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<h1 id="_1">隔离熔断容错</h1>
<p>bizkeeper 模块集成了 <a href="https://github.com/Netflix/Hystrix/wiki/Configuration">Hystrix</a> , 提供隔离、熔断和容错等服务故障保护能力。 Java
Chassis 对 Hystrix 进行了封装， 只需要进行简单的配置即可启用这些功能。 </p>
<p>在项目中引入如下 Handler 模块， </p>
<pre><code class="language-xml"> <dependency>
<groupId>org.apache.servicecomb</groupId>
<artifactId>handler-bizkeeper</artifactId>
</dependency>
</code></pre>
<p>并且增加 Handler 配置：</p>
<pre><code class="language-yaml">servicecomb:
handler:
chain:
Consumer:
default: bizkeeper-consumer
Provider:
default: bizkeeper-provider
</code></pre>
<p><strong> 注意事项： </strong></p>
<p>在实际微服务运维实践中，发现 Hystrix 存在如下一些问题， 开发者可以结合实际业务情况，谨慎的使用这个模块。 Java Chassis 的
<a href="loadbalance.html">负载均衡</a> 也提供了实例隔离和错误重试能力， 加上内置的<a href="../build-provider/thread-pool.html">线程池隔离</a>， 能够
实现大部分 Hystrix 核心治理能力。 </p>
<ul>
<li>调用栈很深，在某些异常情况下，可能屏蔽底层异常，导致问题原因分析困难。</li>
<li>超时机制、重试机制和线程池隔离机制和 Java Chassis 的一些内部机制的协作存在一些不友好的情况，需要特别注意合理的开启超时和重试。 默认情况下是禁用的。Java Chassis 只能使用信号量隔离机制。</li>
<li>Hystrix 的引入对性能影响很大， 会有 20% 以上的框架性能损失。 </li>
<li>Hystrix 项目目前已经停止维护。 </li>
</ul>
<h2 id="_2">概念阐述</h2>
<p>服务故障保护指服务请求异常时，微服务所采用的异常处理策略， 根据处理机制和效果把它分解为三个技术概念：“隔离”、“熔断”、“容错”：</p>
<ul>
<li>“隔离”是一种异常检测机制，常用的检测方法是请求超时、流量过大等。一般的设置参数包括超时时间、同时并发请求个数等。</li>
<li>“熔断”是一种异常反应机制，“熔断”依赖于“隔离”。熔断通常基于错误率来实现。一般的设置参数包括统计请求的个数、错误率等。</li>
<li>“容错”是一种异常处理机制，“容错”依赖于“熔断”。熔断以后，会调用“容错”的方法。一般的设置参数包括调用容错方法的次数等。</li>
</ul>
<p>当前 ServiceComb 提供两种容错方式，分别为返回 null 值和抛出异常。</p>
<p><strong><em> 注意 ： </em></strong> 在不同的上下文，“隔离”、“熔断”、“容错”、“降级” 等概念可能存在不一样的含义，发现这里的定义和其他地方存在不一样的
的时候不用感到意外， 但是不用担心， 当服务出现故障的时候， 需要一定的措施进行应对， 对于措施和应对方式的理解胜于对于名词的理解。</p>
<h2 id="_3">配置说明</h2>
<p>配置项支持对所有接口生效，或者对某个微服务的某个具体方法生效。</p>
<ul>
<li>配置项生效范围<ul>
<li>按照类型(type)：配置项能够针对Provider, Consumer进行配置</li>
<li>按照范围(scope)：配置项能够针对服务进行配置, 也可以针对接口进行配置</li>
</ul>
</li>
</ul>
<p>本章节如果没有特殊说明，所有的配置项都支持按照下面的格式进行配置：</p>
<pre><code>servicecomb.[namespace].[type].[scope].[property name]
</code></pre>
<p>type 指 Provider 或者 Consumser。 scope 指配置项生效范围， 针对特定的微服务的配置，需要增加 MicroServiceName, 针对接口配置的，需要指定接口名称，接口名
称由 <code>schemaId</code> 和 <code>operationId</code> 组成。</p>
<p>下面是一些配置示例：</p>
<pre><code>servicecomb.isolation.Consumer.timeout.enabled # 全局配置
servicecomb.isolation.Consumer.DemoService.timeout.enabled # 服务配置
servicecomb.isolation.Consumer.DemoService.hello.sayHello.timeout.enabled # 接口配置
servicecomb.isolation.Provider.timeout.enabled # 全局配置
servicecomb.isolation.Provider.DemoService.timeout.enabled # 服务配置
servicecomb.isolation.Provider.DemoService.hello.sayHello.timeout.enabled # 接口配置
</code></pre>
<ul>
<li>配置项列表</li>
</ul>
<table>
<thead>
<tr>
<th align="left">配置项</th>
<th align="left">默认值</th>
<th align="left">取值范围</th>
<th align="left">是否必选</th>
<th align="left">含义</th>
<th align="left">注意</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">servicecomb.isolation.[type].[scope].timeout.enabled</td>
<td align="left">FALSE</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">是否启用超时检测</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.isolation.[type].[scope].timeoutInMilliseconds</td>
<td align="left">30000</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">超时时间阈值</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.isolation.[type].[scope].maxConcurrentRequests</td>
<td align="left">1000</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">最大并发数阈值</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].[scope].enabled</td>
<td align="left">TRUE</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">是否启用熔断措施</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].[scope].forceOpen</td>
<td align="left">FALSE</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">不管失败次数，都进行熔断</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].[scope].forceClosed</td>
<td align="left">FALSE</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">任何时候都不熔断</td>
<td align="left">当与forceOpen同时配置时，forceOpen优先。</td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].[scope].sleepWindowInMilliseconds</td>
<td align="left">15000</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">熔断后，多长时间恢复</td>
<td align="left">恢复后，会重新计算失败情况。注意：如果恢复后的调用立即失败，那么会立即重新进入熔断。</td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].[scope].requestVolumeThreshold</td>
<td align="left">20</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">10s内请求数需要大于等于这个参数值，才开始计算错误率和判断是否进行熔断。</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.circuitBreaker.[type].[scope].errorThresholdPercentage</td>
<td align="left">50</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">错误率阈值，达到阈值则触发熔断</td>
<td align="left">由于10秒还会被划分为10个1秒的统计周期，经过1s中后才会开始计算错误率，因此从调用开始至少经过1s，才会发生熔断。</td>
</tr>
<tr>
<td align="left">servicecomb.fallback.[type].[scope].maxConcurrentRequests</td>
<td align="left">10</td>
<td align="left">-</td>
<td align="left">否</td>
<td align="left">并发调用容错处理措施（servicecomb.fallbackpolicy.policy）的请求数，超过这个值则不再调用处理措施，直接返回异常</td>
<td align="left"></td>
</tr>
<tr>
<td align="left">servicecomb.fallbackpolicy.[type].[scope].policy</td>
<td align="left">throwException</td>
<td align="left">returnNull | throwexception</td>
<td align="left">否</td>
<td align="left">出错后的处理策略</td>
<td align="left"></td>
</tr>
</tbody>
</table>
<p><strong>注意</strong>：谨慎使用 <code>servicecomb.isolation.timeout.enabled=true</code> 。因为系统处理链都是异步执行，中间处理链的返回，会导致
后面处理链的逻辑处理效果丢失。尽可能将 <code>servicecomb.isolation.timeout.enabled</code> 保持默认值false，并且正确设置网络层超时时
间 <code>servicecomb.request.timeout=30000</code> 。 配置范围不支持 Schema 级别的配置。</p>
<ul>
<li>示例代码</li>
</ul>
<pre><code class="language-yaml">servicecomb:
handler:
chain:
Consumer:
default: bizkeeper-consumer
isolation:
Consumer:
timeout:
enabled: true
timeoutInMilliseconds: 30000
circuitBreaker:
Consumer:
enabled: true
sleepWindowInMilliseconds: 15000
requestVolumeThreshold: 20
fallback:
Consumer:
enabled: true
fallbackpolicy:
Consumer:
policy: throwException
</code></pre>
<blockquote>
<p><strong>说明：</strong><br />
降级策略需要启用服务治理能力，对应的服务提供者的handler是<code>bizkeeper-provider</code>，服务消费者的handler是<code>bizkeeper-consumer</code>。</p>
</blockquote>
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