聊聊eureka client的fetch-remote-regions-registry属性

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本文主要研究一下eureka client的fetch-remote-regions-registry属性

配置fetch-remote-regions-registry

    {
      "sourceType": "org.springframework.cloud.netflix.eureka.EurekaClientConfigBean",
      "name": "eureka.client.fetch-remote-regions-registry",
      "description": "Comma separated list of regions for which the eureka registry information will be\n fetched. It is mandatory to define the availability zones for each of these regions\n as returned by availabilityZones. Failing to do so, will result in failure of\n discovery client startup.",
      "type": "java.lang.String"
    }

fetchRemoteRegionsRegistry

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

    /**
     * Comma separated list of regions for which the eureka registry information will be
     * fetched. It is mandatory to define the availability zones for each of these regions
     * as returned by availabilityZones. Failing to do so, will result in failure of
     * discovery client startup.
     *
     */
    private String fetchRemoteRegionsRegistry;

    @Override
    public String fetchRegistryForRemoteRegions() {
        return this.fetchRemoteRegionsRegistry;
    }

    public String getFetchRemoteRegionsRegistry() {
        return fetchRemoteRegionsRegistry;
    }

    public void setFetchRemoteRegionsRegistry(String fetchRemoteRegionsRegistry) {
        this.fetchRemoteRegionsRegistry = fetchRemoteRegionsRegistry;
    }

从文字可以看出，如果配置了fetch-remote-regions-registry，则需要配置对应remote region的availability-zones，不然就会抛异常，代码详见

AbstractAzToRegionMapper.setRegionsToFetch

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

    /**
     * A default for the mapping that we know of, if a remote region is configured to be fetched but does not have
     * any availability zone mapping, we will use these defaults. OTOH, if the remote region has any mapping defaults
     * will not be used.
     */
    private final Multimap<String, String> defaultRegionVsAzMap =
            Multimaps.newListMultimap(new HashMap<String, Collection<String>>(), new Supplier<List<String>>() {
                @Override
                public List<String> get() {
                    return new ArrayList<String>();
                }
            });

    public synchronized void setRegionsToFetch(String[] regionsToFetch) {
        if (null != regionsToFetch) {
            this.regionsToFetch = regionsToFetch;
            logger.info("Fetching availability zone to region mapping for regions {}", (Object) regionsToFetch);
            availabilityZoneVsRegion.clear();
            for (String remoteRegion : regionsToFetch) {
                Set<String> availabilityZones = getZonesForARegion(remoteRegion);
                if (null == availabilityZones
                        || (availabilityZones.size() == 1 && availabilityZones.contains(DEFAULT_ZONE))
                        || availabilityZones.isEmpty()) {
                    logger.info("No availability zone information available for remote region: {}"
                            + ". Now checking in the default mapping.", remoteRegion);
                    if (defaultRegionVsAzMap.containsKey(remoteRegion)) {
                        Collection<String> defaultAvailabilityZones = defaultRegionVsAzMap.get(remoteRegion);
                        for (String defaultAvailabilityZone : defaultAvailabilityZones) {
                            availabilityZoneVsRegion.put(defaultAvailabilityZone, remoteRegion);
                        }
                    } else {
                        String msg = "No availability zone information available for remote region: " + remoteRegion
                                + ". This is required if registry information for this region is configured to be "
                                + "fetched.";
                        logger.error(msg);
                        throw new RuntimeException(msg);
                    }
                } else {
                    for (String availabilityZone : availabilityZones) {
                        availabilityZoneVsRegion.put(availabilityZone, remoteRegion);
                    }
                }
            }

            logger.info("Availability zone to region mapping for all remote regions: {}", availabilityZoneVsRegion);
        } else {
            logger.info("Regions to fetch is null. Erasing older mapping if any.");
            availabilityZoneVsRegion.clear();
            this.regionsToFetch = EMPTY_STR_ARRAY;
        }
    }

可以看到，如果对应region的availabilityZones为空或者是defaultZone的话，则会判断defaultRegionVsAzMap.containsKey(remoteRegion)，如果为false的话，进入的分支会抛出RuntimeException

使用fetch-remote-regions-registry

refreshRegistry

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

    void refreshRegistry() {
        try {
            boolean isFetchingRemoteRegionRegistries = isFetchingRemoteRegionRegistries();

            boolean remoteRegionsModified = false;
            // This makes sure that a dynamic change to remote regions to fetch is honored.
            String latestRemoteRegions = clientConfig.fetchRegistryForRemoteRegions();
            if (null != latestRemoteRegions) {
                String currentRemoteRegions = remoteRegionsToFetch.get();
                if (!latestRemoteRegions.equals(currentRemoteRegions)) {
                    // Both remoteRegionsToFetch and AzToRegionMapper.regionsToFetch need to be in sync
                    synchronized (instanceRegionChecker.getAzToRegionMapper()) {
                        if (remoteRegionsToFetch.compareAndSet(currentRemoteRegions, latestRemoteRegions)) {
                            String[] remoteRegions = latestRemoteRegions.split(",");
                            remoteRegionsRef.set(remoteRegions);
                            instanceRegionChecker.getAzToRegionMapper().setRegionsToFetch(remoteRegions);
                            remoteRegionsModified = true;
                        } else {
                            logger.info("Remote regions to fetch modified concurrently," +
                                    " ignoring change from {} to {}", currentRemoteRegions, latestRemoteRegions);
                        }
                    }
                } else {
                    // Just refresh mapping to reflect any DNS/Property change
                    instanceRegionChecker.getAzToRegionMapper().refreshMapping();
                }
            }

            boolean success = fetchRegistry(remoteRegionsModified);
            if (success) {
                registrySize = localRegionApps.get().size();
                lastSuccessfulRegistryFetchTimestamp = System.currentTimeMillis();
            }

            if (logger.isDebugEnabled()) {
                StringBuilder allAppsHashCodes = new StringBuilder();
                allAppsHashCodes.append("Local region apps hashcode: ");
                allAppsHashCodes.append(localRegionApps.get().getAppsHashCode());
                allAppsHashCodes.append(", is fetching remote regions? ");
                allAppsHashCodes.append(isFetchingRemoteRegionRegistries);
                for (Map.Entry<String, Applications> entry : remoteRegionVsApps.entrySet()) {
                    allAppsHashCodes.append(", Remote region: ");
                    allAppsHashCodes.append(entry.getKey());
                    allAppsHashCodes.append(" , apps hashcode: ");
                    allAppsHashCodes.append(entry.getValue().getAppsHashCode());
                }
                logger.debug("Completed cache refresh task for discovery. All Apps hash code is {} ",
                        allAppsHashCodes);
            }
        } catch (Throwable e) {
            logger.error("Cannot fetch registry from server", e);
        }        
    }

    private boolean isFetchingRemoteRegionRegistries() {
        return null != remoteRegionsToFetch.get();
    }

这里可以看到，如果remoteRegion没有变化的话，那么走的逻辑是instanceRegionChecker.getAzToRegionMapper().refreshMapping();
refreshRegistry这个方法有个定时任务会定时调用

refreshMapping

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

    private final Map<String, String> availabilityZoneVsRegion = new ConcurrentHashMap<String, String>();

    public synchronized void refreshMapping() {
        logger.info("Refreshing availability zone to region mappings.");
        setRegionsToFetch(regionsToFetch);
    }

refreshMapping调用的是setRegionsToFetch，它的作用主要是将配置文件中的remote region及其availabilityZone的映射信息更新到availabilityZoneVsRegion中。

PropertyBasedAzToRegionMapper

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

public class PropertyBasedAzToRegionMapper extends AbstractAzToRegionMapper {

    public PropertyBasedAzToRegionMapper(EurekaClientConfig clientConfig) {
        super(clientConfig);
    }

    @Override
    protected Set<String> getZonesForARegion(String region) {
        return new HashSet<String>(Arrays.asList(clientConfig.getAvailabilityZones(region)));
    }
}

EurekaClientConfigBean.getAvailabilityZones

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

    /**
     * Gets the list of availability zones (used in AWS data centers) for the region in
     * which this instance resides.
     *
     * The changes are effective at runtime at the next registry fetch cycle as specified
     * by registryFetchIntervalSeconds.
     */
    private Map<String, String> availabilityZones = new HashMap<>();

    public String[] getAvailabilityZones(String region) {
        String value = this.availabilityZones.get(region);
        if (value == null) {
            value = DEFAULT_ZONE;
        }
        return value.split(",");
    }

这里的availabilityZones就是配置文件指定的eureka.client.availability-zones属性

使用availabilityZoneVsRegion

AbstractAzToRegionMapper.getRegionForAvailabilityZone

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

    public String getRegionForAvailabilityZone(String availabilityZone) {
        String region = availabilityZoneVsRegion.get(availabilityZone);
        if (null == region) {
            return parseAzToGetRegion(availabilityZone);
        }
        return region;
    }

    /**
     * Tries to determine what region we're in, based on the provided availability zone.
     * @param availabilityZone the availability zone to inspect
     * @return the region, if available; null otherwise
     */
    protected String parseAzToGetRegion(String availabilityZone) {
        // Here we see that whether the availability zone is following a pattern like <region><single letter>
        // If it is then we take ignore the last letter and check if the remaining part is actually a known remote
        // region. If yes, then we return that region, else null which means local region.
        if (!availabilityZone.isEmpty()) {
            String possibleRegion = availabilityZone.substring(0, availabilityZone.length() - 1);
            if (availabilityZoneVsRegion.containsValue(possibleRegion)) {
                return possibleRegion;
            }
        }
        return null;
    }

InstanceRegionChecker.getInstanceRegion

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

    public String getInstanceRegion(InstanceInfo instanceInfo) {
        if (instanceInfo.getDataCenterInfo() == null || instanceInfo.getDataCenterInfo().getName() == null) {
            logger.warn("Cannot get region for instance id:{}, app:{} as dataCenterInfo is null. Returning local:{} by default",
                    instanceInfo.getId(), instanceInfo.getAppName(), localRegion);

            return localRegion;
        }
        if (DataCenterInfo.Name.Amazon.equals(instanceInfo.getDataCenterInfo().getName())) {
            AmazonInfo amazonInfo = (AmazonInfo) instanceInfo.getDataCenterInfo();
            Map<String, String> metadata = amazonInfo.getMetadata();
            String availabilityZone = metadata.get(AmazonInfo.MetaDataKey.availabilityZone.getName());
            if (null != availabilityZone) {
                return azToRegionMapper.getRegionForAvailabilityZone(availabilityZone);
            }
        }

        return null;
    }

InstanceRegionChecker.getInstanceRegion用到了azToRegionMapper.getRegionForAvailabilityZone(availabilityZone)
不过要注意，这个也仅仅是在dataCenterInfo是Amazon的时候，才会使用

小结

eureka client的fetch-remote-regions-registry属性，看似好像有什么大用处，实际代码一看，只有dataCenterInfo是Amazon的时候，才用到。普通非Amazon环境的，默认的dataCenterInfo是MyOwn，因此也用不上这个属性。

doc

• 聊聊eureka server的RemoteRegionRegistry
• 聊聊eureka client的backup-registry-impl属性