/*
* The MIT License
*
* Copyright (c) 2004-2009, Sun Microsystems, Inc.
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
package hudson.util;
import com.trilead.ssh2.crypto.digest.MD5;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import hudson.util.Iterators.DuplicateFilterIterator;
/**
* Consistent hash.
*
* <p>
* This implementation is concurrency safe; additions and removals are serialized, but look up
* can be performed concurrently even when modifications is in progress.
*
* <p>
* Since typical hash functions we use in {@link Object#hashCode()} isn't random enough to
* evenly populate the 2^32 ring space, we only ask the user to give us
* <a href="http://en.wikipedia.org/wiki/Injective_function">an injective function</a> to a string,
* and then we use MD5 to create random enough distribution.
*
* <p>
* This consistent hash implementaiton is consistent both to the addition/removal of Ts, as well
* as increase/decrease of the replicas.
*
* <p>
* See http://en.wikipedia.org/wiki/Consistent_hashing for references, and
* http://weblogs.java.net/blog/tomwhite/archive/2007/11/consistent_hash.html is probably a reasonable depiction.
* If we trust his experiments, creating 100 replicas will reduce the stddev to 10% of the mean for 10 nodes.
*
* @author Kohsuke Kawaguchi
* @since 1.302
*/
public class ConsistentHash<T> {
/**
* All the items in the hash, to their replication factors.
*/
private final Map<T,Point[]> items = new HashMap<T,Point[]>();
private final int defaultReplication;
private final Hash<T> hash;
/**
* Used for remembering the computed MD5 hash, since it's bit expensive to do it all over again.
*/
private static final class Point implements Comparable<Point> {
final int hash;
final Object item;
private Point(int hash, Object item) {
this.hash = hash;
this.item = item;
}
public int compareTo(Point that) {
if(this.hash<that.hash) return -1;
if(this.hash==that.hash) return 0;
return 1;
}
}
/**
* Table that gets atomically replaced for concurrency safe operation.
*/
private volatile Table table;
/**
* Immutable consistent hash table.
*/
private final class Table {
private final int[] hash;
private final Object[] owner; // really T[]
private Table() {
// merge all points from all nodes and sort them into a single array
Point[] allPoints = new Point[countAllPoints()];
int p=0;
for (Point[] v : items.values()) {
System.arraycopy(v,0,allPoints,p,v.length);
p+=v.length;
}
Arrays.sort(allPoints);
hash = new int[allPoints.length];
owner = new Object[allPoints.length];
for (int i=0; i<allPoints.length; i++) {
Point pt = allPoints[i];
hash[i]=pt.hash;
owner[i]=pt.item;
}
}
T lookup(int queryPoint) {
int i = index(queryPoint);
if(i<0) return null;
return (T)owner[i];
}
/**
* Returns a consistent stream of nodes starting the given query point.
*
* <p>
* This is a permutation of all the nodes, where nodes with more replicas
* are more likely to show up early on.
*/
Iterator<T> list(int queryPoint) {
final int start = index(queryPoint);
return new DuplicateFilterIterator<T>(new Iterator<T>() {
int pos=0;
public boolean hasNext() {
return pos<owner.length;
}
public T next() {
if(!hasNext()) throw new NoSuchElementException();
return (T)owner[(start+(pos++))%owner.length];
}
public void remove() {
throw new UnsupportedOperationException();
}
});
}
private int index(int queryPoint) {
int idx = Arrays.binarySearch(hash, queryPoint);
if(idx<0) {
idx = -idx-1; // idx is now 'insertion point'
if(hash.length==0) return -1;
idx %= hash.length; // make it a circle
}
return idx;
}
}
/**
* Hashes an object to some value.
*
* <p>
* By default, {@link ConsistentHash} uses {@link Object#toString()} on 'T' to
* obtain the hash, but that behavior can be changed by providing
* a {@link Hash} implementation.
*
* <p>
* This hash function need not produce a very uniform distribution, as the
* output is rehashed with MD5. But it does need to make sure it doesn't
* produce the same value for two different 'T's (and that's why this returns
* String, not the usual int.)
*/
public interface Hash<T> {
/**
* @param t
* The object to be hashed. Never null.
* @return
* The hash value.
*/
String hash(T t);
}
private static final Hash DEFAULT_HASH = new Hash() {
public String hash(Object o) {
return o.toString();
}
};
public ConsistentHash() {
this(DEFAULT_HASH);
}
public ConsistentHash(int defaultReplication) {
this(DEFAULT_HASH,defaultReplication);
}
public ConsistentHash(Hash<T> hash) {
this(hash,100);
}
public ConsistentHash(Hash<T> hash, int defaultReplication) {
this.hash = hash;
this.defaultReplication = defaultReplication;
this.table = new Table(); // initial empty table
}
public int countAllPoints() {
int r=0;
for (Point[] v : items.values())
r+=v.length;
return r;
}
/**
* Adds a new node with the default number of replica.
*/
public void add(T node) {
add(node,defaultReplication);
}
/**
* Calls {@link #add(Object)} with all the arguments.
*/
public void addAll(T... nodes) {
for (T node : nodes)
add(node);
}
/**
* Calls {@link #add(Object)} with all the arguments.
*/
public void addAll(Collection<? extends T> nodes) {
for (T node : nodes)
add(node);
}
/**
* Removes the node entirely. This is the same as {@code add(node,0)}
*/
public void remove(T node) {
add(node,0);
}
/**
* Adds a new node with the given number of replica.
*
* <p>
* This is the only function that manipulates {@link #items}.
*/
public synchronized void add(T node, int replica) {
if(replica==0) {
items.remove(node);
} else {
Point[] points = new Point[replica];
String seed = hash.hash(node);
for (int i=0; i<replica; i++)
points[i] = new Point(md5(seed+':'+i),node);
items.put(node,points);
}
table = new Table();
}
/**
* Compresses a string into an integer with MD5.
*/
private int md5(String s) {
MD5 md5 = new MD5();
md5.update(s.getBytes());
byte[] digest = new byte[16];
md5.digest(digest);
// 16 bytes -> 4 bytes
for (int i=0; i<4; i++)
digest[i] ^= digest[i+4]+digest[i+8]+digest[i+12];
return (b2i(digest[0])<< 24)|(b2i(digest[1])<<16)|(b2i(digest[2])<< 8)|b2i(digest[3]);
}
/**
* unsigned byte->int.
*/
private int b2i(byte b) {
return ((int)b)&0xFF;
}
/**
* Looks up a consistent hash with the given data point.
*
* <p>
* The whole point of this class is that if the same query point is given,
* it's likely to return the same result even when other nodes are added/removed,
* or the # of replicas for the given node is changed.
*
* @return
* null if the consistent hash is empty. Otherwise always non-null.
*/
public T lookup(int queryPoint) {
return table.lookup(queryPoint);
}
/**
* Takes a string, hash it with MD5, then calls {@link #lookup(int)}.
*/
public T lookup(String queryPoint) {
return lookup(md5(queryPoint));
}
/**
* Creates a permutation of all the nodes for the given data point.
*
* <p>
* The returned pemutation is consistent, in the sense that small change
* to the consitent hash (like addition/removal/change of replicas) only
* creates a small change in the permutation.
*
* <p>
* Nodes with more replicas are more likely to show up early in the list
*/
public Iterable<T> list(final int queryPoint) {
return new Iterable<T>() {
public Iterator<T> iterator() {
return table.list(queryPoint);
}
};
}
/**
* Takes a string, hash it with MD5, then calls {@link #list(int)}.
*/
public Iterable<T> list(String queryPoint) {
return list(md5(queryPoint));
}
}