[GitHub] [dubbo] thisiswanghy commented on issue #5424: 为什么即使在服务端没有上下线操作的时候一致性hash负载均衡算法每次都需要重新进行hash环的映射？

["thisiswanghy (GitHub)" <gi...@apache.org>]

自定义的负载均衡算法和Dubbo 2.7.4.1 版本的一样，除了加了几句输出语句：
```
public class WhyConsistentHashLoadBalance extends AbstractLoadBalance {
    public static final String NAME = "consistenthash";

    /**
     * Hash nodes name
     */
    public static final String HASH_NODES = "hash.nodes";

    /**
     * Hash arguments name
     */
    public static final String HASH_ARGUMENTS = "hash.arguments";

    private final ConcurrentMap<String, WhyConsistentHashLoadBalance.ConsistentHashSelector<?>> selectors = new ConcurrentHashMap<String, WhyConsistentHashLoadBalance.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;
        int identityHashCode = System.identityHashCode(invokers);
        WhyConsistentHashLoadBalance.ConsistentHashSelector<T> selector = (WhyConsistentHashLoadBalance.ConsistentHashSelector<T>) selectors.get(key);
        if (selector == null || selector.identityHashCode != identityHashCode) {
            System.out.println("是新的invoker :" + identityHashCode + ",原:" + (selector == null ? "null" : selector.identityHashCode));
            selectors.put(key, new WhyConsistentHashLoadBalance.ConsistentHashSelector<T>(invokers, methodName, identityHashCode));
            selector = (WhyConsistentHashLoadBalance.ConsistentHashSelector<T>) selectors.get(key);
        }
        for (Map.Entry<Long, Invoker<T>> entry : selector.virtualInvokers.entrySet()) {
            System.out.println("key=" + entry.getKey() + ",value=" + entry.getValue());
        }
        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) {
            this.virtualInvokers = new TreeMap<Long, Invoker<T>>();
            this.identityHashCode = identityHashCode;
            URL url = invokers.get(0).getUrl();
            this.replicaNumber = url.getMethodParameter(methodName, HASH_NODES, 160);
            String[] index = COMMA_SPLIT_PATTERN.split(url.getMethodParameter(methodName, HASH_ARGUMENTS, "0"));
            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();
                for (int i = 0; i < replicaNumber / 4; i++) {
                    byte[] digest = md5(address + i);
                    for (int h = 0; h < 4; h++) {
                        long m = hash(digest, h);
                        virtualInvokers.put(m, invoker);
                    }
                }
            }
        }

        public Invoker<T> select(Invocation invocation) {
            String key = toKey(invocation.getArguments());
            byte[] digest = md5(key);
            return selectForKey(hash(digest, 0));
        }

        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();
        }

        private Invoker<T> selectForKey(long hash) {
            Map.Entry<Long, Invoker<T>> entry = virtualInvokers.ceilingEntry(hash);
            if (entry == null) {
                entry = virtualInvokers.firstEntry();
            }
            return entry.getValue();
        }

        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;
        }

        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 = value.getBytes(StandardCharsets.UTF_8);
            md5.update(bytes);
            return md5.digest();
        }

    }

}
```

[ Full content available at: https://github.com/apache/dubbo/issues/5424 ]
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