Dubbo负载均衡（一致性哈希算法）代码摘录

Dubbo负载均衡（一致性哈希算法）代码摘录

https://www.zsythink.net/archives/1182
https://hub.fastgit.org/apache/dubbo/blob/master/dubbo-cluster/src/main/java/org/apache/dubbo/rpc/cluster/loadbalance/ConsistentHashLoadBalance.java

package org.apache.dubbo.rpc.cluster.loadbalance;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.io.Bytes;
import org.apache.dubbo.rpc.Invocation;
import org.apache.dubbo.rpc.Invoker;
import org.apache.dubbo.rpc.support.RpcUtils;
import java.util.List;
import java.util.Map;
import java.util.TreeMap;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import static org.apache.dubbo.common.constants.CommonConstants.COMMA_SPLIT_PATTERN;


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

    
    public static final String HASH_NODES = "hash.nodes";

    
    public static final String HASH_ARGUMENTS = "hash.arguments";

    private final ConcurrentMap> selectors = new ConcurrentHashMap>();

    @SuppressWarnings("unchecked")
    @Override
    protected  Invoker doSelect(List> invokers, URL url, Invocation invocation) {
        String methodName = RpcUtils.getMethodName(invocation);
        String key = invokers.get(0).getUrl().getServiceKey() + "." + methodName;
        // using the hashcode of list to compute the hash only pay attention to the elements in the list
        int invokersHashCode = invokers.hashCode();
        ConsistentHashSelector selector = (ConsistentHashSelector) selectors.get(key);
        if (selector == null || selector.identityHashCode != invokersHashCode) {
            selectors.put(key, new ConsistentHashSelector(invokers, methodName, invokersHashCode));
            selector = (ConsistentHashSelector) selectors.get(key);
        }
        return selector.select(invocation);
    }

    private static final class ConsistentHashSelector {

        private final TreeMap> virtualInvokers;

        private final int replicaNumber;

        private final int identityHashCode;

        private final int[] argumentIndex;

        ConsistentHashSelector(List> invokers, String methodName, int identityHashCode) {
            this.virtualInvokers = new TreeMap>();
            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]);
            }
            // 设置replicaNumber数量的虚拟节点，以防止节点偏斜现象
            for (Invoker invoker : invokers) {
                String address = invoker.getUrl().getAddress();
                for (int i = 0; i < replicaNumber / 4; i++) {
                    byte[] digest = Bytes.getMD5(address + i);
                    for (int h = 0; h < 4; h++) {
                        long m = hash(digest, h);
                        virtualInvokers.put(m, invoker);
                    }
                }
            }
        }

        public Invoker select(Invocation invocation) {
            String key = toKey(invocation.getArguments());
            byte[] digest = Bytes.getMD5(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 selectForKey(long hash) {
        	// 返回给定key大于或等于的最小entry
            Map.Entry> 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;
        }
    }

}