by Example
Request-Reply in Messaging
The request-reply pattern allows a client to send a message and expect a reply of some kind. In practice, the request message will either be a command, which is an intention for service to carry out some work that results in a state change, or a query, which is a request for information.
Unlike request-reply constrained protocols like HTTP, NATS is not limited to a strict point-to-point interaction between a client and server. The request-reply pattern is built on top of the core publish-subscribe model.
By default, this means that any one of subscribers could be a responder and reply to the client. However, because NATS is not limited to point-to-point interactions, the client could indicate to NATS that multiple replies should be allowed.
This example shows the basics of the request-reply pattern including the standard “no responders” error if there are no subscribers available to handle and reply to the requesting message.
Code
package main
import (
"fmt"
"os"
"time"
"github.com/nats-io/nats.go"
)
func main() {Use the env variable if running in the container, otherwise use the default.
url := os.Getenv("NATS_URL")
if url == "" {
url = nats.DefaultURL
}
Create an unauthenticated connection to NATS.
nc, _ := nats.Connect(url)
defer nc.Drain()
In addition to vanilla publish-request, NATS supports request-reply interactions as well. Under the covers, this is just an optimized pair of publish-subscribe operations. The request handler is just a subscription that responds to a message sent to it. This kind of subscription is called a service. For this example, we can use the built-in asynchronous subscription in the Go SDK.
sub, _ := nc.Subscribe("greet.*", func(msg *nats.Msg) {Parse out the second token in the subject (everything after greet.) and use it as part of the response message.
name := msg.Subject[6:]
msg.Respond([]byte("hello, " + name))
})
Now we can use the built-in Request method to do the service request.
We simply pass a nil body since that is being used right now. In addition,
we need to specify a timeout since with a request we are waiting for the
reply and we likely don’t want to wait forever.
rep, _ := nc.Request("greet.joe", nil, time.Second)
fmt.Println(string(rep.Data))
rep, _ = nc.Request("greet.sue", nil, time.Second)
fmt.Println(string(rep.Data))
rep, _ = nc.Request("greet.bob", nil, time.Second)
fmt.Println(string(rep.Data))
What happens if the service is unavailable? We can simulate this by unsubscribing our handler from above. Now if we make a request, we will expect an error.
sub.Unsubscribe()
_, err := nc.Request("greet.joe", nil, time.Second)
fmt.Println(err)
}Output
hello, joe hello, sue hello, bob nats: no responders available for request