[[introduction]]
= Spring Cloud Circuit Breaker

Spring Cloud Circuit breaker provides an abstraction across different circuit breaker implementations.
It provides a consistent API to use in your applications, letting you, the developer, choose the circuit breaker implementation that best fits your needs for your application.

[[supported-implementations]]
== Supported Implementations

Spring Cloud supports the following circuit-breaker implementations:

* https://github.com/resilience4j/resilience4j[Resilience4J]
* https://github.com/alibaba/Sentinel[Sentinel]
* https://github.com/spring-projects/spring-retry[Spring Retry]

[[core-concepts]]
== Core Concepts

To create a circuit breaker in your code, you can use the `CircuitBreakerFactory` API. When you include a Spring Cloud Circuit Breaker starter on your classpath, a bean that implements this API is automatically created for you.
The following example shows a simple example of how to use this API:

[source,java]
----
@Service
public static class DemoControllerService {
	private RestTemplate rest;
	private CircuitBreakerFactory cbFactory;

	public DemoControllerService(RestTemplate rest, CircuitBreakerFactory cbFactory) {
		this.rest = rest;
		this.cbFactory = cbFactory;
	}

	public String slow() {
		return cbFactory.create("slow").run(() -> rest.getForObject("/slow", String.class), throwable -> "fallback");
	}

}
----

The `CircuitBreakerFactory.create` API creates an instance of a class called `CircuitBreaker`.
The `run` method takes a `Supplier` and a `Function`.
The `Supplier` is the code that you are going to wrap in a circuit breaker.
The `Function` is the fallback that is run if the circuit breaker is tripped.
The function is passed the `Throwable` that caused the fallback to be triggered.
You can optionally exclude the fallback if you do not want to provide one.

[[circuit-breakers-in-reactive-code]]
=== Circuit Breakers In Reactive Code

If Project Reactor is on the class path, you can also use `ReactiveCircuitBreakerFactory` for your reactive code.
The following example shows how to do so:

[source,java]
----
@Service
public static class DemoControllerService {
	private ReactiveCircuitBreakerFactory cbFactory;
	private WebClient webClient;


	public DemoControllerService(WebClient webClient, ReactiveCircuitBreakerFactory cbFactory) {
		this.webClient = webClient;
		this.cbFactory = cbFactory;
	}

	public Mono<String> slow() {
		return webClient.get().uri("/slow").retrieve().bodyToMono(String.class).transform(
		it -> cbFactory.create("slow").run(it, throwable -> return Mono.just("fallback")));
	}
}
----

The `ReactiveCircuitBreakerFactory.create` API creates an instance of a class called `ReactiveCircuitBreaker`.
The `run` method takes a `Mono` or a `Flux` and wraps it in a circuit breaker.
You can optionally profile a fallback `Function`, which will be called if the circuit breaker is tripped and is passed the `Throwable`
that caused the failure.

[[configuration]]
== Configuration

You can configure your circuit breakers by creating beans of type `Customizer`.
The `Customizer` interface has a single method (called `customize`) that takes the `Object` to customize.

For detailed information on how to customize a given implementation see
the following documentation:

* link:../../../../spring-cloud-circuitbreaker/current/reference/html/spring-cloud-circuitbreaker.html#configuring-resilience4j-circuit-breakers[Resilience4J]
* link:https://github.com/alibaba/spring-cloud-alibaba/blob/master/spring-cloud-alibaba-docs/src/main/asciidoc/circuitbreaker-sentinel.adoc#circuit-breaker-spring-cloud-circuit-breaker-with-sentinel--configuring-sentinel-circuit-breakers[Sentinel]
* link:../../../../../spring-cloud-circuitbreaker/docs/current/reference/html/spring-cloud-circuitbreaker.html#configuring-spring-retry-circuit-breakers[Spring Retry]

Some `CircuitBreaker` implementations such as `Resilience4JCircuitBreaker` call `customize` method every time `CircuitBreaker#run` is called.
It can be inefficient. In that case, you can use `CircuitBreaker#once` method. It is useful where calling `customize` many times doesn't make sense,
for example, in case of https://resilience4j.readme.io/docs/circuitbreaker#section-consume-emitted-circuitbreakerevents[consuming Resilience4j's  events].

The following example shows the way for each `io.github.resilience4j.circuitbreaker.CircuitBreaker` to consume events.

[source,java]
----
Customizer.once(circuitBreaker -> {
  circuitBreaker.getEventPublisher()
    .onStateTransition(event -> log.info("{}: {}", event.getCircuitBreakerName(), event.getStateTransition()));
}, CircuitBreaker::getName)
----
