# CircuitBreaker
resilience4j是一个轻量级的fault tolerance library，其针对函数式编程进行设计。resilence4j提供了更高阶的函数（`decorator`）来对`function interface, lambda expression, method reference`等内容进行增强。

decorators包含如下分类：
- CircuitBreaker
- Rate Limiter
- Retry
- Bulkhead

对于任何`function interface, lambda expression, method reference`，都可以使用多个decorators进行装饰。

## Introduction
在如下示例中，会展示如何通过CircuitBreaker和Retry来对lambda expression进行装饰，令lambda在发生异常时最多重试3次。

可以针对多次retry之间的interval进行配置，也支持自定义的backoff algorithm。

```java
// Create a CircuitBreaker with default configuration
CircuitBreaker circuitBreaker = CircuitBreaker
  .ofDefaults("backendService");

// Create a Retry with default configuration
// 3 retry attempts and a fixed time interval between retries of 500ms
Retry retry = Retry
  .ofDefaults("backendService");

// Create a Bulkhead with default configuration
Bulkhead bulkhead = Bulkhead
  .ofDefaults("backendService");

Supplier<String> supplier = () -> backendService
  .doSomething(param1, param2)

// Decorate your call to backendService.doSomething() 
// with a Bulkhead, CircuitBreaker and Retry
// **note: you will need the resilience4j-all dependency for this
Supplier<String> decoratedSupplier = Decorators.ofSupplier(supplier)
  .withCircuitBreaker(circuitBreaker)
  .withBulkhead(bulkhead)
  .withRetry(retry)  
  .decorate();

// When you don't want to decorate your lambda expression,
// but just execute it and protect the call by a CircuitBreaker.
String result = circuitBreaker
  .executeSupplier(backendService::doSomething);

// You can also run the supplier asynchronously in a ThreadPoolBulkhead
 ThreadPoolBulkhead threadPoolBulkhead = ThreadPoolBulkhead
  .ofDefaults("backendService");

// The Scheduler is needed to schedule a timeout 
// on a non-blocking CompletableFuture
ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3);
TimeLimiter timeLimiter = TimeLimiter.of(Duration.ofSeconds(1));

CompletableFuture<String> future = Decorators.ofSupplier(supplier)
    .withThreadPoolBulkhead(threadPoolBulkhead)
    .withTimeLimiter(timeLimiter, scheduledExecutorService)
    .withCircuitBreaker(circuitBreaker)
    .withFallback(asList(TimeoutException.class, 
                         CallNotPermittedException.class, 
                         BulkheadFullException.class),  
                  throwable -> "Hello from Recovery")
    .get().toCompletableFuture();
```
## maven
resilence4j需要jdk17及以上，如果使用maven，可以按照如下方式来引入

引入所有包的方式如下
```xml
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-all</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
```

按需引入方式如下
```xml
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-circuitbreaker</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-ratelimiter</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-retry</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-bulkhead</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-cache</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-timelimiter</artifactId>
    <version>${resilience4jVersion}</version>
</dependency>
```