该cloud.google.com/go/pubsub库最近发布（在 v1.5.0 中，参见https://github.com/googleapis/google-cloud-go/releases/tag/pubsub%2Fv1.5.0）支持新的RetryPolicy服务器端功能。当前的文档（https://godoc.org/cloud.google.com/go/pubsub#RetryPolicy）为
我读过维基百科的文章，虽然它描述了离散时间的指数退避，但我看不出这篇文章与MinimumBackoff和MaximumBackoff参数的具体关系。有关这方面的指导，我参考了github.com/cenkalti/backoffhttps://pkg.go.dev/github.com/cenkalti/backoff/v4?tab=doc#ExponentialBackOff的文档。该库定义ExponentialBackoff为
type ExponentialBackOff struct {
InitialInterval time.Duration
RandomizationFactor float64
Multiplier float64
MaxInterval time.Duration
// After MaxElapsedTime the ExponentialBackOff returns Stop.
// It never stops if MaxElapsedTime == 0.
MaxElapsedTime time.Duration
Stop time.Duration
Clock Clock
// contains filtered or unexported fields
}
其中每个随机间隔计算为
randomized interval =
RetryInterval * (random value in range [1 - RandomizationFactor, 1 + RandomizationFactor])
RetryInterval当前的重试间隔在哪里，据我所知，它从 的值开始并以InitialInterval为上限MaxInterval。
我是否正确理解MinimumBackoffandMaximumBackoff对应于InitialIntervaland MaxIntervalin github.com/cenkalti/backoff？也就是说，MinimumBackoff是初始等待时间，MaximumBackoff是重试之间允许的最大时间量？
为了测试我的理论，我编写了以下简化程序：
package main
import (
"context"
"flag"
"fmt"
"log"
"os"
"time"
"cloud.google.com/go/pubsub"
"google.golang.org/grpc/codes"
"google.golang.org/grpc/status"
)
var (
projectID string
minimumBackoff, maximumBackoff time.Duration
)
const (
topicName = "test-topic"
subName = "test-subscription"
defaultMinimumBackoff = 10 * time.Second
defaultMaximumBackoff = 10 * time.Minute
)
func main() {
flag.StringVar(&projectID, "projectID", "my-project", "Google Project ID")
flag.DurationVar(&minimumBackoff, "minimumBackoff", 5*time.Second, "minimum backoff")
flag.DurationVar(&maximumBackoff, "maximumBackoff", 60*time.Second, "maximum backoff")
flag.Parse()
log.Printf("Running with minumum backoff %v and maximum backoff %v...", minimumBackoff, maximumBackoff)
retryPolicy := &pubsub.RetryPolicy{MinimumBackoff: minimumBackoff, MaximumBackoff: maximumBackoff}
client, err := pubsub.NewClient(context.Background(), projectID)
if err != nil {
log.Fatalf("NewClient: %v", err)
}
topic, err := client.CreateTopic(context.Background(), topicName)
if err != nil {
log.Fatalf("CreateTopic: %v", err)
}
log.Printf("Created topic %q", topicName)
defer func() {
topic.Stop()
if err := topic.Delete(context.Background()); err != nil {
log.Fatalf("Delete topic: %v", err)
}
log.Printf("Deleted topic %s", topicName)
}()
sub, err := client.CreateSubscription(context.Background(), subName, pubsub.SubscriptionConfig{
Topic: topic,
RetryPolicy: retryPolicy,
})
if err != nil {
log.Fatalf("CreateSubscription: %v", err)
}
log.Printf("Created subscription %q", subName)
defer func() {
if err := sub.Delete(context.Background()); err != nil {
log.Fatalf("Delete subscription: %v", err)
}
log.Printf("Deleted subscription %q", subName)
}()
go func() {
sub.Receive(context.Background(), func(ctx context.Context, msg *pubsub.Message) {
log.Printf("Nacking message: %s", msg.Data)
msg.Nack()
})
}()
topic.Publish(context.Background(), &pubsub.Message{Data: []byte("Hello, world!")})
log.Println("Published message")
time.Sleep(60 * time.Second)
}
如果我分别使用 flag-defaultMinimumBackoff和MaximumBackoff5s 和 60s 运行它，我会得到以下输出：
> go run main.go
2020/07/29 18:49:32 Running with minumum backoff 5s and maximum backoff 1m0s...
2020/07/29 18:49:33 Created topic "test-topic"
2020/07/29 18:49:34 Created subscription "test-subscription"
2020/07/29 18:49:34 Published message
2020/07/29 18:49:36 Nacking message: Hello, world!
2020/07/29 18:49:45 Nacking message: Hello, world!
2020/07/29 18:49:56 Nacking message: Hello, world!
2020/07/29 18:50:06 Nacking message: Hello, world!
2020/07/29 18:50:17 Nacking message: Hello, world!
2020/07/29 18:50:30 Nacking message: Hello, world!
2020/07/29 18:50:35 Deleted subscription "test-subscription"
2020/07/29 18:50:35 Deleted topic test-topic
而如果我分别用 1s 和 2s 运行它MinimumBackoff，MaximumBackoff我得到
> go run main.go --minimumBackoff=1s --maximumBackoff=2s
2020/07/29 18:50:42 Running with minumum backoff 1s and maximum backoff 2s...
2020/07/29 18:51:11 Created topic "test-topic"
2020/07/29 18:51:12 Created subscription "test-subscription"
2020/07/29 18:51:12 Published message
2020/07/29 18:51:15 Nacking message: Hello, world!
2020/07/29 18:51:18 Nacking message: Hello, world!
2020/07/29 18:51:21 Nacking message: Hello, world!
2020/07/29 18:51:25 Nacking message: Hello, world!
2020/07/29 18:51:28 Nacking message: Hello, world!
2020/07/29 18:51:31 Nacking message: Hello, world!
2020/07/29 18:51:35 Nacking message: Hello, world!
2020/07/29 18:51:38 Nacking message: Hello, world!
2020/07/29 18:51:40 Nacking message: Hello, world!
2020/07/29 18:51:44 Nacking message: Hello, world!
2020/07/29 18:51:47 Nacking message: Hello, world!
2020/07/29 18:51:50 Nacking message: Hello, world!
2020/07/29 18:51:52 Nacking message: Hello, world!
2020/07/29 18:51:54 Nacking message: Hello, world!
2020/07/29 18:51:57 Nacking message: Hello, world!
2020/07/29 18:52:00 Nacking message: Hello, world!
2020/07/29 18:52:03 Nacking message: Hello, world!
2020/07/29 18:52:06 Nacking message: Hello, world!
2020/07/29 18:52:09 Nacking message: Hello, world!
2020/07/29 18:52:12 Nacking message: Hello, world!
2020/07/29 18:52:13 Deleted subscription "test-subscription"
2020/07/29 18:52:13 Deleted topic test-topic
在后一个示例中，nack 之间的时间似乎非常一致~3s，这大概代表了在MaximumBackoff2s 内完成它的“最大努力”？我仍然不清楚的是是否有任何随机化，是否有乘数（从第一个例子来看，重试之间的时间似乎不是每次都加倍），以及是否有等价的MaxElapsedTime没有更多的重试？