Add exponential backoff functions (#152)

* Add exponential backoff functions

* Move backoff exponent to backoffutils

* Add an exampe for the exponential backoff

Author: polyfloyd

retry/backoff.go

@@ -24,3 +24,21 @@ func BackoffLinearWithJitter(waitBetween time.Duration, jitterFraction float64)
 		return backoffutils.JitterUp(waitBetween, jitterFraction)
 	}
 }
+
+// BackoffExponential produces increasing intervals for each attempt.
+//
+// The scalar is multiplied times 2 raised to the current attempt. So the first
+// retry with a scalar of 100ms is 100ms, while the 5th attempt would be 3.2s.
+func BackoffExponential(scalar time.Duration) BackoffFunc {
+	return func(attempt uint) time.Duration {
+		return scalar * time.Duration(backoffutils.ExponentBase2(attempt))
+	}
+}
+
+// BackoffExponentialWithJitter creates an exponential backoff like
+// BackoffExponential does, but adds jitter.
+func BackoffExponentialWithJitter(scalar time.Duration, jitterFraction float64) BackoffFunc {
+	return func(attempt uint) time.Duration {
+		return backoffutils.JitterUp(scalar*time.Duration(backoffutils.ExponentBase2(attempt)), jitterFraction)
+	}
+}

retry/examples_test.go

@@ -42,6 +42,19 @@ func Example_initializationWithOptions() {
 	)
 }
 
+// Example with an exponential backoff starting with 100ms.
+//
+// Each next interval is the previous interval multiplied by 2.
+func Example_initializationWithExponentialBackoff() {
+	opts := []grpc_retry.CallOption{
+		grpc_retry.WithBackoff(grpc_retry.BackoffExponential(100 * time.Millisecond)),
+	}
+	grpc.Dial("myservice.example.com",
+		grpc.WithStreamInterceptor(grpc_retry.StreamClientInterceptor(opts...)),
+		grpc.WithUnaryInterceptor(grpc_retry.UnaryClientInterceptor(opts...)),
+	)
+}
+
 // Simple example of an idempotent `ServerStream` call, that will be retried automatically 3 times.
 func Example_simpleCall() {
 	client := pb_testproto.NewTestServiceClient(cc)

util/backoffutils/backoff.go

@@ -21,3 +21,8 @@ func JitterUp(duration time.Duration, jitter float64) time.Duration {
 	multiplier := jitter * (rand.Float64()*2 - 1)
 	return time.Duration(float64(duration) * (1 + multiplier))
 }
+
+// ExponentBase2 computes 2^(a-1) where a >= 1. If a is 0, the result is 0.
+func ExponentBase2(a uint) uint {
+	return (1 << a) >> 1
+}