Talk about Preferential preferSameZoneEureka Parameters of eureka

  springcloud

Order

This paper mainly studies the preferences Samezoneeureka parameter of eureka

Configuration

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;
    }
    //......
}

Use

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;
    }

Here, the preferSameZone parameter is passed into the getZoneOffset method to obtain the offset of the zone, and then the name of the zone is retrieved through the availZones array.

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;
    }

Here, for all availZones, it is determined one by one whether they are equal to myZone. If yes, it is returned. If none is found, log warn and then return to the first availZone.

Summary

Eureka’s preferSameZoneEureka defaults to true, which is similar to Session Affinity and gives priority to the service in the zone where the client instance is located.

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