stability/circuit-breaker: revamp the old example

stability/circuit-breaker.md

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+# 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 throught 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.
+
+```go
+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, cancelation signals, and
+other request-scoped values across API boundaries and between processes.
+
+```go
+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](https://github.com/sony/go-breaker) is a well-tested and intuitive circuit breaker implementation for real-world use cases.