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.
$ nbe run messaging/request-reply/dotnetView the source code or learn how to run this example yourself
Code
using System;
using System.Text;
using System.Threading.Tasks;
using NATS.Client;
string natsUrl = Environment.GetEnvironmentVariable("NATS_URL");
if (natsUrl == null)
{
natsUrl = "nats://127.0.0.1:4222";
}
Create a new connection factory to create a connection.
Options opts = ConnectionFactory.GetDefaultOptions();
opts.Url = natsUrl;
Creates a connection to nats server at the natsUrl
An IConnection is IDisposable so it can be used
within a using statement.
ConnectionFactory cf = new ConnectionFactory();
IConnection c = cf.CreateConnection(opts);
Reply
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 replier simply publishes a message to that reply-to.
EventHandler<MsgHandlerEventArgs> handler = (sender, args) =>
{
string name = args.Message.Subject.Substring(6);
string response = $"hello {name}";
c.Publish(args.Message.Reply, Encoding.UTF8.GetBytes(response));
};
IAsyncSubscription sub = c.SubscribeAsync("greet.*", handler);
Request
Make a request and wait a most 1 second for a response.
try
{
Msg m0 = c.Request("greet.bob", null, 1000);
Console.WriteLine("Response received: " + Encoding.UTF8.GetString(m0.Data));
}
catch (NATSTimeoutException)
{
Console.WriteLine($"NATSTimeoutException: The request did not complete in time.");
}
A request can also be made asynchronously
Task<Msg> task1 = c.RequestAsync("greet.pam", null);
task1.Wait(1000);
Msg m1 = task1.Result;
Console.WriteLine("Response received: " + Encoding.UTF8.GetString(m1.Data));
Once we unsubscribe there will be no subscriptions to reply.
sub.Unsubscribe();
If there are no-responders to a synchronous request
we get a NATSNoRespondersException.
try
{
c.Request("greet.fred", null, 1000);
}
catch (NATSNoRespondersException)
{
Console.WriteLine($"NATSNoRespondersException: There were no responders listening for the subject.");
}
catch (NATSTimeoutException)
{
Console.WriteLine($"NATSTimeoutException: The request did not complete in time.");
}
If there are no-responders to an asynchronous request
we get a NATSNoRespondersException wrapped inside the AggregateException
try
{
Task<Msg> task2 = c.RequestAsync("greet.sue", null);
task2.Wait(1000);
}
catch (AggregateException ae)
{
if (ae.InnerExceptions[0] is NATSNoRespondersException)
{
Console.WriteLine($"NATSNoRespondersException: There were no responders listening for the subject.");
}
else
{
Console.WriteLine($"Some exception: " + ae.Message);
}
}Output
Response received: hello bob Response received: hello pam NATSNoRespondersException: There were no responders listening for the subject. NATSNoRespondersException: There were no responders listening for the subject.