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<title>13. Circuit Breaker: Hystrix Clients</title><link rel="stylesheet" type="text/css" href="css/manual-multipage.css"><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="multi_spring-cloud.html" title="Spring Cloud"><link rel="up" href="multi__spring_cloud_netflix.html" title="Part III. Spring Cloud Netflix"><link rel="prev" href="multi_spring-cloud-eureka-server.html" title="12. Service Discovery: Eureka Server"><link rel="next" href="multi__circuit_breaker_hystrix_dashboard.html" title="14. Circuit Breaker: Hystrix Dashboard"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">13. Circuit Breaker: Hystrix Clients</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="multi_spring-cloud-eureka-server.html">Prev</a> </td><th width="60%" align="center">Part III. Spring Cloud Netflix</th><td width="20%" align="right"> <a accesskey="n" href="multi__circuit_breaker_hystrix_dashboard.html">Next</a></td></tr></table><hr></div><div class="chapter"><div class="titlepage"><div><div><h2 class="title"><a name="_circuit_breaker_hystrix_clients" href="#_circuit_breaker_hystrix_clients"></a>13. Circuit Breaker: Hystrix Clients</h2></div></div></div><p>Netflix has created a library called <a class="link" href="https://github.com/Netflix/Hystrix" target="_top">Hystrix</a> that implements the <a class="link" href="https://martinfowler.com/bliki/CircuitBreaker.html" target="_top">circuit breaker pattern</a>. In a microservice architecture it is common to have multiple layers of service calls.</p><div class="figure"><a name="d0e3379" href="#d0e3379"></a><p class="title"><b>Figure 13.1. Microservice Graph</b></p><div class="figure-contents"><div class="mediaobject"><img src="https://raw.githubusercontent.com/spring-cloud/spring-cloud-netflix/1.4.x/docs/src/main/asciidoc/images/Hystrix.png" alt="Hystrix"></div></div></div><br class="figure-break"><p>A service failure in the lower level of services can cause cascading failure all the way up to the user. When calls to a particular service is greater than <code class="literal">circuitBreaker.requestVolumeThreshold</code> (default: 20 requests) and failue percentage is greater than <code class="literal">circuitBreaker.errorThresholdPercentage</code> (default: >50%) in a rolling window defined by <code class="literal">metrics.rollingStats.timeInMilliseconds</code> (default: 10 seconds), the circuit opens and the call is not made. In cases of error and an open circuit a fallback can be provided by the developer.</p><div class="figure"><a name="d0e3399" href="#d0e3399"></a><p class="title"><b>Figure 13.2. Hystrix fallback prevents cascading failures</b></p><div class="figure-contents"><div class="mediaobject"><img src="https://raw.githubusercontent.com/spring-cloud/spring-cloud-netflix/1.4.x/docs/src/main/asciidoc/images/HystrixFallback.png" alt="HystrixFallback"></div></div></div><br class="figure-break"><p>Having an open circuit stops cascading failures and allows overwhelmed or failing services time to heal. The fallback can be another Hystrix protected call, static data or a sane empty value. Fallbacks may be chained so the first fallback makes some other business call which in turn falls back to static data.</p><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="netflix-hystrix-starter" href="#netflix-hystrix-starter"></a>13.1 How to Include Hystrix</h2></div></div></div><p>To include Hystrix in your project use the starter with group <code class="literal">org.springframework.cloud</code>
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and artifact id <code class="literal">spring-cloud-starter-netflix-hystrix</code>. See the <a class="link" href="https://projects.spring.io/spring-cloud/" target="_top">Spring Cloud Project page</a>
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for details on setting up your build system with the current Spring Cloud Release Train.</p><p>Example boot app:</p><pre class="screen">@SpringBootApplication
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@EnableCircuitBreaker
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public class Application {
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public static void main(String[] args) {
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new SpringApplicationBuilder(Application.class).web(true).run(args);
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}
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}
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@Component
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public class StoreIntegration {
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@HystrixCommand(fallbackMethod = "defaultStores")
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public Object getStores(Map<String, Object> parameters) {
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//do stuff that might fail
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}
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public Object defaultStores(Map<String, Object> parameters) {
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return /* something useful */;
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}
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}</pre><p>The <code class="literal">@HystrixCommand</code> is provided by a Netflix contrib library called
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<a class="link" href="https://github.com/Netflix/Hystrix/tree/master/hystrix-contrib/hystrix-javanica" target="_top">"javanica"</a>.
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Spring Cloud automatically wraps Spring beans with that
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annotation in a proxy that is connected to the Hystrix circuit
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breaker. The circuit breaker calculates when to open and close the
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circuit, and what to do in case of a failure.</p><p>To configure the <code class="literal">@HystrixCommand</code> you can use the <code class="literal">commandProperties</code>
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attribute with a list of <code class="literal">@HystrixProperty</code> annotations. See
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<a class="link" href="https://github.com/Netflix/Hystrix/tree/master/hystrix-contrib/hystrix-javanica#configuration" target="_top">here</a>
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for more details. See the <a class="link" href="https://github.com/Netflix/Hystrix/wiki/Configuration" target="_top">Hystrix wiki</a>
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for details on the properties available.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_propagating_the_security_context_or_using_spring_scopes" href="#_propagating_the_security_context_or_using_spring_scopes"></a>13.2 Propagating the Security Context or using Spring Scopes</h2></div></div></div><p>If you want some thread local context to propagate into a <code class="literal">@HystrixCommand</code> the default declaration will not work because it executes the command in a thread pool (in case of timeouts). You can switch Hystrix to use the same thread as the caller using some configuration, or directly in the annotation, by asking it to use a different "Isolation Strategy". For example:</p><pre class="programlisting"><em><span class="hl-annotation" style="color: gray">@HystrixCommand(fallbackMethod = "stubMyService",
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commandProperties = {
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@HystrixProperty(name="execution.isolation.strategy", value="SEMAPHORE")
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}
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)</span></em>
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...</pre><p>The same thing applies if you are using <code class="literal">@SessionScope</code> or <code class="literal">@RequestScope</code>. You will know when you need to do this because of a runtime exception that says it can’t find the scoped context.</p><p>You also have the option to set the <code class="literal">hystrix.shareSecurityContext</code> property to <code class="literal">true</code>. Doing so will auto configure an Hystrix concurrency strategy plugin hook who will transfer the <code class="literal">SecurityContext</code> from your main thread to the one used by the Hystrix command. Hystrix does not allow multiple hystrix concurrency strategy to be registered so an extension mechanism is available by declaring your own <code class="literal">HystrixConcurrencyStrategy</code> as a Spring bean. Spring Cloud will lookup for your implementation within the Spring context and wrap it inside its own plugin.</p></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_health_indicator_4" href="#_health_indicator_4"></a>13.3 Health Indicator</h2></div></div></div><p>The state of the connected circuit breakers are also exposed in the
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<code class="literal">/health</code> endpoint of the calling application.</p><pre class="programlisting"><span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">{</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"hystrix"</span>: <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">{</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"openCircuitBreakers"</span>: <span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">[</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"StoreIntegration::getStoresByLocationLink"</span>
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]<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">,</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"status"</span>: <span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"CIRCUIT_OPEN"</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">},</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"status"</span>: <span xmlns:d="http://docbook.org/ns/docbook" class="hl-string">"UP"</span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-keyword">}</span></pre></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="_hystrix_metrics_stream" href="#_hystrix_metrics_stream"></a>13.4 Hystrix Metrics Stream</h2></div></div></div><p>To enable the Hystrix metrics stream include a dependency on <code class="literal">spring-boot-starter-actuator</code>. This will expose the <code class="literal">/hystrix.stream</code> as a management endpoint.</p><pre class="programlisting"> <span xmlns:d="http://docbook.org/ns/docbook" class="hl-tag"><dependency></span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-tag"><groupId></span>org.springframework.boot<span xmlns:d="http://docbook.org/ns/docbook" class="hl-tag"></groupId></span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-tag"><artifactId></span>spring-boot-starter-actuator<span xmlns:d="http://docbook.org/ns/docbook" class="hl-tag"></artifactId></span>
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<span xmlns:d="http://docbook.org/ns/docbook" class="hl-tag"></dependency></span></pre></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="multi_spring-cloud-eureka-server.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="multi__spring_cloud_netflix.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="multi__circuit_breaker_hystrix_dashboard.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">12. Service Discovery: Eureka Server </td><td width="20%" align="center"><a accesskey="h" href="multi_spring-cloud.html">Home</a></td><td width="40%" align="right" valign="top"> 14. Circuit Breaker: Hystrix Dashboard</td></tr></table></div></body></html> |