聊聊eureka的ZoneAffinityServerListFilter

序

本文主要研究一下eureka的ZoneAffinityServerListFilter

ZoneAffinityServerListFilter

ribbon-loadbalancer-2.2.5-sources.jar!/com/netflix/loadbalancer/ZoneAffinityServerListFilter.java

/**
 * This server list filter deals with filtering out servers based on the Zone affinity. 
 * This filtering will be turned on if either {@link CommonClientConfigKey#EnableZoneAffinity} 
 * or {@link CommonClientConfigKey#EnableZoneExclusivity} is set to true in {@link IClientConfig} object
 * passed into this class during initialization. When turned on, servers outside the same zone (as 
 * indicated by {@link Server#getZone()}) will be filtered out. By default, zone affinity 
 * and exclusivity are turned off and nothing is filtered out.
 * 
 * @author stonse
 *
 */
public class ZoneAffinityServerListFilter<T extends Server> extends
        AbstractServerListFilter<T> implements IClientConfigAware {
     //......
    @Override
    public List<T> getFilteredListOfServers(List<T> servers) {
        if (zone != null && (zoneAffinity || zoneExclusive) && servers !=null && servers.size() > 0){
            List<T> filteredServers = Lists.newArrayList(Iterables.filter(
                    servers, this.zoneAffinityPredicate.getServerOnlyPredicate()));
            if (shouldEnableZoneAffinity(filteredServers)) {
                return filteredServers;
            } else if (zoneAffinity) {
                overrideCounter.increment();
            }
        }
        return servers;
    }   
}

能够看到这里首先调用了zoneAffinityPredicate.getServerOnlyPredicate()进行过滤；而后再调用shouldEnableZoneAffinity判断是否真的须要返回过滤后的数据。

ZoneAffinityPredicate

ribbon-loadbalancer-2.2.5-sources.jar!/com/netflix/loadbalancer/ZoneAffinityPredicate.java

/**
 * A predicate the filters out servers that are not in the same zone as the client's current
 * zone. The current zone is determined from the call
 * 
 * <pre>{@code
 * ConfigurationManager.getDeploymentContext().getValue(ContextKey.zone);
 * }</pre>
 * 
 * @author awang
 *
 */
public class ZoneAffinityPredicate extends AbstractServerPredicate {

    private final String zone = ConfigurationManager.getDeploymentContext().getValue(ContextKey.zone);
    
    public ZoneAffinityPredicate() {        
    }

    @Override
    public boolean apply(PredicateKey input) {
        Server s = input.getServer();
        String az = s.getZone();
        if (az != null && zone != null && az.toLowerCase().equals(zone.toLowerCase())) {
            return true;
        } else {
            return false;
        }
    }
}

能够看到这里对server的zone进行判断，相同的zone才选取出来。

shouldEnableZoneAffinity

private boolean shouldEnableZoneAffinity(List<T> filtered) {    
        if (!zoneAffinity && !zoneExclusive) {
            return false;
        }
        if (zoneExclusive) {
            return true;
        }
        LoadBalancerStats stats = getLoadBalancerStats();
        if (stats == null) {
            return zoneAffinity;
        } else {
            logger.debug("Determining if zone affinity should be enabled with given server list: {}", filtered);
            ZoneSnapshot snapshot = stats.getZoneSnapshot(filtered);
            double loadPerServer = snapshot.getLoadPerServer();
            int instanceCount = snapshot.getInstanceCount();            
            int circuitBreakerTrippedCount = snapshot.getCircuitTrippedCount();
            if (((double) circuitBreakerTrippedCount) / instanceCount >= blackOutServerPercentageThreshold.get() 
                    || loadPerServer >= activeReqeustsPerServerThreshold.get()
                    || (instanceCount - circuitBreakerTrippedCount) < availableServersThreshold.get()) {
                logger.debug("zoneAffinity is overriden. blackOutServerPercentage: {}, activeReqeustsPerServer: {}, availableServers: {}", 
                        new Object[] {(double) circuitBreakerTrippedCount / instanceCount,  loadPerServer, instanceCount - circuitBreakerTrippedCount});
                return false;
            } else {
                return true;
            }
            
        }
    }

这里进行判断，若是目标zone的server统计数据不是太好，达到断路的标准，则不会返回该zone的server。

ZonePreferenceServerListFilter

spring-cloud-netflix-ribbon-2.0.0.RC1-sources.jar!/org/springframework/cloud/netflix/ribbon/ZonePreferenceServerListFilter.java

/**
 * A filter that actively prefers the local zone (as defined by the deployment context, or
 * the Eureka instance metadata).
 *
 * @author Dave Syer
 */
public class ZonePreferenceServerListFilter extends ZoneAffinityServerListFilter<Server> {
    //......
    @Override
    public List<Server> getFilteredListOfServers(List<Server> servers) {
        List<Server> output = super.getFilteredListOfServers(servers);
        if (this.zone != null && output.size() == servers.size()) {
            List<Server> local = new ArrayList<>();
            for (Server server : output) {
                if (this.zone.equalsIgnoreCase(server.getZone())) {
                    local.add(server);
                }
            }
            if (!local.isEmpty()) {
                return local;
            }
        }
        return output;
    }
}

Spring Cloud的ZonePreferenceServerListFilter继承了eureka的ZoneAffinityServerListFilter，重写了getFilteredListOfServers方法，即eureka的ZoneAffinityServerListFilter计算出来没有根据zone过滤的话，那么它会再过滤一次，选出跟实例相同zone的server。注意这里进行了判断，如根据zone过滤出来为空，则返回父类过滤出来server，即再也不根据zone进行过滤。

小结

eureka提供了ZoneAffinityServerListFilter，能够对server进行zone亲和性过滤，同时还会根据server的健康统计判断是否须要使用进行zone亲和过滤后的server，若是不该该开启，则返回没有过滤的server列表。好比没有跟该实例相同zone的server列表，那么很明显就是不该该返回根据zone进行过滤后的空列表。

doc

• HA and Zone Affinity with Spring Cloud Netflix Eureka