聊聊eureka的ZoneAffinityServerListFilter

/**
 * 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;
    }   
}

/**
 * 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;
        }
    }
}

    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;
            }

        }
    }

/**
 * 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;
    }
}