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本文主要研究一下eureka的preferSameZoneEureka参数

配置

eureka.client.prefer-same-zone-eureka

    {
      "sourceType": "org.springframework.cloud.netflix.eureka.EurekaClientConfigBean",
      "defaultValue": true,
      "name": "eureka.client.prefer-same-zone-eureka",
      "description": "Indicates whether or not this instance should try to use the eureka server in the\n same zone for latency and\/or other reason.\n\n Ideally eureka clients are configured to talk to servers in the same zone\n\n The changes are effective at runtime at the next registry fetch cycle as specified\n by registryFetchIntervalSeconds",
      "type": "java.lang.Boolean"
    }

EurekaClientConfigBean

spring-cloud-netflix-eureka-client-2.0.0.RC1-sources.jar!/org/springframework/cloud/netflix/eureka/EurekaClientConfigBean.java

@ConfigurationProperties(EurekaClientConfigBean.PREFIX)
public class EurekaClientConfigBean implements EurekaClientConfig {
    /**
     * Indicates whether or not this instance should try to use the eureka server in the
     * same zone for latency and/or other reason.
     *
     * Ideally eureka clients are configured to talk to servers in the same zone
     *
     * The changes are effective at runtime at the next registry fetch cycle as specified
     * by registryFetchIntervalSeconds
     */
    private boolean preferSameZoneEureka = true;

    public boolean shouldPreferSameZoneEureka() {
        return this.preferSameZoneEureka;
    }
    //......
}

使用

EndpointUtils.getDiscoveryServiceUrls

eureka-client-1.8.8-sources.jar!/com/netflix/discovery/endpoint/EndpointUtils.java

    /**
     * Get the list of all eureka service urls for the eureka client to talk to.
     *
     * @param clientConfig the clientConfig to use
     * @param zone the zone in which the client resides
     * @param randomizer a randomizer to randomized returned urls, if loading from dns
     *
     * @return The list of all eureka service urls for the eureka client to talk to.
     */
    public static List<String> getDiscoveryServiceUrls(EurekaClientConfig clientConfig, String zone, ServiceUrlRandomizer randomizer) {
        boolean shouldUseDns = clientConfig.shouldUseDnsForFetchingServiceUrls();
        if (shouldUseDns) {
            return getServiceUrlsFromDNS(clientConfig, zone, clientConfig.shouldPreferSameZoneEureka(), randomizer);
        }
        return getServiceUrlsFromConfig(clientConfig, zone, clientConfig.shouldPreferSameZoneEureka());
    }

getServiceUrlsFromConfig

   /**
     * Get the list of all eureka service urls from properties file for the eureka client to talk to.
     *
     * @param clientConfig the clientConfig to use
     * @param instanceZone The zone in which the client resides
     * @param preferSameZone true if we have to prefer the same zone as the client, false otherwise
     * @return The list of all eureka service urls for the eureka client to talk to
     */
    public static List<String> getServiceUrlsFromConfig(EurekaClientConfig clientConfig, String instanceZone, boolean preferSameZone) {
        List<String> orderedUrls = new ArrayList<String>();
        String region = getRegion(clientConfig);
        String[] availZones = clientConfig.getAvailabilityZones(clientConfig.getRegion());
        if (availZones == null || availZones.length == 0) {
            availZones = new String[1];
            availZones[0] = DEFAULT_ZONE;
        }
        logger.debug("The availability zone for the given region {} are {}", region, availZones);
        int myZoneOffset = getZoneOffset(instanceZone, preferSameZone, availZones);

        List<String> serviceUrls = clientConfig.getEurekaServerServiceUrls(availZones[myZoneOffset]);
        if (serviceUrls != null) {
            orderedUrls.addAll(serviceUrls);
        }
        int currentOffset = myZoneOffset == (availZones.length - 1) ? 0 : (myZoneOffset + 1);
        while (currentOffset != myZoneOffset) {
            serviceUrls = clientConfig.getEurekaServerServiceUrls(availZones[currentOffset]);
            if (serviceUrls != null) {
                orderedUrls.addAll(serviceUrls);
            }
            if (currentOffset == (availZones.length - 1)) {
                currentOffset = 0;
            } else {
                currentOffset++;
            }
        }

        if (orderedUrls.size() < 1) {
            throw new IllegalArgumentException("DiscoveryClient: invalid serviceUrl specified!");
        }
        return orderedUrls;
    }

这里把preferSameZone参数传入getZoneOffset方法获取zone的offset，然后通过availZones数组取出zone的名称

getZoneOffset

    /**
     * Gets the zone to pick up for this instance.
     */
    private static int getZoneOffset(String myZone, boolean preferSameZone, String[] availZones) {
        for (int i = 0; i < availZones.length; i++) {
            if (myZone != null && (availZones[i].equalsIgnoreCase(myZone.trim()) == preferSameZone)) {
                return i;
            }
        }
        logger.warn("DISCOVERY: Could not pick a zone based on preferred zone settings. My zone - {}," +
                " preferSameZone - {}. Defaulting to {}", myZone, preferSameZone, availZones[0]);

        return 0;
    }

这里对于所有的availZones，挨个判断是否跟myZone相等，如果是则返回，如果都没找到则日志warn一下然后返回第一个availZone。

小结

eureka的preferSameZoneEureka默认为true，类似Session Affinity，优先选择client实例所在的zone的服务。

doc

• Understanding eureka client server communication