From 6252cc02e00309feec3ca053728afd95895330a4 Mon Sep 17 00:00:00 2001
From: Ken Hibino <ken.hibino7@gmail.com>
Date: Sun, 17 Nov 2019 18:44:40 -0800
Subject: [PATCH] Implement exponential backoff retry

---
 asynq.go | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/asynq.go b/asynq.go
index 9d4f4df..620373e 100644
--- a/asynq.go
+++ b/asynq.go
@@ -15,6 +15,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"math"
+	"math/rand"
 	"strconv"
 	"time"
 
@@ -59,7 +61,7 @@ type taskMessage struct {
 	// queue name this message should be enqueued to
 	Queue string
 
-	// remainig retry count
+	// max number of retry for this task.
 	Retry int
 
 	// number of times we've retried so far
@@ -151,16 +153,17 @@ func (w *Workers) Run(handler TaskHandler) {
 		go func(task *Task) {
 			err := handler(task)
 			if err != nil {
-				if msg.Retry == 0 {
+				if msg.Retried >= msg.Retry {
 					// TODO(hibiken): Add the task to "dead" collection
 					fmt.Println("Retry exausted!!!")
 					return
 				}
 				fmt.Println("RETRY!!!")
-				delay := 10 * time.Second // TODO(hibiken): Implement exponential backoff.
-				msg.Retry--
+				retryAt := time.Now().Add(delaySeconds((msg.Retried)))
+				fmt.Printf("[DEBUG] retying the task in %v\n", retryAt.Sub(time.Now()))
+				msg.Retried++
 				msg.ErrorMsg = err.Error()
-				if err := zadd(w.rdb, retry, float64(time.Now().Add(delay).Unix()), &msg); err != nil {
+				if err := zadd(w.rdb, retry, float64(retryAt.Unix()), &msg); err != nil {
 					// TODO(hibiken): Not sure how to handle this error
 					log.Printf("[SEVERE ERROR] could not add msg %+v to 'retry' set: %v\n", msg, err)
 					return
@@ -243,3 +246,11 @@ func zadd(rdb *redis.Client, zset string, zscore float64, msg *taskMessage) erro
 func listQueues(rdb *redis.Client) []string {
 	return rdb.SMembers(allQueues).Val()
 }
+
+// delaySeconds returns a number seconds to delay before retrying.
+// Formula taken from https://github.com/mperham/sidekiq.
+func delaySeconds(count int) time.Duration {
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	s := int(math.Pow(float64(count), 4)) + 15 + (r.Intn(30) * (count + 1))
+	return time.Duration(s) * time.Second
+}