undefined## Circuit Breaker: Hystrix Clients {#circuit-breaker-hystrix-clients}

Netflix has created a library called Hystrix that implements the circuit breaker pattern. In a microservice architecture it is common to have multiple layers of service calls.

Figure 1. Microservice Graph

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 reach a certain threshold (20 failures in 5 seconds is the default in Hystrix), 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.

Figure 2. Hystrix fallback prevents cascading failures

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.

Example boot app:

<pre>@SpringBootApplication @EnableCircuitBreaker public class Application {

public static void main(String[] args) {
    new SpringApplicationBuilder(Application.class).web(true).run(args);
}

}

@Component public class StoreIntegration {

@HystrixCommand(fallbackMethod = "defaultStores")
public Object getStores(Map<String, Object> parameters) {
    //do stuff that might fail
}

public Object defaultStores(Map<String, Object> parameters) {
    return /* something useful */;
}

}</pre>

The @HystrixCommand is provided by a Netflix contrib library called "javanica". Spring Cloud automatically wraps Spring beans with that annotation in a proxy that is connected to the Hystrix circuit breaker. The circuit breaker calculates when to open and close the circuit, and what to do in case of a failure.

To configure the @HystrixCommand you can use the commandProperties attribute with a list of @HystrixProperty annotations. See here for more details. See the Hystrix wiki for details on the properties available.

Propagating the Security Context or using Spring Scopes

If you want some thread local context to propagate into a @HystrixCommand 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:

@HystrixCommand(fallbackMethod = "stubMyService",
    commandProperties = {
      @HystrixProperty(name="execution.isolation.strategy", value="SEMAPHORE")
    }
)
...

The same thing applies if you are using @SessionScope or @RequestScope. You will know when you need to do this because of a runtime exception that says it can’t find the scoped context.

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Health Indicator

The state of the connected circuit breakers are also exposed in the /health endpoint of the calling application.

{
    "hystrix": {
        "openCircuitBreakers": [
            "StoreIntegration::getStoresByLocationLink"
        ],
        "status": "CIRCUIT_OPEN"
    },
    "status": "UP"
}

Hystrix Metrics Stream

To enable the Hystrix metrics stream include a dependency on spring-boot-starter-actuator. This will expose the /hystrix.stream as a management endpoint.

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>