException
```
    Can be used so that only the necessary log records are sent when a flush(LogInstant flush_to_this) is called in the log factory. Returns the highest log instant in the chunk of log that can be read with getData().
    
    Returns:
    
    The highest log instant in the chunk of log returned by getData().
    
    Throws:
    
    java.util.NoSuchElementException - if there was no log in the buffer the last time next() was called.
  - switchDirtyBuffer
```
private void switchDirtyBuffer()
                        throws LogBufferFullException
```
    Appends the currentDirtyBuffer to dirtyBuffers, and makes a fresh buffer element from freeBuffers the currentDirtyBuffer. Note: this method is not synchronized since all uses of it is inside synchronized(listLatch) code blocks.
    
    Throws:
    
    LogBufferFullException - if the freeBuffers list is empty
  - getFillInformation
```
public int getFillInformation()
```
    Used to calculate the Fill Information. The fill information is a indicator of how full the buffer is at any point of time fill information = (full buffers/Total Buffers)*100. The Fill information ranges between 0-100 (both 0 and 100 inclusive).
    
    Returns:
    
    an integer value between 0-100 representing the fill information.

Class ReplicationLogBuffer

Field Summary

Constructor Summary

Method Summary

Methods inherited from class java.lang.Object

Field Detail

DEFAULT_NUMBER_LOG_BUFFERS

dirtyBuffers

freeBuffers

currentDirtyBuffer

validOutBuffer

outBufferData

outBufferStored

outBufferLastInstant

listLatch

outputLatch

defaultBufferSize

mf

Constructor Detail

ReplicationLogBuffer

Method Detail

appendLog

next

getData

validData

getSize

getLastInstant

switchDirtyBuffer

getFillInformation