org.rrd4j.core
Class RrdBackend

java.lang.Object
  org.rrd4j.core.RrdBackend

Direct Known Subclasses:

RrdByteArrayBackend, RrdFileBackend

public abstract class RrdBackend
extends Object

Base implementation class for all backend classes. Each Round Robin Database object (RrdDb object) is backed with a single RrdBackend object which performs actual I/O operations on the underlying storage. Rrd4j supports multiple backends out of the box. E.g.:

• RrdRandomAccessFileBackend: objects of this class are created from the RrdRandomAccessFileBackendFactory class. This was the default backend used in all Rrd4j releases prior to 1.4.0. It uses java.io.* package and RandomAccessFile class to store RRD data in files on the disk.
• RrdNioBackend: objects of this class are created from the RrdNioBackendFactory class. The backend uses java.io.* and java.nio.* classes (mapped ByteBuffer) to store RRD data in files on the disk. This backend is fast, very fast, but consumes a lot of memory (borrowed not from the JVM but from the underlying operating system directly). This is the default backend used in Rrd4j since 1.4.0 release.
• RrdMemoryBackend: objects of this class are created from the RrdMemoryBackendFactory class. This backend stores all data in memory. Once JVM exits, all data gets lost. The backend is extremely fast and memory hungry.

To create your own backend in order to provide some custom type of RRD storage, you should do the following:

• Create your custom RrdBackend class (RrdCustomBackend, for example) by extending RrdBackend class. You have to implement all abstract methods defined in the base class.
• Create your custom RrdBackendFactory class (RrdCustomBackendFactory, for example) by extending RrdBackendFactory class. You have to implement all abstract methods defined in the base class. Your custom factory class will actually create custom backend objects when necessary.
• Create instance of your custom RrdBackendFactory and register it as a regular factory available to Rrd4j framework. See javadoc for RrdBackendFactory to find out how to do this.

Constructor Summary

protected

RrdBackend(String path) Creates backend for a RRD storage with the given path.

Method Summary

void	close() Closes the underlying backend.
abstract long	getLength() Returns the number of RRD bytes in the underlying storage.
String	getPath() Returns path to the storage.
protected boolean	isCachingAllowed() This method suggests the caching policy to the Rrd4j frontend (high-level) classes.
protected abstract void	read(long offset, byte[] b) Reads an array of bytes from the underlying storage starting from the given storage offset.
byte[]	readAll() Reads all RRD bytes from the underlying storage.
protected abstract void	setLength(long length) Sets the number of bytes in the underlying RRD storage.
protected abstract void	write(long offset, byte[] b) Writes an array of bytes to the underlying storage starting from the given storage offset.

Methods inherited from class java.lang.Object

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

Constructor Detail

RrdBackend

protected RrdBackend(String path)

Creates backend for a RRD storage with the given path.

Parameters:

path - String identifying RRD storage. For files on the disk, this argument should represent file path. Other storage types might interpret this argument differently.

Method Detail

getPath

public String getPath()

Returns path to the storage.

Returns:

Storage path

write

protected abstract void write(long offset,
                              byte[] b)
                       throws IOException

Writes an array of bytes to the underlying storage starting from the given storage offset.

Parameters:

offset - Storage offset.

b - Array of bytes that should be copied to the underlying storage

Throws:

IOException - Thrown in case of I/O error

read

protected abstract void read(long offset,
                             byte[] b)
                      throws IOException

Reads an array of bytes from the underlying storage starting from the given storage offset.

Parameters:

offset - Storage offset.

b - Array which receives bytes from the underlying storage

Throws:

IOException - Thrown in case of I/O error

getLength

public abstract long getLength()
                        throws IOException

Returns the number of RRD bytes in the underlying storage.

Returns:

Number of RRD bytes in the storage.

Throws:

IOException - Thrown in case of I/O error.

setLength

protected abstract void setLength(long length)
                           throws IOException

Sets the number of bytes in the underlying RRD storage. This method is called only once, immediately after a new RRD storage gets created.

Parameters:

length - Length of the underlying RRD storage in bytes.

Throws:

IOException - Thrown in case of I/O error.

close

public void close()
           throws IOException

Closes the underlying backend.

Throws:

IOException - Thrown in case of I/O error

isCachingAllowed

protected boolean isCachingAllowed()

This method suggests the caching policy to the Rrd4j frontend (high-level) classes. If true is returned, frontend classes will cache frequently used parts of a RRD file in memory to improve performance. If false is returned, high level classes will never cache RRD file sections in memory.

Returns:

true if file caching is enabled, false otherwise. By default, the method returns true but it can be overriden in subclasses.

readAll

public final byte[] readAll()
                     throws IOException

Reads all RRD bytes from the underlying storage.

Returns:

RRD bytes

Throws:

IOException - Thrown in case of I/O error