Add support for Jitter (#28)

* Expose Rate for better scaling control
* Expose other Retry values to allow for direct instantiation
* Use Phi as default scaling rate.

README.md

@@ -8,7 +8,7 @@ An exponentially backing off retry package for Go.
 go get github.com/coder/retry@latest
 ```
 
-`retry` promotes control flow using `for`/`goto` instead of callbacks, which are unwieldy in Go.
+`retry` promotes control flow using `for`/`goto` instead of callbacks.
 
 ## Examples
 
@@ -21,6 +21,12 @@ func pingGoogle(ctx context.Context) error {
 
 	r := retry.New(time.Second, time.Second*10);
 
+	// Jitter is useful when the majority of clients to a service use
+	// the same backoff policy.
+	//
+	// It is provided as a standard deviation.
+	r.Jitter = 0.1
+
   retry:
 	_, err = http.Get("https://google.com")
 	if err != nil {

retrier.go

@@ -2,35 +2,73 @@ package retry
 
 import (
 	"context"
+	"math"
+	"math/rand"
 	"time"
 )
 
 // Retrier implements an exponentially backing off retry instance.
 // Use New instead of creating this object directly.
 type Retrier struct {
-	delay       time.Duration
-	floor, ceil time.Duration
+	// Delay is the current delay between attempts.
+	Delay time.Duration
+
+	// Floor and Ceil are the minimum and maximum delays.
+	Floor, Ceil time.Duration
+
+	// Rate is the rate at which the delay grows.
+	// E.g. 2 means the delay doubles each time.
+	Rate float64
+
+	// Jitter determines the level of indeterminism in the delay.
+	//
+	// It is the standard deviation of the normal distribution of a random variable
+	// multiplied by the delay. E.g. 0.1 means the delay is normally distributed
+	// with a standard deviation of 10% of the delay. Floor and Ceil are still
+	// respected, making outlandish values impossible.
+	//
+	// Jitter can help avoid thundering herds.
+	Jitter float64
 }
 
 // New creates a retrier that exponentially backs off from floor to ceil pauses.
 func New(floor, ceil time.Duration) *Retrier {
 	return &Retrier{
-		delay: 0,
-		floor: floor,
-		ceil:  ceil,
+		Delay: 0,
+		Floor: floor,
+		Ceil:  ceil,
+		// Phi scales more calmly than 2, but still has nice pleasing
+		// properties.
+		Rate: math.Phi,
+	}
+}
+
+func applyJitter(d time.Duration, jitter float64) time.Duration {
+	if jitter == 0 {
+		return d
+	}
+	d *= time.Duration(1 + jitter*rand.NormFloat64())
+	if d < 0 {
+		return 0
 	}
+	return d
 }
 
+// Wait returns after min(Delay*Growth, Ceil) or ctx is cancelled.
+// The first call to Wait will return immediately.
 func (r *Retrier) Wait(ctx context.Context) bool {
-	const growth = 2
-	r.delay *= growth
-	if r.delay > r.ceil {
-		r.delay = r.ceil
+	r.Delay *= time.Duration(float64(r.Delay) * r.Rate)
+
+	r.Delay = applyJitter(r.Delay, r.Jitter)
+
+	if r.Delay > r.Ceil {
+		r.Delay = r.Ceil
 	}
+
 	select {
-	case <-time.After(r.delay):
-		if r.delay < r.floor {
-			r.delay = r.floor
+	case <-time.After(r.Delay):
+		if r.Delay < r.Floor {
+			r.Delay = r.Floor
 		}
 		return true
 	case <-ctx.Done():
@@ -40,5 +78,5 @@ func (r *Retrier) Wait(ctx context.Context) bool {
 
 // Reset resets the retrier to its initial state.
 func (r *Retrier) Reset() {
-	r.delay = 0
+	r.Delay = 0
 }

retrier_test.go

@@ -2,6 +2,7 @@ package retry
 
 import (
 	"context"
+	"math"
 	"testing"
 	"time"
 )
@@ -38,3 +39,53 @@ func TestReset(t *testing.T) {
 	r.Reset()
 	r.Wait(ctx)
 }
+
+func TestJitter_Normal(t *testing.T) {
+	t.Parallel()
+
+	r := New(time.Millisecond, time.Millisecond)
+	r.Jitter = 0.5
+
+	var (
+		sum   time.Duration
+		waits []float64
+		ctx   = context.Background()
+	)
+	for i := 0; i < 1000; i++ {
+		start := time.Now()
+		r.Wait(ctx)
+		took := time.Since(start)
+		waits = append(waits, (took.Seconds() * 1000))
+		sum += took
+	}
+
+	avg := float64(sum) / float64(len(waits))
+	std := stdDev(waits)
+	if std > avg*0.1 {
+		t.Fatalf("standard deviation too high: %v", std)
+	}
+
+	t.Logf("average: %v", time.Duration(avg))
+	t.Logf("std dev: %v", std)
+	t.Logf("sample: %v", waits[len(waits)-10:])
+}
+
+// stdDev returns the standard deviation of the sample.
+func stdDev(sample []float64) float64 {
+	if len(sample) == 0 {
+		return 0
+	}
+	mean := 0.0
+	for _, v := range sample {
+		mean += v
+	}
+	mean /= float64(len(sample))
+
+	variance := 0.0
+	for _, v := range sample {
+		variance += math.Pow(v-mean, 2)
+	}
+	variance /= float64(len(sample))
+
+	return math.Sqrt(variance)
+}