前言

之前講了 一致性hash演算法的原理，現在擼程式碼。

1. 環的資料結構，可能首先想到的是常用的List。一致性hash需要找比當前元素大的節點，那麼list要麼排好序然後比較找到對應的節點，要麼是不排序直接遍歷算差值。顯然在這種查詢比較頻繁的場景樹結構更適合，所以我們用有序的樹結構TreeMap。
2. 節點的hash衝突時我們再取一次hash值

/**一致性hash
 * Created by szk on 2018/9/10.
 */
public class ConsistentHashWithVirtualNode {

    /**
     * 待新增入Hash環的伺服器列表
     */
    private static String[] SERVERS = {"192.168.1.2:6379", "192.168.1.3:6379", "192.168.1.4:6379"};

    /**
     * key表示伺服器的hash值，value表示虛擬節點的名稱
     */
    private static SortedMap<Integer, String> HASH_CIRCLE = new TreeMap<Integer, String>();
    /**
     * 用於結果統計
     */
    private static Map<String,Integer> result = new HashMap<String,Integer>();
    /**
     * 每個真實節點對應虛擬節點數
     */
    private static Integer VIRTUAL_NODES_NUM = 150;

    /**
     * 使用FNV1_32_HASH演算法計算伺服器的Hash值
     */
    private static int getHash(String str) {
        final int p = 16777619;
        int hash = (int) 2166136261L;
        for (int i = 0; i < str.length(); i++)
            hash = (hash ^ str.charAt(i)) * p;
        hash += hash << 13;
        hash ^= hash >> 7;
        hash += hash << 3;
        hash ^= hash >> 17;
        hash += hash << 5;

        // 如果值為負數則取其絕對值
        if (hash < 0)
            hash = Math.abs(hash);
        return hash;
    }

    static {
        for (int i = 0; i < SERVERS.length; i++)
        {
            for (Integer j = 0; j < VIRTUAL_NODES_NUM; j++) {
                setServer(SERVERS[i]+"vn"+j);
            }
        }
    }

    private static void setServer(String ip){
        setServer(ip,null);
    }


    private static void setServer(String ip, Integer hash){
        hash = hash != null ?getHash(hash.toString()) : getHash(ip);
        if(StringUtils.isBlank(HASH_CIRCLE.get(hash))){
            HASH_CIRCLE.put(hash, ip);
            System.out.println("[" + ip + "]加入sortedMap中, 其Hash值為" + hash);
        }else {
            //解決hash碰撞
            setServer(ip,hash);
        }
    }


    public static void main(String[] args) {
        for (int i = 0; i < 50; i++) {
            long nodes = RandomUtils.nextLong();
            String server = getServer(nodes);
            String realServer = server.split("vn")[0];
            System.out.println("[" + nodes + "]的hash值為"+ getHash(""+nodes) + ", 被路由到虛擬結點[" + server+ "], 真實結點[" + realServer + "]");
            result.put(realServer,(result.get(realServer)==null?0:result.get(realServer))+1);
        }
        result.forEach((k,v)->{
            System.out.println("結點["+k+"]上有"+v);
        });
    }

    public static String getServer(Object node) {
        String ip = HASH_CIRCLE.get(HASH_CIRCLE.firstKey());
        // 得到帶路由的結點的Hash值
        int hash = getHash(node.toString());
        // 得到大於該Hash值的所有Map
        SortedMap<Integer, String> subMap = HASH_CIRCLE.tailMap(hash);

        if(!subMap.isEmpty()){
            // 第一個Key就是順時針過去離node最近的那個結點
            Integer i = subMap.firstKey();
            ip = subMap.get(i);
        }
        // 返回對應的伺服器名稱
        return ip;
    }
}
 

150個虛擬節點，50個元素去匹配，結果如下

[4506423623011963877]的hash值為2076651453, 被路由到虛擬結點[192.168.1.2:6379vn31], 真實結點[192.168.1.2:6379]
[5570050191736927528]的hash值為852645907, 被路由到虛擬結點[192.168.1.3:6379vn3], 真實結點[192.168.1.3:6379]
[8129699418679179907]的hash值為487796751, 被路由到虛擬結點[192.168.1.2:6379vn40], 真實結點[192.168.1.2:6379]
[477939724802787766]的hash值為1916940123, 被路由到虛擬結點[192.168.1.2:6379vn139], 真實結點[192.168.1.2:6379]
[2237405063640190973]的hash值為1033214343, 被路由到虛擬結點[192.168.1.2:6379vn65], 真實結點[192.168.1.2:6379]
[3378009287555310246]的hash值為1664158122, 被路由到虛擬結點[192.168.1.3:6379vn123], 真實結點[192.168.1.3:6379]
[8357430545483250147]的hash值為1498249592, 被路由到虛擬結點[192.168.1.2:6379vn66], 真實結點[192.168.1.2:6379]
[6575662240925929638]的hash值為299745323, 被路由到虛擬結點[192.168.1.2:6379vn147], 真實結點[192.168.1.2:6379]
[5834994408022173056]的hash值為1060475722, 被路由到虛擬結點[192.168.1.4:6379vn118], 真實結點[192.168.1.4:6379]
[1553332217396956129]的hash值為401533392, 被路由到虛擬結點[192.168.1.3:6379vn122], 真實結點[192.168.1.3:6379]
[7171179586727756331]的hash值為1578420313, 被路由到虛擬結點[192.168.1.2:6379vn81], 真實結點[192.168.1.2:6379]
[72466124520548464]的hash值為1891754379, 被路由到虛擬結點[192.168.1.4:6379vn10], 真實結點[192.168.1.4:6379]
[4099801649503227100]的hash值為618254540, 被路由到虛擬結點[192.168.1.2:6379vn61], 真實結點[192.168.1.2:6379]
[7654612135449452049]的hash值為1208151474, 被路由到虛擬結點[192.168.1.2:6379vn63], 真實結點[192.168.1.2:6379]
結點[192.168.1.2:6379]上有18
結點[192.168.1.3:6379]上有15
結點[192.168.1.4:6379]上有17