Dubbo 的负载均衡策略：一致性哈希策略

public class ConsistentHashLoadBalance extends AbstractLoadBalance {
    public static final String NAME = "consistenthash";

    // 记录所有
    private final ConcurrentMap<String, ConsistentHashSelector<?>> selectors = new ConcurrentHashMap<String, ConsistentHashSelector<?>>();

    @SuppressWarnings("unchecked")
    @Override
    protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
        String methodName = RpcUtils.getMethodName(invocation);
        String key = invokers.get(0).getUrl().getServiceKey() + "." + methodName;
        // 计算哈希值 key，通过这个字段来识别出调用实例是否有变化，有变化则重新创建 ConsistentHashSelector
        int identityHashCode = System.identityHashCode(invokers);
        ConsistentHashSelector<T> selector = (ConsistentHashSelector<T>) selectors.get(key);
        if (selector == null || selector.identityHashCode != identityHashCode) {
            // 没有存在 selector 或者 invokers 实例有变化，重新创建
            selectors.put(key, new ConsistentHashSelector<T>(invokers, methodName, identityHashCode));
            selector = (ConsistentHashSelector<T>) selectors.get(key);
        }
        return selector.select(invocation);
    }

    private static final class ConsistentHashSelector<T> {

        private final TreeMap<Long, Invoker<T>> virtualInvokers;

        private final int replicaNumber;

        private final int identityHashCode;

        private final int[] argumentIndex;

        ConsistentHashSelector(List<Invoker<T>> invokers, String methodName, int identityHashCode) {
            // 虚拟节点
            this.virtualInvokers = new TreeMap<Long, Invoker<T>>();
            this.identityHashCode = identityHashCode;
            URL url = invokers.get(0).getUrl();
            // 虚拟节点数量
            this.replicaNumber = url.getMethodParameter(methodName, "hash.nodes", 160);
            // 需要哈希的参数，默认是第一个参数
            String[] index = Constants.COMMA_SPLIT_PATTERN.split(url.getMethodParameter(methodName, "hash.arguments", "0"));
            // 记录需要哈希的参数的下标
            argumentIndex = new int[index.length];
            for (int i = 0; i < index.length; i++) {
                argumentIndex[i] = Integer.parseInt(index[i]);
            }
            for (Invoker<T> invoker : invokers) {
                String address = invoker.getUrl().getAddress();
                // 每一个实例存 replicaNumber 份虚拟节点
                for (int i = 0; i < replicaNumber / 4; i++) {
                    // 先将虚拟节点分为 (replicaNumber/4) 份，先做 MD5
                    byte[] digest = md5(address + i);
                    for (int h = 0; h < 4; h++) {
                        //再做哈希计算，得到 m，作为 Key
                        long m = hash(digest, h);
                        // 存到虚拟节点
                        virtualInvokers.put(m, invoker);
                    }
                }
            }
        }

        public Invoker<T> select(Invocation invocation) {
            // 得出要进行哈希的字符串
            String key = toKey(invocation.getArguments());
            // 进行 MD5
            byte[] digest = md5(key);
            return selectForKey(hash(digest, 0));
        }

        // 根据 hash.arguments 配置的对多少个参数进行哈希，得出拼接后的字符串
        private String toKey(Object[] args) {
            StringBuilder buf = new StringBuilder();
            for (int i : argumentIndex) {
                if (i >= 0 && i < args.length) {
                    buf.append(args[i]);
                }
            }
            return buf.toString();
        }

        // 选择实例
        private Invoker<T> selectForKey(long hash) {
            // 找出大于或等于 hash 中最小的键值对
            Map.Entry<Long, Invoker<T>> entry = virtualInvokers.ceilingEntry(hash);
            if (entry == null) {
                // 找出最小的键值对
                entry = virtualInvokers.firstEntry();
            }
            return entry.getValue();
        }

        // 计算哈希值
        private long hash(byte[] digest, int number) {
            return (((long) (digest[3 + number * 4] & 0xFF) << 24)
                    | ((long) (digest[2 + number * 4] & 0xFF) << 16)
                    | ((long) (digest[1 + number * 4] & 0xFF) << 8)
                    | (digest[number * 4] & 0xFF))
                    & 0xFFFFFFFFL;
        }

        // MD5 加密
        private byte[] md5(String value) {
            MessageDigest md5;
            try {
                md5 = MessageDigest.getInstance("MD5");
            } catch (NoSuchAlgorithmException e) {
                throw new IllegalStateException(e.getMessage(), e);
            }
            md5.reset();
            byte[] bytes;
            try {
                bytes = value.getBytes("UTF-8");
            } catch (UnsupportedEncodingException e) {
                throw new IllegalStateException(e.getMessage(), e);
            }
            md5.update(bytes);
            return md5.digest();
        }

    }

}