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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"    }复制代码

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;	}复制代码

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
    
      defaultRegionVsAzMap =            Multimaps.newListMultimap(new HashMap
     
      >(), new Supplier
      
       
        >() {                @Override                public List
        
          get() {                    return new ArrayList
         
          (); } }); 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
          
            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
           
             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; } }复制代码
           
          
         
        
       
      
     
    

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
    
      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();    }复制代码
    

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
    
      availabilityZoneVsRegion = new ConcurrentHashMap
     
      ();    public synchronized void refreshMapping() {        logger.info("Refreshing availability zone to region mappings.");        setRegionsToFetch(regionsToFetch);    }复制代码
     
    

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
    
      getZonesForARegion(String region) {        return new HashSet
     
      (Arrays.asList(clientConfig.getAvailabilityZones(region)));    }}复制代码
     
    

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
    
      availabilityZones = new HashMap<>();	public String[] getAvailabilityZones(String region) {		String value = this.availabilityZones.get(region);		if (value == null) {			value = DEFAULT_ZONE;		}		return value.split(",");	}复制代码
    

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 
    
     
              // 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;    }复制代码
     
    

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
    
      metadata = amazonInfo.getMetadata();            String availabilityZone = metadata.get(AmazonInfo.MetaDataKey.availabilityZone.getName());            if (null != availabilityZone) {                return azToRegionMapper.getRegionForAvailabilityZone(availabilityZone);            }        }        return null;    }复制代码
    

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。

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