awesome-patterns/stability/circuit-breaker.md

Circuit Breaker Pattern

Similar to electrical fuses that prevent fires when a circuit that is connected to the electrical grid starts drawing a high amount of power which causes the wires to heat up and combust, the circuit breaker design pattern is a fail-first mechanism that shuts down the circuit, request/response relationship or a service in the case of software development, to prevent bigger failures.

Note: The words "circuit" and "service" are used synonymously through this document.

Implementation

Below is the implementation of a very simple circuit breaker to illustrate the purpose of the circuit breaker design pattern.

Operation Counter

circuit.Counter is a simple counter that records success and failure states of a circuit along with a timestamp and calculates the consecutive number of failures.

package circuit

import (
    "time"
)

type State int

const (
    UnknownState State = iota
    FailureState
    SuccessState
)

type Counter interface {
    Count(State)
    ConsecutiveFailures() uint32
    LastActivity() time.Time
    Reset()
}

Circuit Breaker

Circuit is wrapped using the circuit.Breaker closure that keeps an internal operation counter. It returns a fast error if the circuit has failed consecutively more than the specified threshold. After a while it retries the request and records it.

Note: Context type is used here to carry deadlines, cancellation signals, and other request-scoped values across API boundaries and between processes.

package circuit

import (
    "context"
    "time"
)

type Circuit func(context.Context) error

func Breaker(c Circuit, failureThreshold uint32) Circuit {
    cnt := NewCounter()

    return func(ctx context) error {
        if cnt.ConsecutiveFailures() >= failureThreshold {
            canRetry := func(cnt Counter) {
                backOffLevel := Cnt.ConsecutiveFailures() - failureThreshold

                // Calculates when should the circuit breaker resume propagating requests
                // to the service
                shouldRetryAt := cnt.LastActivity().Add(time.Seconds * 2 << backOffLevel)

                return time.Now().After(shouldRetryAt)
            }

            if !canRetry(cnt) {
                // Fails fast instead of propagating requests to the circuit since
                // not enough time has passed since the last failure to retry
                return ErrServiceUnavailable
            }
        }

        // Unless the failure threshold is exceeded the wrapped service mimics the
        // old behavior and the difference in behavior is seen after consecutive failures
        if err := c(ctx); err != nil {
            cnt.Count(FailureState)
            return err
        }

        cnt.Count(SuccessState)
        return nil
    }
}

Related Works

• sony/gobreaker is a well-tested and intuitive circuit breaker implementation for real-world use cases.