负载均衡策略PredicateBasedRule

小知识，大挑战！本文正在参与「程序员必备小知识」创作活动

19负载均衡策略PredicateBasedRule

PredicateBasedRule：

/*
 *
 * Copyright 2013 Netflix, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */
package com.netflix.loadbalancer;

import com.google.common.base.Optional;

/**
 * A rule which delegates the server filtering logic to an instance of {@link AbstractServerPredicate}.
 * After filtering, a server is returned from filtered list in a round robin fashion.
 * 
 * 
 * @author awang
 *
 */
public abstract class PredicateBasedRule extends ClientConfigEnabledRoundRobinRule {
   
    /**
     * Method that provides an instance of {@link AbstractServerPredicate} to be used by this class.
     * 
     */
    public abstract AbstractServerPredicate getPredicate();
        
    /**
     * Get a server by calling {@link AbstractServerPredicate#chooseRandomlyAfterFiltering(java.util.List, Object)}.
     * The performance for this method is O(n) where n is number of servers to be filtered.
     */
    @Override
    public Server choose(Object key) {
        ILoadBalancer lb = getLoadBalancer();
        Optional<Server> server = getPredicate().chooseRoundRobinAfterFiltering(lb.getAllServers(), key);
        if (server.isPresent()) {
            return server.get();
        } else {
            return null;
        }       
    }
}

继承ClientConfigEnabledRoundRobinRule，先通过chooseRoundRobinAfterFiltering来选出具体的服务实例

public Optional<Server> chooseRoundRobinAfterFiltering(List<Server> servers, Object loadBalancerKey) {
    List<Server> eligible = getEligibleServers(servers, loadBalancerKey);
    if (eligible.size() == 0) {
        return Optional.absent();
    }
    return Optional.of(eligible.get(nextIndex.getAndIncrement() % eligible.size()));
}

先通过getEligibleServers获取备选的实例清单，如果返回的清单为空，则用Optional.absent()表示不存在，反正以线性轮询方式从备选清单中获取实例

public List<Server> getEligibleServers(List<Server> servers, Object loadBalancerKey) {
    if (loadBalancerKey == null) {
        return ImmutableList.copyOf(Iterables.filter(servers, this.getServerOnlyPredicate()));            
    } else {
        List<Server> results = Lists.newArrayList();
        for (Server server: servers) {
            if (this.apply(new PredicateKey(loadBalancerKey, server))) {
                results.add(server);
            }
        }
        return results;            
    }
}

没有apply方法的具体实现，而是定义了一个先过滤后轮询选择的模板，对于如何过滤，需要通过AbstractServerPredicate的子类中实现appl方法来确定具体的过滤策略。