by Example
Request-Reply in JetStream
This is an experimental preview of a possible request-reply API for JetStream. With core NATS, if a given client is not showing interest in a subject at the time a message is published on that subject, the message will not be received.
In general, this behavior is expected of request-reply since these are expected to behave like a phone call where someone needs to pick up in order to talk. The alternative is relying on a mailbox which is essentially what a peristent stream enables.
The question is what happens with the reply if a message can be sent, queued, and the responder comes online later when the original requester is now offline?
These are some active questions the NATS team are evaluating if you want to show your interest and use cases!
Code
package main
import (
"fmt"
"os"
"time"
"github.com/nats-io/nats.go"
)
func main() {
natsURL := os.Getenv("NATS_URL")
nc, _ := nats.Connect(natsURL)
defer nc.Drain()
js, _ := nc.JetStream()
Create a [work-queue][wq] stream that will act as a buffer for requests.
js.AddStream(&nats.StreamConfig{
Name: "REQUESTS",
Subjects: []string{"requests.*"},
Retention: nats.WorkQueuePolicy,
})
Create an ephemeral consumer + subscription responsible for replying.
sub, _ := js.Subscribe("requests.*", func(msg *nats.Msg) {
var r string
switch msg.Subject {
case "requests.order-sandwich":
r = "🥪"
case "requests.order-bagel":
r = "🥯"
case "requests.order-flatbread":
r = "🥙"
default:
return
}
msg.Respond([]byte(r))
})
defer sub.Drain()
Send some requests.
rep, _ := js.Request("requests.order-sandwich", nil, time.Second)
fmt.Println(string(rep.Data))
rep, _ = js.Request("requests.order-flatbread", nil, time.Second)
fmt.Println(string(rep.Data))
If a request cannot be fulfilled, the message is terminated.
_, err := js.Request("requests.order-drink", nil, time.Second)
fmt.Printf("timeout? %v\n", err == nats.ErrTimeout)
info, _ := js.StreamInfo("REQUESTS")
fmt.Printf("%d remaining in the stream\n", info.State.Msgs)
}Output
Network 7b333570_default Creating Network 7b333570_default Created Container 7b333570-nats-1 Creating Container 7b333570-nats-1 Created Container 7b333570-nats-1 Starting Container 7b333570-nats-1 Started 🥪 🥙 timeout? true 0 remaining in the stream