[SCB-1491] Sample introduction (#590)
* Sample introduction
* Fix spelling bug in image
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+异构服务中心
+-------
+[Introduction to English](./multi-servicecenters.md)
+
+本案例模拟了异构服务中心之间的通讯场景，包括以下四个部分：
+- EurekaServer： Eureka服务中心
+- AccountServer：注册到Eureka的账号服务
+- Servicecomb-ServiceCenter：Servicecomb-ServiceCenter服务中心
+- HelloServer：注册到Servicecomb-ServiceCenter，需要使用AccountServer的服务
+
+在传统的环境中，不同服务中心之间的实例是不能互相发现并访问的。当我们使用Syncer后，这将很容易实现。如下图：
+
+
+### 运行环境
+ 1. 2台linux机器: （假设为：10.0.0.10 和 10.0.0.11）
+ 2. [JDK 1.8](http://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html)
+ 3. [Maven 3.x](https://maven.apache.org/install.html)
+ 4. [Go 1.11.4](https://golang.google.cn/dl/)
+ 5. [ServiceComb Service-Center 1.2.0](https://apache.org/dyn/closer.cgi/servicecomb/servicecomb-service-center/1.2.0/)
+
+### 步骤1：启动Eureka环境和服务
+机器： 10.0.0.10
+#### 1. 编译项目
+```bash
+# 记录当前目录
+$ project_dir=`pwd`
+
+# 下载源码
+$ git clone https://github.com/apache/servicecomb-service-center.git
+
+# 编译 Syncer
+$ cd servicecomb-service-center/syncer
+$ GO111MODULE=on go build
+
+# 编译 EurekaServer 和 AccountServer
+$ cd samples/multi-servicecenters/eureka
+$ mvn clean install
+```
+#### 2. 启动EurekaServer：
+- 修改启动配置
+ 文件位置：${project_dir}/servicecomb-service-center/syncer/samples/multi-servicecenters/eureka/eureka-server/src/main/resources/application.yaml
+```yml
+spring:
+ application:
+ name: eureka-server
+server:
+ port : 8761
+# servlet:
+# context-path: /eureka
+eureka:
+ instance:
+ hostname : 10.0.0.10
+ client:
+ registerWithEureka : false
+ fetchRegistry : false
+ serviceUrl:
+ defaultZone : http://${eureka.instance.hostname}:${server.port}/eureka/
+management:
+ endpoints:
+ web:
+ exposure:
+ include: "*"
+```
+
+- 启动服务
+```bash
+# 启动 EurekaServer
+$ cd ${project_dir}/servicecomb-service-center/syncer/samples/multi-servicecenters/eureka/eureka-server/
+$ nohup mvn spring-boot:run & >> eureka-server.log 2>&1 &
+```
+浏览器打开http://10.0.0.10:8761，若出现如下页面，则启动成功
+
+
+#### 3. 启动AccountServer
+- 修改启动配置
+ 文件位置：${project_dir}/servicecomb-service-center/syncer/samples/multi-servicecenters/eureka/account-server/src/main/resources/application.yaml
+```yml
+spring:
+ application:
+ name: account-server
+server:
+ port: 8090
+eureka:
+ instance:
+ hostname: 10.0.0.10
+ client:
+ service-url:
+ defaultZone: http://${eureka.instance.hostname}:8761/eureka/
+management:
+ endpoints:
+ web:
+ exposure:
+ include: "*"
+```
+- 启动服务
+```bash
+#启动 AccountServer
+$ cd ${project_dir}/servicecomb-service-center/syncer/samples/multi-servicecenters/eureka/account-server
+$ mvn spring-boot:run
+
+# 出现如下字样则为成功
+2019-09-19 17:20:35.534 INFO 20890 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port(s): 8090 (http) with context path ''
+2019-09-19 17:20:35.548 INFO 20890 --- [ main] .s.c.n.e.s.EurekaAutoServiceRegistration : Updating port to 8090
+2019-09-19 17:20:35.551 INFO 20890 --- [ main] o.a.s.s.account.AccountApplication : Started AccountApplication in 3.92 seconds (JVM running for 6.754)
+2019-09-19 17:20:35.617 INFO 20890 --- [nfoReplicator-0] com.netflix.discovery.DiscoveryClient : DiscoveryClient_ACCOUNT-SERVER/10.0.0.10:account-server:8090 - registration status: 204
+```
+此时打开http://10.0.0.10:8761，可以看到AccountServer已经注册成功
+
+
+#### 4. 启动Syncer
+```bash
+$ cd ${project_dir}/servicecomb-service-center/syncer
+$ ./syncer daemon --sc-addr http://10.0.0.10:8761/eureka --bind-addr 10.0.0.10:30190 --rpc-addr 10.0.0.10:30191 --sc-plugin=eureka
+
+# 出现如下字样则为成功
+2019-09-19T17:28:28.809+0800 INFO etcd/agent.go:55 start etcd success
+2019-09-19T17:28:28.809+0800 INFO grpc/server.go:94 start grpc success
+2019-09-19T17:28:28.809+0800 DEBUG server/handler.go:39 is leader: true
+2019-09-19T17:28:28.809+0800 DEBUG server/handler.go:43 Handle Tick
+```
+
+### 步骤2：启动Servicecenter环境和服务
+机器： 10.0.0.11
+#### 1. 编译项目
+```bash
+# 记录当前目录
+$ project_dir=`pwd`
+
+# 下载源码
+$ git clone https://github.com/apache/servicecomb-service-center.git
+
+# 编译 Syncer
+$ cd servicecomb-service-center/syncer
+$ GO111MODULE=on go build
+
+# 编译 HelloServer
+$ cd samples/multi-servicecenters/servicecenter/hello-server/
+$ GO111MODULE=on go build
+```
+#### 2. 启动Servicecenter
+- 下载ServiceCenter项目
+```bash
+$ cd ${project_dir}
+
+# 下载 ServiceCenter 1.2.0版本包
+$ curl -O https://mirrors.tuna.tsinghua.edu.cn/apache/servicecomb/servicecomb-service-center/1.2.0/apache-servicecomb-service-center-1.2.0-linux-amd64.tar.gz
+$ tar -zxvf apache-servicecomb-service-center-1.2.0-linux-amd64.tar.gz
+```
+
+- 修改启动配置：
+ 文件位置：${project_dir}/apache-servicecomb-service-center-1.2.0-linux-amd64/conf/app.conf
+```conf
+frontend_host_ip = 10.0.0.11
+frontend_host_port = 30103
+
+###################################################################
+# sever options
+###################################################################
+# if you want to listen at ipv6 address, then set the httpaddr value like:
+# httpaddr = 2400:A480:AAAA:200::159 (global scope)
+# httpaddr = fe80::f816:3eff:fe17:c38b%eth0 (link-local scope)
+httpaddr = 10.0.0.11
+httpport = 30100
+# ...以下省略...
+```
+
+- 启动 ServiceCenter 和 Frontend
+```bash
+$ cd ${project_dir}/apache-servicecomb-service-center-1.2.0-linux-amd64
+
+# 启动 ServiceCenter
+$ ./start-service-center.sh
+
+# 启动 前端页面 Frontend
+$ ./start-frontend.sh
+```
+浏览器打开http://10.0.0.11:30103,若出现如下页面，则启动成功
+
+
+#### 3. 启动HelloServer
+- 修改启动配置：
+ 文件位置：${project_dir}/servicecomb-service-center/syncer/samples/multi-servicecenters/servicecenter/hello-server/conf/microservice.yaml
+```yml
+service: # 微服务配置
+ appId: eureka # eureka 中同步的实例appID均为：eureka
+ name: hello-server
+ version: 0.0.1
+ instance: # 实例信息
+ protocol: rest
+ listenAddress: 10.0.0.11:8091 #实例监听地址
+provider: # 服务端信息
+ appId: eureka
+ name: account-server
+ version: 0.0.1
+registry:
+ address: http://10.0.0.11:30100
+```
+
+- 启动服务
+```bash
+$ cd ${project_dir}/servicecomb-service-center/syncer/samples/multi-servicecenters/servicecenter/hello-server
+$ ./hello-server
+2019-09-19T18:37:50.645+0800 DEBUG servicecenter/servicecenter.go:163 send heartbeat success
+2019-09-19T18:37:50.645+0800 WARN servicecenter/servicecenter.go:85 discovery provider failed, appID = eureka, name = account-server, version = 0.0.1
+2019-09-19T18:37:50.645+0800 INFO servicecenter/servicecenter.go:87 waiting for retry
+```
+此时打开http://10.0.0.11:30103，可以看到HelloServer已经注册成功
+
+但由于无法发现属于Eureka服务中心的AccountServer实例，HelloServer处于重试状态
+（==注：重试次数为3次，每次间隔30秒，所以我们需要在90秒内完成后面的操作==）
+
+#### 4.启动Syncer
+```bash
+$ cd ${project_dir}/servicecomb-service-center/syncer
+$ ./syncer daemon --sc-addr http://10.0.0.11:30100 --bind-addr 10.0.0.11:30190 --rpc-addr 10.0.0.11:30191 --sc-plugin=servicecenter --join-addr 10.0.0.10:30190
+
+# 出现以下内容则为Syncer成功启动，并同步了对方的实例
+2019-09-19T18:44:35.536+0800 DEBUG server/handler.go:62 is leader: true
+2019-09-19T18:44:35.536+0800 DEBUG server/handler.go:79 Receive serf user event
+2019-09-19T18:44:35.536+0800 DEBUG serf/agent.go:130 member = xxxxxa, groupName = 0204d59328090c2f4449a088d4e0f1d8
+2019-09-19T18:44:35.536+0800 DEBUG serf/agent.go:130 member = xxxxxb, groupName = 34f53a9520a11c01f02f58f733e856b3
+2019-09-19T18:44:35.536+0800 DEBUG server/handler.go:97 Going to pull data from xxxxxb 10.0.0.10:30191
+2019-09-19T18:44:35.536+0800 INFO grpc/client.go:76 Create new grpc connection to 10.0.0.10:30191
+2019-09-19T18:44:35.538+0800 DEBUG servicecenter/servicecenter.go:87 create service success orgServiceID= account-server, curServiceID = 80784229255ec96d90353e3c041bdf3586fdbbae
+2019-09-19T18:44:35.538+0800 DEBUG servicecenter/servicecenter.go:90 trying to do registration of instance, instanceID = 10.0.0.10:account-server:8090
+2019-09-19T18:44:35.540+0800 DEBUG servicecenter/sync.go:63 Registered instance successful, instanceID = 78bca3e2daca11e99638fa163eca30e0
+```
+
+### 步骤3：结果验证
+1. 此时的HelloServer获取实例成功，并调用了AccountServer的CheckHealth接口
+
+
+2. 分别打开Euraka和ServiceCenter的网页，两个服务中心里均包含了所有的实例信息
+
+
+3. curl命令调用HelloServer的Login接口
+```bash
+$ curl -X POST \
+http://192.168.88.75:8091/login \
+-H 'Content-Type: application/json' \
+-d '{
+ "user":"Jack",
+ "password":"123456"
+}'
+welcome Jack
+```
+HelloServer与AccountServer分别会打印如下的信息
+AccountServer
+
+HelloServer
+
