哈希大杂烩

栏目: 数据库 · 发布时间: 5年前

内容简介：一致性哈希一般用来做分布式缓存将各服务器的哈希值映射到一个圆环上(一般用0~2^32-1)，缓存寻址时，将缓存的键取哈希值放到圆环上，然后顺时针寻找第一个服务器节点，用找到的这个节点做缓存服务器有缓存服务器5台，缓存键30个，分别对每个缓存键在服务器上寻址

一致性哈希一般用来做分布式缓存

将各服务器的哈希值映射到一个圆环上(一般用0~2^32-1)，缓存寻址时，将缓存的键取哈希值放到圆环上，然后顺时针寻找第一个服务器节点，用找到的这个节点做缓存服务器

Java 示例

有缓存服务器5台，缓存键30个，分别对每个缓存键在服务器上寻址

Java代码

package bj;

import lombok.RequiredArgsConstructor;
import org.apache.commons.codec.digest.DigestUtils;
import org.junit.Test;

import java.nio.ByteBuffer;
import java.util.TreeSet;
import java.util.stream.IntStream;

/**
 * Created by BaiJiFeiLong@gmail.com at 2018/12/6 下午2:11
 */
public class LinearHashingTest {

    @RequiredArgsConstructor
    static class Node implements Comparable<Node> {
        private final String name;

        long digest() {
            byte[] digest = DigestUtils.md5(name);
            return Integer.toUnsignedLong(ByteBuffer.wrap(digest, 0, 4).getInt());
        }

        @Override
        public int compareTo(Node o) {
            return Long.compare(this.digest(), o.digest());
        }

        @Override
        public String toString() {
            return String.format("Node(name=%s, digest=%d)", name, digest());
        }
    }

    @Test
    public void testAlpha() {
        TreeSet<Node> set = new TreeSet<>();
        for (int i : new int[]{1, 2, 3, 4, 5}) {
            set.add(new Node("server" + i));
        }

        System.out.println("Server nodes:");
        for (Node node : set) {
            System.out.println(node);
        }

        System.out.println("Routing...");
        IntStream.rangeClosed(1, 20).forEach(i -> {
            long hash = new Node("key" + i).digest();
            System.out.printf("key%d\t%d\t => %s\n", i, hash, route(set, hash));
        });

        IntStream.rangeClosed(1, 20).forEach(i -> {
            set.add(new Node("key" + i));
        });
        System.out.println("All nodes:");
        for (Node node : set) {
            System.out.println(node);
        }
    }

    private Node route(TreeSet<Node> nodes, long code) {
        for (Node node : nodes) {
            if (node.digest() > code) {
                return node;
            }
        }
        return nodes.first();
    }
}
``

控制台输出:

```log
Node(name=server3, digest=232443701)
Node(name=server2, digest=424647047)
Node(name=server4, digest=648772546)
Node(name=server5, digest=1375127456)
Node(name=server1, digest=2822999463)
Routing...
key1	3266172564	 => Node(name=server3, digest=232443701)
key2	2029528490	 => Node(name=server1, digest=2822999463)
key3	909203333	 => Node(name=server5, digest=1375127456)
key4	3405308747	 => Node(name=server3, digest=232443701)
key5	1034591130	 => Node(name=server5, digest=1375127456)
key6	1684928417	 => Node(name=server1, digest=2822999463)
key7	4187051213	 => Node(name=server3, digest=232443701)
key8	1564577213	 => Node(name=server1, digest=2822999463)
key9	215402731	 => Node(name=server3, digest=232443701)
key10	4045919512	 => Node(name=server3, digest=232443701)
key11	3930184853	 => Node(name=server3, digest=232443701)
key12	3610429654	 => Node(name=server3, digest=232443701)
key13	4149020260	 => Node(name=server3, digest=232443701)
key14	2302410740	 => Node(name=server1, digest=2822999463)
key15	1605175883	 => Node(name=server1, digest=2822999463)
key16	4248449615	 => Node(name=server3, digest=232443701)
key17	3126975152	 => Node(name=server3, digest=232443701)
key18	533962436	 => Node(name=server4, digest=648772546)
key19	1425581655	 => Node(name=server1, digest=2822999463)
key20	180948674	 => Node(name=server3, digest=232443701)
All nodes:
Node(name=key20, digest=180948674)
Node(name=key9, digest=215402731)
Node(name=server3, digest=232443701)
Node(name=server2, digest=424647047)
Node(name=key18, digest=533962436)
Node(name=server4, digest=648772546)
Node(name=key3, digest=909203333)
Node(name=key5, digest=1034591130)
Node(name=server5, digest=1375127456)
Node(name=key19, digest=1425581655)
Node(name=key8, digest=1564577213)
Node(name=key15, digest=1605175883)
Node(name=key6, digest=1684928417)
Node(name=key2, digest=2029528490)
Node(name=key14, digest=2302410740)
Node(name=server1, digest=2822999463)
Node(name=key17, digest=3126975152)
Node(name=key1, digest=3266172564)
Node(name=key4, digest=3405308747)
Node(name=key12, digest=3610429654)
Node(name=key11, digest=3930184853)
Node(name=key10, digest=4045919512)
Node(name=key13, digest=4149020260)
Node(name=key7, digest=4187051213)
Node(name=key16, digest=4248449615)

文章首发: https://baijifeilong.github.io

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