How Does a Circuit Breaker for a Service Work?

Step	Description
1	The server receives a request to invoke a service. Upon receiving a request to invoke a service, the server first determines whether or not a circuit breaker is configured for the service. If a circuit breaker is configured for the service, the server passes the request to the circuit breaker which checks the state of the circuit as described in step 2, below. If a circuit breaker is not used with the service, the server invokes the service.
2	The circuit breaker examines the state of the circuit for the service. If the circuit is closed, the circuit breaker invokes the service using a new thread from the circuit breaker thread pool, which is separate from the server thread pool. The thread that called the service originally waits for the results of the service execution. If the circuit is open, the circuit breaker does not invoke the service. See step 5, below, for more information about how the circuit breaker responds to a request for service with an open circuit. If the circuit is half-open and this is the first request for the service since the circuit state became half-open, the circuit breaker invokes the service. When executing a service in a half-open state, any other requests for the service wait until the circuit exits the half-open state. For more information, see step 8, below.
3	When the circuit state is closed, upon execution of the service by the circuit breaker, one of the following occurs: The service executes successfully. Note: If the Failure event property is set to Timeout only or Exception or timeout, a successful execution indicates that the service executed successfully within the specified timeout period. The service ends with an exception. If the Failure event property is set to Exception only or Exception or Timeout, the circuit breaker considers the exception to be a failure event. The circuit breaker increments the failure count for the service by 1. The service does not execute to completion before the timeout period elapses. If the Failure event property is set to Timeout only or Exception or Timeout, the circuit breaker considers the timeout to be a failure event. The circuit breaker increments the failure count for the service by 1. If the Cancel thread on timeout property is set to true, the circuit breaker attempts to cancel the thread executing the service. If the service ends with a failure event and it is the first failure event, the circuit breaker starts the failure timeout period. If the number of failure events specified in Failure threshold property occurs before the failure timeout period ends, the circuit opens. Circuit breaker determines the time that the failure period ends by adding the number of seconds specified in the Failure period property to the time the first failure event occurred. If the service executes successfully and completes after the failure timeout period ends, circuit breaker resets the failure count for the service to 0.
4	Subsequent invocations of the service result in the circuit breaker opening or “tripping” the circuit for the service if: The number of failure events for the service equals the Failure threshold property value and The failure events occur within the failure timeout period which circuit breaker determines by adding the value of Failure period property to the time of the first failure event. The circuit breaker starts the reset period which determines the length of time to keep the circuit in an open state. The Circuit reset period property determines the length of the reset period. For example, suppose that circuit breaker treats exceptions as a failure event, the Failure threshold property is set to 5, and the Failure period is set to 60 seconds. Further suppose that the first failure event occurs at 10:07:10. Circuit breaker starts the failure timeout period. The failure timeout period ends at 10:08:10 which circuit breaker determines by adding the Failure period value to the time of the first failure event. If 5 invocations of the service end with an exception in less than 60 seconds, that is, before 10:08:10, the circuit breaker opens the circuit for the service.
5	When the circuit breaker receives a request to execute the service, the circuit breaker does one of the following: If the Circuit open action property is set to Throw exception, the circuit breaker responds to the invoke request by throwing the exception that caused the circuit to open. If the Circuit open action property is set to Invoke service, the circuit breaker executes the alternate service specified in the Circuit open service property and returns the results to the calling service. The circuit breaker places the $circuitBreakerService and $circuitBreakerEvent parameters in the input pipeline for the alternate service. For more information about the circuit open service, see the section Building a Service for Use with an Open Circuit Building a Service for Use with an Open Circuit . Note: When the circuit is open, circuit breaker does not use the circuit breaker thread pool to throw the exception or execute the alternate, open circuit service. Instead, circuit breaker uses the same thread that executes the calling service to return the exception or execute the alternate, open circuit service.
6	The circuit breaker repeats the previous step in response to a request for the service until the time specified in the Circuit reset period property elapses.
7	When the circuit reset period elapses, the circuit breaker sets the circuit to a half-open state.
8	The circuit breaker receives a request for the service. If this is the first request for the service since the circuit state became half-open, the circuit breaker invokes the requested service. One of the following occurs: If the service executes successfully, the circuit breaker closes the circuit. The circuit breaker will invoke the service upon subsequent service executions. The circuit breaker resets the failure count to 0 (zero). If service execution results in a failure event, the circuit breaker re-opens the circuit and restarts the reset period. The circuit breaker proceeds as described in step 5. While the circuit is in a half-open state and circuit breaker is already executing a request for the service, any other requests for the service wait until the circuit exits the half-open state. If service execution is successful, circuit breaker closes the circuit and executes the waiting requests for the service. If service execution is not successful, circuit breaker re-opens the circuit and responds to the waiting services as described in step 5.