spring cloud eureka集群，注册中心再添加一台服务器

Question

spring cloud eureka集群注册中心增加服务器，client除了用config，还可以怎么刷新serverUrls列表?

问题详细描述：
最近在研究spring cloud eureka集群配置的时候碰到问题:多台eureka server如果需要互相注册,需要在配置文件中将其他服务器地址配置写死.同样客户端启用服务发现功能(eureka client)也需要配置服务端地址列表(其实eureka server与其他eureka server通信也是用的是eureka client组件).按照官方案例提供3台server,如果现在需要增加第四台,第五台，该怎么做？

Answer 1

最近在研究spring cloud eureka集群配置的时候碰到问题:多台eureka server如果需要互相注册,需要在配置文件中将其他服务器地址配置写死.同样客户端启用服务发现功能(eureka client)也需要配置服务端地址列表(其实eureka server与其他eureka server通信也是用的是eureka client组件).按照官方案例提供3台server,如果现在需要增加第四台,第五台...那么问题就来了,所有eureka client的serverUrls列表是否都得更新(修改配置文件)?

一句话总结如上问题就是:eureka集群有什么办法能支持动态集群(集群数量可增减客户端不需要改动任何内容)?

经过寻找发现spring cloud eureka client提供一个eureka.client.useDnsForFetchingServiceUrls选项,使用Dns获取服务地址.

经过各种了解,明确了该配置项就是启用dns来存储eureka server列表的,可以实现动态eureka server集群的功能.但是问题又来了,相关属性还有那些?dns又该如何配置呢?

相关属性好找,有网友提供的例子,dns这块没有比较明确的说明,为了弄明白这块自己尝试着看了看源码,结果找到了DNS里面关于具体如何使用DNS结果的相关代码.代码如下:

getDiscoveryServiceUrls的作用是获取所有eureka service urls,该方法首先判断是否需要从DNS获取服务列表,图中红框部分就是从DNS获取服务列表,继续往下分析:

在分析之前,插播一个知识点:eureka集群与region和zone这几个概念的关系.http://www.vccoo.com/v/bqq4vj

继续看代码:

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1 /**
2 * Get the list of all eureka service urls from DNS for the eureka client to
3 * talk to. The client picks up the service url from its zone and then fails over to
4 * other zones randomly. If there are multiple servers in the same zone, the client once
5 * again picks one randomly. This way the traffic will be distributed in the case of failures.
6 *
7 * @param clientConfig the clientConfig to use
8 * @param instanceZone The zone in which the client resides.
9 * @param preferSameZone true if we have to prefer the same zone as the client, false otherwise.
10 * @param randomizer a randomizer to randomized returned urls
11 *
12 * @return The list of all eureka service urls for the eureka client to talk to.
13 */
14 public static List<String> getServiceUrlsFromDNS(EurekaClientConfig clientConfig, String instanceZone, boolean preferSameZone, ServiceUrlRandomizer randomizer) {
15 String region = getRegion(clientConfig);
16 // Get zone-specific DNS names for the given region so that we can get a
17 // list of available zones
18 Map<String, List<String>> zoneDnsNamesMap = getZoneBasedDiscoveryUrlsFromRegion(clientConfig, region);
19 Set<String> availableZones = zoneDnsNamesMap.keySet();
20 List<String> zones = new ArrayList<String>(availableZones);
21 if (zones.isEmpty()) {
22 throw new RuntimeException("No available zones configured for the instanceZone " + instanceZone);
23 }
24 int zoneIndex = 0;
25 boolean zoneFound = false;
26 for (String zone : zones) {
27 logger.debug("Checking if the instance zone {} is the same as the zone from DNS {}", instanceZone, zone);
28 if (preferSameZone) {
29 if (instanceZone.equalsIgnoreCase(zone)) {
30 zoneFound = true;
31 }
32 } else {
33 if (!instanceZone.equalsIgnoreCase(zone)) {
34 zoneFound = true;
35 }
36 }
37 if (zoneFound) {
38 Object[] args = {zones, instanceZone, zoneIndex};
39 logger.debug("The zone index from the list {} that matches the instance zone {} is {}", args);
40 break;
41 }
42 zoneIndex++;
43 }
44 if (zoneIndex >= zones.size()) {
45 logger.warn("No match for the zone {} in the list of available zones {}",
46 instanceZone, Arrays.toString(zones.toArray()));
47 } else {
48 // Rearrange the zones with the instance zone first
49 for (int i = 0; i < zoneIndex; i++) {
50 String zone = zones.remove(0);
51 zones.add(zone);
52 }
53 }
54
55 // Now get the eureka urls for all the zones in the order and return it
56 List<String> serviceUrls = new ArrayList<String>();
57 for (String zone : zones) {
58 for (String zoneCname : zoneDnsNamesMap.get(zone)) {
59 List<String> ec2Urls = new ArrayList<String>(getEC2DiscoveryUrlsFromZone(zoneCname, DiscoveryUrlType.CNAME));
60 // Rearrange the list to distribute the load in case of
61 // multiple servers
62 if (ec2Urls.size() > 1) {
63 randomizer.randomize(ec2Urls);
64 }
65 for (String ec2Url : ec2Urls) {
66 String serviceUrl = "http://" + ec2Url + ":"
67 + clientConfig.getEurekaServerPort()
68 + "/" + clientConfig.getEurekaServerURLContext()
69 + "/";
70 logger.debug("The EC2 url is {}", serviceUrl);
71 serviceUrls.add(serviceUrl);
72 }
73 }
74 }
75 // Rearrange the fail over server list to distribute the load
76 String primaryServiceUrl = serviceUrls.remove(0);
77 randomizer.randomize(serviceUrls);
78 serviceUrls.add(0, primaryServiceUrl);
79
80 logger.debug("This client will talk to the following serviceUrls in order : {} ",
81 Arrays.toString(serviceUrls.toArray()));
82 return serviceUrls;
83 }
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从代码中可以看到,首先获取当前eureka-client所在的region,然后根据region获取所有zone以及对应的域名信息,然后循环所有zone域名信息获取eureka-server地址,拼接成完整的serviceUrl并加入serviceUrls列表中.

拼接的serviceUrl格式为:"http://" + ec2Url + ":" + clientConfig.getEurekaServerPort() + "/" + clientConfig.getEurekaServerURLContext() + "/";

另外在代码中有标出两处重点,红色背景的是根据region获取zone的具体方法,蓝色背景的是根据zone具体地址获取eureka地址列表的方法.重点就在这两个方法中.

首先看第一个,获取zone的逻辑:

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1 /**
2 * Get the zone based CNAMES that are bound to a region.
3 *
4 * @param region
5 * - The region for which the zone names need to be retrieved
6 * @return - The list of CNAMES from which the zone-related information can
7 * be retrieved
8 */
9 public static Map<String, List<String>> getZoneBasedDiscoveryUrlsFromRegion(EurekaClientConfig clientConfig, String region) {
10 String discoveryDnsName = null;
11 try {
12 discoveryDnsName = "txt." + region + "." + clientConfig.getEurekaServerDNSName();
13
14 logger.debug("The region url to be looked up is {} :", discoveryDnsName);
15 Set<String> zoneCnamesForRegion = new TreeSet<String>(DnsResolver.getCNamesFromTxtRecord(discoveryDnsName));
16 Map<String, List<String>> zoneCnameMapForRegion = new TreeMap<String, List<String>>();
17 for (String zoneCname : zoneCnamesForRegion) {
18 String zone = null;
19 if (isEC2Url(zoneCname)) {
20 throw new RuntimeException(
21 "Cannot find the right DNS entry for "
22 + discoveryDnsName
23 + ". "
24 + "Expected mapping of the format <aws_zone>.<domain_name>");
25 } else {
26 String[] cnameTokens = zoneCname.split("\.");
27 zone = cnameTokens[0];
28 logger.debug("The zoneName mapped to region {} is {}", region, zone);
29 }
30 List<String> zoneCnamesSet = zoneCnameMapForRegion.get(zone);
31 if (zoneCnamesSet == null) {
32 zoneCnamesSet = new ArrayList<String>();
33 zoneCnameMapForRegion.put(zone, zoneCnamesSet);
34 }
35 zoneCnamesSet.add(zoneCname);
36 }
37 return zoneCnameMapForRegion;
38 } catch (Throwable e) {
39 throw new RuntimeException("Cannot get cnames bound to the region:" + discoveryDnsName, e);
40 }
41 }
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12行是请求dns中的地址格式:"txt." + region + "." + clientConfig.getEurekaServerDNSName(),例如:txt.region1.baidu.com,txt.region1.163.com,txt.region2.163.com等

17,27,35行是对返回结果的解析逻辑,可以看出返回值应当是多条记录并且以空格分开(在15行方法内),每条记录都应当是一个域名并且第一个.之前的部分作为zone名称,最终按照zone组织成zone:List<区域地址>的结果返回.

再看第二个,获取eureka server地址逻辑:

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1 /**
2 * Get the list of EC2 URLs given the zone name.
3 *
4 * @param dnsName The dns name of the zone-specific CNAME
5 * @param type CNAME or EIP that needs to be retrieved
6 * @return The list of EC2 URLs associated with the dns name
7 */
8 public static Set<String> getEC2DiscoveryUrlsFromZone(String dnsName, DiscoveryUrlType type) {
9 Set<String> eipsForZone = null;
10 try {
11 dnsName = "txt." + dnsName;
12 logger.debug("The zone url to be looked up is {} :", dnsName);
13 Set<String> ec2UrlsForZone = DnsResolver.getCNamesFromTxtRecord(dnsName);
14 for (String ec2Url : ec2UrlsForZone) {
15 logger.debug("The eureka url for the dns name {} is {}", dnsName, ec2Url);
16 ec2UrlsForZone.add(ec2Url);
17 }
18 if (DiscoveryUrlType.CNAME.equals(type)) {
19 return ec2UrlsForZone;
20 }
21 eipsForZone = new TreeSet<String>();
22 for (String cname : ec2UrlsForZone) {
23 String[] tokens = cname.split("\.");
24 String ec2HostName = tokens[0];
25 String[] ips = ec2HostName.split("-");
26 StringBuilder eipBuffer = new StringBuilder();
27 for (int ipCtr = 1; ipCtr < 5; ipCtr++) {
28 eipBuffer.append(ips[ipCtr]);
29 if (ipCtr < 4) {
30 eipBuffer.append(".");
31 }
32 }
33 eipsForZone.add(eipBuffer.toString());
34 }
35 logger.debug("The EIPS for {} is {} :", dnsName, eipsForZone);
36 } catch (Throwable e) {
37 throw new RuntimeException("Cannot get cnames bound to the region:" + dnsName, e);
38 }
39 return eipsForZone;
40 }
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11行代码明确了请求格式为:"txt." + dnsName,例如:txt.zone1.163.com,txt.zone2.baidu.com等

而返回结果则比较复杂,我们只关心cnametype的,从getServiceUrlsFromDNS方法中可以得知,只需要返回服务器地址或域名就可以了(返回多组同样用空格隔开).

看到这里对dns获取eureka地址的过程已经明白了.然后就是配置了

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eureka:
client:

eureka-server-d-n-s-name: relinson.com
use-dns-for-fetching-service-urls: true
region: region1
eureka-server-u-r-l-context: eureka
eureka-server-port: 9999
prefer-same-zone-eureka: true

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启动后可以看到这种方式配置还算是成功的