client: determenistic retry backoff

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

client/backoff.go

@@ -2,6 +2,7 @@ package client
 
 import (
 	"context"
+	"math"
 	"time"
 
 	"go.unistack.org/micro/v3/util/backoff"
@@ -10,6 +11,20 @@ import (
 // BackoffFunc is the backoff call func
 type BackoffFunc func(ctx context.Context, req Request, attempts int) (time.Duration, error)
 
-func exponentialBackoff(ctx context.Context, req Request, attempts int) (time.Duration, error) {
+// BackoffExp using exponential backoff func
+func BackoffExp(_ context.Context, _ Request, attempts int) (time.Duration, error) {
 	return backoff.Do(attempts), nil
 }
+
+// BackoffInterval specifies randomization interval for backoff func
+func BackoffInterval(min time.Duration, max time.Duration) BackoffFunc {
+	return func(_ context.Context, _ Request, attempts int) (time.Duration, error) {
+		td := time.Duration(time.Duration(math.Pow(float64(attempts), math.E)) * time.Millisecond * 100)
+		if td < min {
+			return min, nil
+		} else if td > max {
+			return max, nil
+		}
+		return td, nil
+	}
+}