Hystrix（服务熔断，服务降级）

分布式系统面临的服务雪崩问题

当用户发起请求时，假设处理请求的服务器1运行时需要调用到服务器2，服务器2运行时需要调用到服务器3，而如果服务器2因为某些原因始终不能正常调用到服务器3，这时候后面其他用户的请求也都会阻塞在服务器1，在并发量极大的情况下容易导致多个服务器崩溃已经许多其他级联问题的发生，这就是服务雪崩。

测试熔断机制，

为了方便后续复习，直接新建一个有熔断机制的provider8001模块：

步骤：导包->

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.example</groupId>
        <artifactId>springcloud</artifactId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <groupId>com.lian</groupId>
    <artifactId>springcloud-provider-dept-hystrix-8001</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <dependencies>
        <!-- https://mvnrepository.com/artifact/org.springframework.cloud/spring-cloud-starter-netflix-hystrix -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
            <version>2.1.2.RELEASE</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.springframework.cloud/spring-cloud-starter-netflix-eureka-client -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
            <version>2.1.2.RELEASE</version>
        </dependency>
        <!--        #监视器-->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>


        <dependency>
            <groupId>org.example</groupId>
            <artifactId>springcloud-api</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>

        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-core</artifactId>
        </dependency>
        <!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-web -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>2.1.4.RELEASE</version>
        </dependency>

        <!--        spring-boot-test-->
        <!-- https://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-test -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <version>2.1.4.RELEASE</version>
            <scope>test</scope>
        </dependency>



        <!--        jetty-->
        <!--        热部署-->

    </dependencies>

</project>

2.只测试一个get/id的方法，方便对比，在controller类添加备选方法，像我们原来如果没有添加这个备选方法，那么在用户请求不存在的id时，前端就会报错，现在我们有了这个备选方法，当用户请求不存在的id时，前端会打印出提示信息。