reverse_proxy: Add local circuit breaker

This migrates a feature that was previously reserved for enterprise
users, according to https://github.com/caddyserver/caddy/issues/2786.

The local circuit breaker is a simple metrics counter that can cause
the reverse proxy to consider a backend unhealthy before it actually
goes offline, by measuring recent latencies over a sliding window.

Credit to Danny Navarro

modules/caddyhttp/reverseproxy/circuitbreaker.go

@@ -0,0 +1,152 @@
+// Copyright 2015 Matthew Holt and The Caddy Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package reverseproxy
+
+import (
+	"fmt"
+	"sync/atomic"
+	"time"
+
+	"github.com/caddyserver/caddy/v2"
+	"github.com/vulcand/oxy/memmetrics"
+)
+
+func init() {
+	caddy.RegisterModule(localCircuitBreaker{})
+}
+
+// localCircuitBreaker implements circuit breaking functionality
+// for requests within this process over a sliding time window.
+type localCircuitBreaker struct {
+	tripped   int32
+	cbType    int32
+	threshold float64
+	metrics   *memmetrics.RTMetrics
+	tripTime  time.Duration
+	Config
+}
+
+// CaddyModule returns the Caddy module information.
+func (localCircuitBreaker) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		Name: "http.handlers.reverse_proxy.circuit_breakers.local",
+		New:  func() caddy.Module { return new(localCircuitBreaker) },
+	}
+}
+
+// Provision sets up a configured circuit breaker.
+func (c *localCircuitBreaker) Provision(ctx caddy.Context) error {
+	t, ok := typeCB[c.Type]
+	if !ok {
+		return fmt.Errorf("type is not defined")
+	}
+
+	if c.TripTime == "" {
+		c.TripTime = defaultTripTime
+	}
+
+	tw, err := time.ParseDuration(c.TripTime)
+	if err != nil {
+		return fmt.Errorf("cannot parse trip_time duration, %v", err.Error())
+	}
+
+	mt, err := memmetrics.NewRTMetrics()
+	if err != nil {
+		return fmt.Errorf("cannot create new metrics: %v", err.Error())
+	}
+
+	c.cbType = t
+	c.tripTime = tw
+	c.threshold = c.Threshold
+	c.metrics = mt
+	c.tripped = 0
+
+	return nil
+}
+
+// Ok returns whether the circuit breaker is tripped or not.
+func (c *localCircuitBreaker) Ok() bool {
+	tripped := atomic.LoadInt32(&c.tripped)
+	return tripped == 0
+}
+
+// RecordMetric records a response status code and execution time of a request. This function should be run in a separate goroutine.
+func (c *localCircuitBreaker) RecordMetric(statusCode int, latency time.Duration) {
+	c.metrics.Record(statusCode, latency)
+	c.checkAndSet()
+}
+
+// Ok checks our metrics to see if we should trip our circuit breaker, or if the fallback duration has completed.
+func (c *localCircuitBreaker) checkAndSet() {
+	var isTripped bool
+
+	switch c.cbType {
+	case typeErrorRatio:
+		// check if amount of network errors exceed threshold over sliding window, threshold for comparison should be < 1.0 i.e. .5 = 50th percentile
+		if c.metrics.NetworkErrorRatio() > c.threshold {
+			isTripped = true
+		}
+	case typeLatency:
+		// check if threshold in milliseconds is reached and trip
+		hist, err := c.metrics.LatencyHistogram()
+		if err != nil {
+			return
+		}
+
+		l := hist.LatencyAtQuantile(c.threshold)
+		if l.Nanoseconds()/int64(time.Millisecond) > int64(c.threshold) {
+			isTripped = true
+		}
+	case typeStatusCodeRatio:
+		// check ratio of error status codes of sliding window, threshold for comparison should be < 1.0 i.e. .5 = 50th percentile
+		if c.metrics.ResponseCodeRatio(500, 600, 0, 600) > c.threshold {
+			isTripped = true
+		}
+	}
+
+	if isTripped {
+		c.metrics.Reset()
+		atomic.AddInt32(&c.tripped, 1)
+
+		// wait tripTime amount before allowing operations to resume.
+		t := time.NewTimer(c.tripTime)
+		<-t.C
+
+		atomic.AddInt32(&c.tripped, -1)
+	}
+}
+
+// Config represents the configuration of a circuit breaker.
+type Config struct {
+	Threshold float64 `json:"threshold"`
+	Type      string  `json:"type"`
+	TripTime  string  `json:"trip_time"`
+}
+
+const (
+	typeLatency = iota + 1
+	typeErrorRatio
+	typeStatusCodeRatio
+	defaultTripTime = "5s"
+)
+
+var (
+	// typeCB handles converting a Config Type value to the internal circuit breaker types.
+	typeCB = map[string]int32{
+		"latency":      typeLatency,
+		"error_ratio":  typeErrorRatio,
+		"status_ratio": typeStatusCodeRatio,
+	}
+)