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24Mar 2023

实现有限负载一致性哈希

618 words - 4 mins

文章具有负载边界的一致性哈希讨论了具有负载边界的一致性哈希解决的问题、算法原理等理论,本文借助 GPT 谈谈如何 C# 代码实现。

若通过 C# 代码实现具有有界负载的一致哈希,可使用 MurmurHash3 哈希函数为系统中的每个节点生成哈希值,然后将每个键映射到其哈希值最接近键的哈希值的节点。下面是 GPT 提供的一个实现示例。

有界负载一致性哈希实现 C# 代码:

using System;
using System.Collections.Generic;

public class BoundedLoadConsistentHash<T>
{
    private readonly Func<T, uint> _hashFunction;
    private readonly SortedDictionary<uint, T> _ring = new SortedDictionary<uint, T>();
    private readonly Dictionary<T, int> _loadMap = new Dictionary<T, int>();
    private readonly int _maxLoad;

    public BoundedLoadConsistentHash(Func<T, uint> hashFunction, int maxLoad)
    {
        _hashFunction = hashFunction;
        _maxLoad = maxLoad;
    }

    public void Add(T node)
    {
        for (int i = 0; i < _maxLoad; i++)
        {
            uint hash = MurmurHash3.Hash(node.GetHashCode() + i);
            _ring[hash] = node;
            _loadMap[node] = i + 1;
        }
    }

    public void Remove(T node)
    {
        for (int i = 0; i < _maxLoad; i++)
        {
            uint hash = MurmurHash3.Hash(node.GetHashCode() + i);
            _ring.Remove(hash);
            _loadMap.Remove(node);
        }
    }

    public T GetNode(string key)
    {
        if (_ring.Count == 0)
        {
            throw new InvalidOperationException("No nodes available");
        }

        uint hash = MurmurHash3.Hash(key);
        foreach (var nodeHash in _ring.Keys)
        {
            if (hash <= nodeHash)
            {
                return _ring[nodeHash];
            }
        }

        return _ring[_ring.Keys[0]];
    }

    public int GetLoad(T node)
    {
        return _loadMap[node];
    }
}

MurmurHash3 哈希函数实现 C# 代码:

public static class MurmurHash3
{
    private const uint Seed = 0xc58f1a7b;

    public static uint Hash(int input)
    {
        return Hash(BitConverter.GetBytes(input));
    }

    public static uint Hash(string input)
    {
        return Hash(System.Text.Encoding.UTF8.GetBytes(input));
    }

    private static uint Hash(byte[] input)
    {
        const uint c1 = 0xcc9e2d51;
        const uint c2 = 0x1b873593;
        const int r1 = 15;
        const int r2 = 13;
        const uint m = 5;
        const uint n = 0xe6546b64;

        uint hash = Seed;

        int len = input.Length / 4;
        for (int i = 0; i < len; i++)
        {
            uint k = BitConverter.ToUInt32(input, i * 4);

            k *= c1;
            k = (k << r1) | (k >> (32 - r1));
            k *= c2;

            hash ^= k;
            hash = (hash << r2) | (hash >> (32 - r2));
            hash = hash * m + n;
        }

        uint remainingBytes = (uint)(input.Length & 3);
        if (remainingBytes > 0)
        {
            uint k = 0;
            for (int i = 0; i < remainingBytes; i++)
            {
                k <<= 8;
                k |= input[input.Length - remainingBytes + i];
            }

            k *= c1;
            k = (k << r1) | (k >> (32 - r1));
            k *= c2;

            hash ^= k;
        }

        hash ^= (uint)input.Length;
        hash ^= hash >> 16;
        hash *= 0x85ebca6b;
        hash ^= hash >> 13;
        hash *= 0xc2b2ae35;
        hash ^= hash >> 16;
        return hash;
    }
}

使用示例:

class Program
{
    static void Main(string[] args)
    {
        BoundedLoadConsistentHash<string> ch = new BoundedLoadConsistentHash<string>(MurmurHash3.Hash, 3);
        ch.Add("node1");
        ch.Add("node2");
        ch.Add("node3");

        string key = "example_key";
        string node = ch.GetNode(key);
        int load = ch.GetLoad(node);

        Console.WriteLine("Key: {0}, Node: {1}, Load: {2}", key, node, load);
    }
}

在上面的代码示例中,创建了一个 ConsistentHash 实例,并向其添加了三个节点。然后,使用 GetNode 方法获取一个 key 所属的节点,并使用 GetLoad 方法获取该节点的负载。默认情况下,每个节点最大负载为 3,因此当负载达到 3 时,不会再向该节点分配更多的 key。