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]

Environment

• Dubbo version: 2.7.4.1

为了更好的研究一致性hash负载均衡策略，我自定义了一个SPI。
whyConsistentHash=com.example.dubbo.loadbalance.provider.loadbalance.WhyConsistentHashLoadBalance

我加入了几行输出代码如下:

 @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) {
             //我加的
            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);
        }
       //我加的
        for (Map.Entry<Long, Invoker<T>> entry : selector.virtualInvokers.entrySet()) {
            System.out.println("key=" + entry.getKey() + ",value=" + entry.getValue());
        }
        return selector.select(invocation);
    }

同时为了演示方便，我调整了虚拟节点数为4:
<dubbo:parameter key="hash.nodes" value="4"></dubbo:parameter>

这样就是2个服务端，

启动两个服务端。这样就是2个服务端，一个服务端有4个虚拟节点，共计8个节点。

客户端调用如下:

    @Test
    public void testLeastActiveLoadBalance() {
        for (int i = 0; i < 5; i++) {
            demoService.sayHello("公众号why技术-" + i);
        }
    }

最后输出结果为:

是新的invoker :418299920,原:null
key=447653163,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=488050138,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=724509126,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=2033828652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=2496774652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=3155964929,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4068105014,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4223326152,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
是新的invoker :105393903,原:418299920
key=447653163,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=488050138,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=724509126,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=2033828652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=2496774652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=3155964929,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4068105014,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4223326152,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
是新的invoker :533698361,原:105393903
key=447653163,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=488050138,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=724509126,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=2033828652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=2496774652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=3155964929,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4068105014,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4223326152,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
是新的invoker :1531545666,原:533698361
key=447653163,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=488050138,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=724509126,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=2033828652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=2496774652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=3155964929,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4068105014,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4223326152,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
是新的invoker :481525476,原:1531545666
key=447653163,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=488050138,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=724509126,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=2033828652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=2496774652,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@67c119b7
key=3155964929,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4068105014,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed
key=4223326152,value=org.apache.dubbo.registry.integration.RegistryDirectory$InvokerDelegate@2ca5f1ed

在过程中并没有服务上下线的操作，为什么5次调用，每次都会重新进行hash环的映射。而且从输出可以看出，每次映射的结果是一样的。

我看了invokers在这五次调用的时候地址值确实不一样。但是我不明白为什么？求解答。

[thisiswanghy]

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();
        }

    }

}