1，Eureka作为注册中心，掌管者服务治理的功能，十分重要，如果注册中心的服务一旦宕机，所有的服务就会挂了，为此，实现注册中心的集群（高可用）就显得十分必要了
2，Eureka 搭建集群
实现原理就是注册中心的相互注册，实现集群。
例子：
有两个Eureka服务，分别为eureka和eureka_backup
eureka的配置文件：8200端口是eureka_backup的端口，将8100的eureka服务注册到eureka_backup注册中心上
###服务端口号
server:
port: 8100
###eureka 基本信息配置
spring:
application:
name: eureka-server
eureka:
instance:
###注册到eurekaip地址
hostname: 127.0.0.1
client:
serviceUrl:
defaultZone: http://127.0.0.1:8200/eureka/
###因为是集群，注册中心也需要注册到其他的注册中心
register-with-eureka: true
fetch-registry: true
eureka_backup的配置文件：8100端口是eureka_backup的端口，将8200的eureka服务注册到eureka注册中心上
###服务端口号
server:
port: 8200
###eureka 基本信息配置
spring:
application:
name: eureka-server
eureka:
instance:
###注册到eurekaip地址
hostname: 127.0.0.1
client:
serviceUrl:
defaultZone: http://127.0.0.1:8100/eureka/
###因为是集群，注册中心也需要注册到其他的注册中心
register-with-eureka: true
fetch-registry: true
其他的服务：member,order,分别需要注册到这个两个注册中心上
member:
###服务启动端口号
server:
port: 8000
###服务名称(服务注册到eureka名称)
spring:
application:
name: app-aiyuesheng-member
###服务注册到eureka地址
eureka:
client:
service-url:
defaultZone: http://localhost:8100/eureka,http://localhost:8200/eureka
###需要注册到eureka
register-with-eureka: true
###是否需要从eureka上获取注册信息
fetch-registry: true
order:
###服务启动端口号
server:
port: 8001
###服务名称(服务注册到eureka名称)
spring:
application:
name: app-aiyuesheng-order
###服务注册到eureka地址
eureka:
client:
service-url:
defaultZone: http://localhost:8100/eureka,http://localhost:8200/eureka
###需要注册到eureka
register-with-eureka: true
###是否需要从eureka上获取注册信息
fetch-registry: true
配置完毕：
先启动eureka 和 eureka_backup,因为这两个注册中心需要相互注册，当先启动eureka 的时候，eureka_backup 还没有启动，所有eureka 启动会报错，不用管，错误如下：
com.netflix.discovery.shared.transport.TransportException: Cannot execute request on any known server
at com.netflix.discovery.shared.transport.decorator.RetryableEurekaHttpClient.execute(RetryableEurekaHttpClient.java:111) ~[eureka-client-1.8.6.jar:1.8.6]
at com.netflix.discovery.shared.transport.decorator.EurekaHttpClientDecorator.register(EurekaHttpClientDecorator.java:56) ~[eureka-client-1.8.6.jar:1.8.6]
at com.netflix.discovery.shared.transport.decorator.EurekaHttpClientDecorator$1.execute(EurekaHttpClientDecorator.java:59) ~[eureka-client-1.8.6.jar:1.8.6]
at com.netflix.discovery.shared.transport.decorator.SessionedEurekaHttpClient.execute(SessionedEurekaHttpClient.java:77) ~[eureka-client-1.8.6.jar:1.8.6]
at com.netflix.discovery.shared.transport.decorator.EurekaHttpClientDecorator.register(EurekaHttpClientDecorator.java:56) ~[eureka-client-1.8.6.jar:1.8.6]
at com.netflix.discovery.DiscoveryClient.register(DiscoveryClient.java:829) ~[eureka-client-1.8.6.jar:1.8.6]
at com.netflix.discovery.InstanceInfoReplicator.run(InstanceInfoReplicator.java:104) [eureka-client-1.8.6.jar:1.8.6]
at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [na:1.8.0_201]
at java.util.concurrent.FutureTask.run(Unknown Source) [na:1.8.0_201]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(Unknown Source) [na:1.8.0_201]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(Unknown Source) [na:1.8.0_201]
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [na:1.8.0_201]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [na:1.8.0_201]
at java.lang.Thread.run(Unknown Source) [na:1.8.0_201]
再启动member,order 服务会注册到注册中心上，因为先启动8100，再启动8200，所以8200 上有8100，8200，8100上只有8200
下面是8100的地址：
如果当8200挂了之后，大约30秒后，注册信息会转移到8100.。。。时间可以配置
至此，实现了Eureka集群高可用