Hystrix源码解析--HystrixCircuitBreaker--从抽象接口说起（二）

一、HystrixCircuitBreaker接口的起源

断路器模式的思想来源于Netflix这家公司。Spring cloud是非常贱的，他搞了一个CircuitBreaker接口，搞得好像HystrixCircuitBreaker是继承自Spring cloud的CircuitBreaker一样，其实并不是这样的，HystrixCircuitBreaker跟Spring cloud的CircuitBreaker半毛钱的关系都没有，按照出生时间来算，是先有HystrixCircuitBreaker接口，然后才有Spring cloud的CircuitBreaker。

二、HystrixCircuitBreaker的接口定义

/**
 * Circuit-breaker logic that is hooked into {@link HystrixCommand} execution and will stop allowing executions if failures have gone past the defined threshold.
 * <p>
 * The default (and only) implementation  will then allow a single retry after a defined sleepWindow until the execution
 * succeeds at which point it will again close the circuit and allow executions again.
 */
public interface HystrixCircuitBreaker {

    /**
     * Every {@link HystrixCommand} requests asks this if it is allowed to proceed or not.  It is idempotent and does
     * not modify any internal state, and takes into account the half-open logic which allows some requests through
     * after the circuit has been opened
     * 
     * @return boolean whether a request should be permitted
     */
    boolean allowRequest();

    /**
     * Whether the circuit is currently open (tripped).
     * 
     * @return boolean state of circuit breaker
     */
    boolean isOpen();

    /**
     * Invoked on successful executions from {@link HystrixCommand} as part of feedback mechanism when in a half-open state.
     */
    void markSuccess();

    /**
     * Invoked on unsuccessful executions from {@link HystrixCommand} as part of feedback mechanism when in a half-open state.
     */
    void markNonSuccess();

    /**
     * Invoked at start of command execution to attempt an execution.  This is non-idempotent - it may modify internal
     * state.
     */
    boolean attemptExecution();
}

HystrixCircuitBreaker接口定义的方法非常的简单，通过源码分析，我们知道，其实第一个方法allowRequest已经不再使用了，取而代之的是attemptExecution方法。allowRequest() 和attemptExecution() 方法，方法目的基本类似，差别在于当断路器满足尝试关闭条件时，前者不会将断路器不会修改状态( CLOSE => HALF-OPEN )，而后者会。很多人估计都很疑惑，HystrixCircuitBreaker到底是被谁调用的呢？ 其实很简单，肯定是被Command调用的。至于为什么？我会在后续的博文中给出解释的。从抽象角度看，调用的流程图就是如上所示。