org.apache.jackrabbit.oak.util

Class ApproximateCounter

java.lang.Object

org.apache.jackrabbit.oak.util.ApproximateCounter

public class ApproximateCounter
extends java.lang.Object

An approximate counter algorithm.

Field Summary

Fields

Modifier and Type	Field and Description
static int	COUNT_MAX
static java.lang.String	COUNT_PROPERTY_PREFIX
static int	COUNT_RESOLUTION

Constructor Summary

Constructors

Constructor and Description
ApproximateCounter()

Method Summary

Modifier and Type	Method and Description
static void	adjustCountSync(NodeBuilder builder, long offset) Adjust a counter in the given node.
static long	adjustOffset(long oldCount, long calculatedOffset, int resolution) This method ensures that the new approximate count (the old count plus the calculated offset) does not go below 0.
static long	calculateOffset(long offset, int resolution) Calculate the approximate offset from a given offset.
static long	getCountSync(NodeState node) Get the count estimation.

Methods inherited from class java.lang.Object

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

Field Detail

COUNT_PROPERTY_PREFIX

public static final java.lang.String COUNT_PROPERTY_PREFIX

See Also:

Constant Field Values

COUNT_RESOLUTION

public static final int COUNT_RESOLUTION

See Also:

Constant Field Values

COUNT_MAX

public static final int COUNT_MAX

See Also:

Constant Field Values

Constructor Detail

ApproximateCounter

public ApproximateCounter()

Method Detail

calculateOffset

public static long calculateOffset(long offset,
                                   int resolution)

Calculate the approximate offset from a given offset. The offset is the number of added or removed entries. The result is 0 in most of the cases, but sometimes it is a (positive or negative) multiple of the resolution, such that on average, the sum of the returned value matches the sum of the passed offsets.

Parameters:

offset - the high-resolution input offset

resolution - the resolution

Returns:

the low-resolution offset (most of the time 0)

adjustOffset

public static long adjustOffset(long oldCount,
                                long calculatedOffset,
                                int resolution)

This method ensures that the new approximate count (the old count plus the calculated offset) does not go below 0. Also, for large counts and resolutions larger than 10, it reduces the resolution by a factor of 10 (further reducing the number of updates needed by a factor of 10).

Parameters:

oldCount - the old count

calculatedOffset - the calculated offset (may not be 0)

resolution - the new (lower) resolution

Returns:

the new offset

adjustCountSync

public static void adjustCountSync(NodeBuilder builder,
                                   long offset)

Adjust a counter in the given node. This method supports concurrent changes. It uses multiple properties, and is less accurate, but can be used in a multi-threaded environment, as it uses unique property names.

Parameters:

builder - the node builder

offset - the offset

getCountSync

public static long getCountSync(NodeState node)

Get the count estimation.

Parameters:

node - the node

Returns:

the estimation (-1 if no estimation is available)