If the circuit breaker has only been open for a short time, less than the OpenToHalfOpenWaitTime value, the ExecuteAction method simply throws a CircuitBreakerOpenException exception and returns the error that caused the circuit breaker to transition to the open state.
Additionally, it uses a lock to prevent the circuit breaker from trying to perform concurrent calls to the operation while it's half open. A concurrent attempt to invoke the operation will be handled as if the circuit breaker was open, and it'll fail with an exception as described later.
... if (IsOpen) { // The circuit breaker is Open. Check if the Open timeout has expired. // If it has, set the state to HalfOpen. Another approach might be to // check for the HalfOpen state that had be set by some other operation. if (stateStore.LastStateChangedDateUtc + OpenToHalfOpenWaitTime < DateTime.UtcNow) { // The Open timeout has expired. Allow one operation to execute. Note that, in // this example, the circuit breaker is set to HalfOpen after being // in the Open state for some period of time. An alternative would be to set // this using some other approach such as a timer, test method, manually, and // so on, and check the state here to determine how to handle execution // of the action. // Limit the number of threads to be executed when the breaker is HalfOpen. // An alternative would be to use a more complex approach to determine which // threads or how many are allowed to execute, or to execute a simple test // method instead. bool lockTaken = false; try { Monitor.TryEnter(halfOpenSyncObject, ref lockTaken); if (lockTaken) { // Set the circuit breaker state to HalfOpen. stateStore.HalfOpen(); // Attempt the operation. action(); // If this action succeeds, reset the state and allow other operations. // In reality, instead of immediately returning to the Closed state, a counter // here would record the number of successful operations and return the // circuit breaker to the Closed state only after a specified number succeed. this.stateStore.Reset(); return; } catch (Exception ex) { // If there's still an exception, trip the breaker again immediately. this.stateStore.Trip(ex); // Throw the exception so that the caller knows which exception occurred. throw; } finally { if (lockTaken) { Monitor.Exit(halfOpenSyncObject); } } } } // The Open timeout hasn't yet expired. Throw a CircuitBreakerOpen exception to // inform the caller that the call was not actually attempted, // and return the most recent exception received. throw new CircuitBreakerOpenException(stateStore.LastException); } ...To use a CircuitBreaker object to protect an operation, an application creates an instance of the CircuitBreaker class and invokes the ExecuteAction method, specifying the operation to be performed as the parameter. The application should be prepared to catch the CircuitBreakerOpenException exception if the operation fails because the circuit breaker is open. The following code shows an example:
var breaker = new CircuitBreaker(); try { breaker.ExecuteAction(() => { // Operation protected by the circuit breaker. ... }); } catch (CircuitBreakerOpenException ex) { // Perform some different action when the breaker is open. // Last exception details are in the inner exception. ... } catch (Exception ex) { ... } 相关模式与指导在实现该模式时，以下模式也会有帮助：
重试模式。
当应用尝试连接服务或网络资源时遇到临时性错误时，简单的重试之前失败的操作。
健康状态监控模式。
断路器应该能够通过给服务端点发送请求来测试服务的健康状况。服务应该能够返回信息来表明自身状况。
翻译自 Azure Cloud Design Patterns