Nats Jetstream Exactly Once Delivery

I want to implement an exactly once delivery system with Nats Jetstream. Documentation says that Jetstream has this option, but there is no samples or details about that how it's work and how clients can implement this. I know that in publisher side we can set MsgId and specify duplication window when creating Stream, but what about consumer side?

Here are the docs for exactly-once delivery. This is a bit of a misnomer since what is actually needed (and what this feature provides) is exactly-once processing.

As you point out, it is a combination of de-duplication by the server when receiving a published message as well as a double ack call by the subscription that had received the message (plus retries if necessary).

Here is an example (excess error handling elided for brevity). Start the server with JetStream enabled: nats-server --js and then run this code (it assuming nats.go v1.16+).

package main

import (
	"log"
	"time"

	"github.com/nats-io/nats.go"
)

func failOnErr(err error) {
	if err != nil {
		log.Fatal(err)
	}
}

func main() {
	// Connect and get the JetStream context.
	nc, _ := nats.Connect(nats.DefaultURL)
	js, _ := nc.JetStream()

	// Create a test stream.
	_, err := js.AddStream(&nats.StreamConfig{
		Name:       "test",
		Storage:    nats.MemoryStorage,
		Subjects:   []string{"test.>"},
		Duplicates: time.Minute,
	})
	failOnErr(err)

	defer js.DeleteStream("test")

	// Publish some messages with duplicates.
	js.Publish("test.1", []byte("hello"), nats.MsgId("1"))
	js.Publish("test.2", []byte("world"), nats.MsgId("2"))
	js.Publish("test.1", []byte("hello"), nats.MsgId("1"))
	js.Publish("test.1", []byte("hello"), nats.MsgId("1"))
	js.Publish("test.2", []byte("world"), nats.MsgId("2"))
	js.Publish("test.2", []byte("world"), nats.MsgId("2"))

	// Create an explicit pull consumer on the stream.
	_, err = js.AddConsumer("test", &nats.ConsumerConfig{
		Durable:       "test",
		AckPolicy:     nats.AckExplicitPolicy,
		DeliverPolicy: nats.DeliverAllPolicy,
	})
	failOnErr(err)
	defer js.DeleteConsumer("test", "test")

	// Create a subscription on the pull consumer.
	// Subject can be empty since it defaults to all subjects bound to the stream.
	sub, err := js.PullSubscribe("", "test", nats.BindStream("test"))
	failOnErr(err)

	// Only two should be delivered.
	batch, _ := sub.Fetch(10)
	log.Printf("%d messages", len(batch))

	// AckSync both to ensure the server received the ack.
	batch[0].AckSync()
	batch[1].AckSync()

	// Should be zero.
	batch, _ = sub.Fetch(10, nats.MaxWait(time.Second))
	log.Printf("%d messages", len(batch))
}

It is worth noting that if an AckSync does fail (an error can be returned from it) then its on this code to retry the ack again until a response is received. A redundant ack from the client is a no-op.