如何理解DependenciesBasedLoadBalancer
这篇文章给大家介绍如何理解DependenciesBasedLoadBalancer，内容非常详细，感兴趣的小伙伴们可以参考借鉴，希望对大家能有所帮助。
序
本文主要研究一下DependenciesBasedLoadBalancer
DependenciesBasedLoadBalancer
spring-cloud-zookeeper-discovery-2.1.2.RELEASE-sources.jar!/org/springframework/cloud/zookeeper/discovery/dependency/DependenciesBasedLoadBalancer.java
public class DependenciesBasedLoadBalancer extends DynamicServerListLoadBalancer { private static final Log log = LogFactory.getLog(DependenciesBasedLoadBalancer.class); private final Map ruleCache = new ConcurrentHashMap<>(); private final ZookeeperDependencies zookeeperDependencies; public DependenciesBasedLoadBalancer(ZookeeperDependencies zookeeperDependencies, ServerList serverList, IClientConfig config, IPing iPing) { super(config); this.zookeeperDependencies = zookeeperDependencies; setServersList(serverList.getInitialListOfServers()); setPing(iPing); setServerListImpl(serverList); } @Override public Server chooseServer(Object key) { String keyAsString; if ("default".equals(key)) { // this is the default hint, use name instead keyAsString = getName(); } else { keyAsString = (String) key; } ZookeeperDependency dependency = this.zookeeperDependencies .getDependencyForAlias(keyAsString); log.debug(String.format("Current dependencies are [%s]", this.zookeeperDependencies)); if (dependency == null) { log.debug(String.format( "No dependency found for alias [%s] - will use the default rule which is [%s]", keyAsString, this.rule)); return this.rule.choose(key); } cacheEntryIfMissing(keyAsString, dependency); log.debug(String.format( "Will try to retrieve dependency for key [%s]. Current cache contents [%s]", keyAsString, this.ruleCache)); updateListOfServers(); return this.ruleCache.get(keyAsString).choose(key); } private void cacheEntryIfMissing(String keyAsString, ZookeeperDependency dependency) { if (!this.ruleCache.containsKey(keyAsString)) { log.debug(String.format("Cache doesn't contain entry for [%s]", keyAsString)); this.ruleCache.put(keyAsString, chooseRuleForLoadBalancerType(dependency.getLoadBalancerType())); } } private IRule chooseRuleForLoadBalancerType(LoadBalancerType type) { switch (type) { case ROUND_ROBIN: return getRoundRobinRule(); case RANDOM: return getRandomRule(); case STICKY: return getStickyRule(); default: throw new IllegalArgumentException("Unknown load balancer type " + type); } } private RoundRobinRule getRoundRobinRule() { return new RoundRobinRule(this); } private IRule getRandomRule() { RandomRule randomRule = new RandomRule(); randomRule.setLoadBalancer(this); return randomRule; } private IRule getStickyRule() { StickyRule stickyRule = new StickyRule(getRoundRobinRule()); stickyRule.setLoadBalancer(this); return stickyRule; }} DependenciesBasedLoadBalancer继承了com.netflix.loadbalancer.DynamicServerListLoadBalancer
其chooseServer方法会使用zookeeperDependencies.getDependencyForAlias来跟进key获取ZookeeperDependency，如果dependency为null则直接使用rule.choose(key)，不为null则进行chahe，然后更新server列表，最后通过ruleCache.get(keyAsString).choose(key)返回
cacheEntryIfMissing方法会根据ZookeeperDependency.getLoadBalancerType()进行chooseRuleForLoadBalancerType，这里分为了ROUND_ROBIN、RANDOM、STICKY三种
StickyRule
spring-cloud-zookeeper-discovery-2.1.2.RELEASE-sources.jar!/org/springframework/cloud/zookeeper/discovery/dependency/StickyRule.java
public class StickyRule extends AbstractLoadBalancerRule { private static final Log log = LogFactory.getLog(StickyRule.class); private final IRule masterStrategy; private final AtomicReference ourInstance = new AtomicReference<>(null); private final AtomicInteger instanceNumber = new AtomicInteger(-1); public StickyRule(IRule masterStrategy) { this.masterStrategy = masterStrategy; } @Override public void initWithNiwsConfig(IClientConfig iClientConfig) { } @Override public Server choose(Object key) { final List instances = getLoadBalancer().getServerList(true); log.debug(String.format("Instances taken from load balancer [%s]", instances)); Server localOurInstance = this.ourInstance.get(); log.debug(String.format("Current saved instance [%s]", localOurInstance)); if (!instances.contains(localOurInstance)) { this.ourInstance.compareAndSet(localOurInstance, null); } if (this.ourInstance.get() == null) { Server instance = this.masterStrategy.choose(key); if (this.ourInstance.compareAndSet(null, instance)) { this.instanceNumber.incrementAndGet(); } } return this.ourInstance.get(); } /** * Each time a new instance is picked, an internal counter is incremented. This way * you can track when/if the instance changes. The instance can change when the * selected instance is not in the current list of instances returned by the instance * provider * @return instance number */ public int getInstanceNumber() { return this.instanceNumber.get(); }} StickyRule继承了com.netflix.loadbalancer.AbstractLoadBalancerRule；其choose方法首先通过getLoadBalancer().getServerList(true)获取server列表，对于该列表没有localOurInstance的，则更新本地引用为null；然后判断localOurInstance是否为null，为null的话则使用masterStrategy.choose(key)进行选择然后更新；最后返回ourInstance.get()
小结
DependenciesBasedLoadBalancer继承了com.netflix.loadbalancer.DynamicServerListLoadBalancer
其chooseServer方法会使用zookeeperDependencies.getDependencyForAlias来跟进key获取ZookeeperDependency，如果dependency为null则直接使用rule.choose(key)，不为null则进行chahe，然后更新server列表，最后通过ruleCache.get(keyAsString).choose(key)返回
cacheEntryIfMissing方法会根据ZookeeperDependency.getLoadBalancerType()进行chooseRuleForLoadBalancerType，这里分为了ROUND_ROBIN、RANDOM、STICKY三种
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