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Circuit-breaker strategy

To create a circuit-breaker strategy we need to call our policy factory using the createCircuitBreakerHttpExecutor method:

const circuitBreakerPolicyExecutor =
  PolicyExecutorFactory.createCircuitBreakerHttpExecutor({
      maxNumberOfRetriesBeforeCircuitIsOpen: 1,
      retryIntervalStrategy: RetryIntervalStrategy.Constant,
      baseRetryDelayInMilli: 500,
      timeoutPerRetryInMilli: 2000,
      circuitOpenDurationInMilli: 3000,
      onClose: () => console.log("custom close"),
      onHalfOpen: () => console.log("custom half open"),
      onOpen: () => console.log("custom open"),
  });

The createCircuitBreakerHttpExecutor method takes a configuration object to properly configure the circuit-breaker policy.

Let's take a closer look at the fields.

fields to compute retry strategy

The fields mentioned here are: maxNumberOfRetriesBeforeCircuitIsOpen, retryIntervalStrategy, baseRetryDelayInMilli, timeoutPerRetryInMilli, excludeRetriesOnStatusCodes.

To understand these fields better, please refer to the retry documentation.

Our circuit-breaker can be enabled by retry policies. The fields mentioned above will configure such retry policies.

If no retry mechanism is wanted, just place maxNumberOfRetriesBeforeCircuitIsOpen as 1 and ignore all the remaining fields.

A note about the maxNumberOfRetriesBeforeCircuitIsOpen property. As the name suggests, after the amount of retries specified here, the circuit will be open and the circuit-breaker state machine will start. If no value is specified here, the default value of 3 will be assumed.

circuitOpenDurationInMilli

Whenever the maximum amount of retries is reached, the circuit is going from the state CLOSED to the state OPENED, and will remaing in such state for the amount of milliseconds specified here.

As long as the circuit remains on this state, all requests made will be automatically rejects, with an error thrown under the error property on the result object.

After circuitOpenDurationInMilli, the state will transition to HALF-OPEN.

custom state transition functions

As states change, you might want to provide a custom state changing function. This function must return nothing (be void).

If you wish to add any custom functions for state transitioning, please consider doing so under the optional properties onOpen, onHalfOpen and onClose.

If nothing is provided in these fields, a simple console.log is done to inform you about states being transitioned.

Important

To fully achieve the circuit-breaker with our tool, please consider using one circuit-breaker executor per endpoint that you want to manage the circuit state for. Be consistent in using one state machine for a given endpoint.