Request-Reply (C# .NET V2)

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.

CLI Go Python JavaScript Rust C# .NET V2 Java Ruby Elixir Crystal C

Jump to the output or the recording

$ nbe run messaging/request-reply/dotnet2

View the source code or learn how to run this example yourself

Code

Install NATS.Net from NuGet.

using System;
using System.Diagnostics;
using System.Threading.Tasks;
using NATS.Client.Core;


var stopwatch = Stopwatch.StartNew();

NATS_URL environment variable can be used to pass the locations of the NATS servers.

var url = Environment.GetEnvironmentVariable("NATS_URL") ?? "127.0.0.1:4222";
Log($"[CON] Connecting to {url}...");

Connect to NATS server. Since connection is disposable at the end of our scope we should flush our buffers and close connection cleanly.

var opts = NatsOpts.Default with { Url = url };
await using var nats = new NatsConnection(opts);

Create a message event handler and then subscribe to the target subject which leverages a wildcard greet.*. When a user makes a “request”, the client populates the reply-to field and then listens (subscribes) to that as a subject. The responder simply publishes a message to that reply-to.

 await using var sub = await nats.SubscribeCoreAsync<int>("greet.*");


 var reader = sub.Msgs;
 var responder = Task.Run(async () =>
 {
     await foreach (var msg in reader.ReadAllAsync())
     {
         var name = msg.Subject.Split('.')[1];
         Log($"[REP] Received {msg.Subject}");
         await Task.Delay(500);
         await msg.ReplyAsync($"Hello {name}!");
     }
 });

Make a request and wait a most 1 second for a response.

var replyOpts = new NatsSubOpts { Timeout = TimeSpan.FromSeconds(2) };


Log("[REQ] From joe");
var reply = await nats.RequestAsync<int, string>("greet.joe", 0, replyOpts: replyOpts);
Log($"[REQ] {reply.Data}");


Log("[REQ] From sue");
reply = await nats.RequestAsync<int, string>("greet.sue", 0, replyOpts: replyOpts);
Log($"[REQ] {reply.Data}");


Log("[REQ] From bob");
reply = await nats.RequestAsync<int, string>("greet.bob", 0, replyOpts: replyOpts);
Log($"[REQ] {reply.Data}");

Once we unsubscribe there will be no subscriptions to reply.

await sub.UnsubscribeAsync();


await responder;

Now there is no responder our request will timeout.

try
{
    reply = await nats.RequestAsync<int, string>("greet.joe", 0, replyOpts: replyOpts);
    Log($"[REQ] {reply.Data} - This will timeout. We should not see this message.");
}
catch (NatsNoReplyException)
{
    Log("[REQ] timed out!");
}

That’s it! We saw how we can create a responder and request data from it. We also set request timeouts to make sure we can move on when there is no response to our requests.

Log("Bye!");


return;


void Log(string log) => Console.WriteLine($"{stopwatch.Elapsed} {log}");

Output

00:00:00.0008194 [CON] Connecting to nats://nats:4222...
00:00:00.1225386 [REQ] From joe
00:00:00.1374172 [REP] Received greet.joe
00:00:00.6489792 [REQ] Hello joe!
00:00:00.6490418 [REQ] From sue
00:00:00.6498312 [REP] Received greet.sue
00:00:01.1512245 [REQ] Hello sue!
00:00:01.1512731 [REQ] From bob
00:00:01.1518886 [REP] Received greet.bob
00:00:01.6531626 [REQ] Hello bob!
00:00:03.6546766 [REQ] timed out!
00:00:03.6547387 Bye!

Recording

Note, playback is half speed to make it a bit easier to follow.