org.apache.activemq.apollo.util

Class HashRing<Node,Resource>

java.lang.Object

org.apache.activemq.apollo.util.HashRing<Node,Resource>

public class HashRing<Node,Resource>
extends Object

A hash ring is used to map resources to a to a set of nodes. This hash ring implements Consistent Hashing and therefore adding a removing nodes minimally changes how resources are map to the nodes.

This implementation also allows you to apply non-uniform node weighting. This feature is useful when you want to allocate more resources to some nodes and fewer to others.

The default weight of node is 200. The weight of a node determines how many points on the hash ring the node is allocated. Higher node weights increases the uniform distribution of resources.

Note that the order that nodes are added to the ring impact how resources map to the nodes due to node hash collisions.

Field Summary

Fields

Modifier and Type	Field and Description
static int	DEFAULT_WEIGHT
static Hasher	TO_STRING_HASHER The Hasher.ToStringHasher implemenation is as the hasher when none is specifed in the constructor.

Constructor Summary

Constructors

Constructor and Description
HashRing() Constructs a HashRing which uses the OBJECT_HASHER to hash the nodes and values.
HashRing(Hasher hasher) Constructs a HashRing with the specified hasher.

Method Summary

Methods

Modifier and Type	Method and Description
void	add(Node... nodes) Adds all the specified nodes to the HashRing using the default weight of 200 for each node.
ArrayList<Iterator<Node>>	add(Node node) Adds a node to the HashRing using the default weight of 200 for the node.
ArrayList<Iterator<Node>>	add(Node node, int weight) Adds a node to the HashRing using the specified weight.
void	addAll(Iterable<Node> nodes) Adds all the specified nodes to the HashRing using the default weight of 200 for each node.
void	clear() Removes all previously added nodes.
Node	get(Resource resource) Maps a resource value to a node.
List<Node>	getNodes()
Iterator<Node>	iterator(Resource resource) Maps a resource value to an interator to the nodes in the HashRing starting at the Node which resource maps to.
boolean	remove(Node node) Removes a previously added node from the HashRing

Methods inherited from class java.lang.Object

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

Field Detail

DEFAULT_WEIGHT

public static int DEFAULT_WEIGHT

TO_STRING_HASHER

public static Hasher TO_STRING_HASHER

The Hasher.ToStringHasher implemenation is as the hasher when none is specifed in the constructor.

Constructor Detail

HashRing

public HashRing()

Constructs a HashRing which uses the OBJECT_HASHER to hash the nodes and values.

HashRing

public HashRing(Hasher hasher)

Constructs a HashRing with the specified hasher.

Parameters:

hasher -

Method Detail

addAll

public void addAll(Iterable<Node> nodes)

Adds all the specified nodes to the HashRing using the default weight of 200 for each node.

Parameters:

nodes - the nodes to add

add

public void add(Node... nodes)

Adds all the specified nodes to the HashRing using the default weight of 200 for each node.

Parameters:

nodes - the nodes to add

add

public ArrayList<Iterator<Node>> add(Node node)

Adds a node to the HashRing using the default weight of 200 for the node.

Parameters:

node - the node to add

add

public ArrayList<Iterator<Node>> add(Node node,
                            int weight)

Adds a node to the HashRing using the specified weight.

Parameters:

node - the node to add

weight - the number of hash replicas to create the node in the HashRing

Throws:

IllegalArgumentException - if the weight is less than 1

remove

public boolean remove(Node node)

Removes a previously added node from the HashRing

Parameters:

node - the node to remove

Returns:

true if the node was previously added

clear

public void clear()

Removes all previously added nodes.

getNodes

public List<Node> getNodes()

Returns:

all the previously added nodes.

get

public Node get(Resource resource)

Maps a resource value to a node.

Parameters:

resource - the resource to map

Returns:

the Node that the resource maps to or null if the HashRing is empty.

iterator

public Iterator<Node> iterator(Resource resource)

Maps a resource value to an interator to the nodes in the HashRing starting at the Node which resource maps to. Note that duplicate node objects may be returned. This is because

Parameters:

resource - the resource to map

Returns:

a Iterator