Server errors aren't throwing exceptions

[mtmk]

Observed behavior

When server responds with an error -ERR we're not propagating the error as an exception to relevant calls. One use case is the 'Permissions Violation' error for example when permissions are set on the server per subject.

Expected behavior

Relevant calls should throw an exception. This should also propagate through to the higher level APIs like Services.

workaround: You should observe the errors in logs

Server and client version

any

Host environment

any

Steps to reproduce

Setup server with JWT auth and permission sub/pub.

var opts = new NatsOpts
{
    LoggerFactory = // register your factory here
    Url = "tls://connect.ngs.global:4222",
    Name = "test-service",
    AuthOpts = new NatsAuthOpts { Jwt = "xyz..." }
};
var cnn = new NatsConnection(opts);
var svc = new NatsSvcContext(cnn);
var cfg = new NatsSvcConfig("foo", "1.0.0") { Description = "test service" };
var service = await svc.AddServiceAsync(cfg);
await service.AddEndpointAsync<string>((x) => { return x.ReplyAsync("test"); }, "getstring", "foo.aaa.bbb.test");

Code above runs without any exceptions and it actually doesn't work. you can observe the errors in log.

Also if the $SVC subject is allowed but the endpoint subject isn't, service and the endpoint is registered but the endpoint does not respond.

Thanks @mkobaly for finding the issue and digging into it.

• see also [FEAT] document -ERR protocol messages nats-architecture-and-design#282

[Tiebe-Vercoutter]

Hi all, is there an update for this?

I would expect this program to throw an error when receiving an error back from the server.

using NATS.Client.Services;
using NATS.Net;

var url = Environment.GetEnvironmentVariable("NATS_URL") ?? "nats://127.0.0.1:4222";

await using var nc = new NatsClient(url);
var svc = nc.CreateServicesContext();

var service = await svc.AddServiceAsync(new NatsSvcConfig("divide", "0.0.1")
{
    Description = "A simple division service",
});

await service.AddEndpointAsync<int>(name: "divide42", handler: async m =>
{
    if (m.Exception != null)
    {
        await m.ReplyErrorAsync(500, m.Exception.Message);
        return;
    }

    if (m.Data == 0)
    {
        await m.ReplyErrorAsync(400, "Division by zero");
        return;
    }

    await m.ReplyAsync(42 / m.Data);
});

// Send to the service
try
{
    var result = await nc.RequestAsync<int, int>("divide42", data: 0);
    Console.WriteLine($"Result: {result}");
}
catch (Exception ex)
{
    Console.WriteLine($"An error occurred: {ex.Message}");
}

await service.StopAsync();

Thanks in advance!

[mtmk]

@Tiebe-Vercoutter thanks for the report. Services errors passed down using message headers Nats-Service-Error. So it's services api convention and not a core NATS error, that's why you don't see exceptions. Having said that you're correct we currently don't have a mechanism to detect the error. We can include an Ensure kind of extension to promote an exception e.g.:

try
{
    var result = await nc.RequestAsync<int, int>("divide42", data: 0);

    // PROPOSAL
    result.EnsureServiceSuccess(); // <-- extension to check for Nats-Service-Error headers

    Console.WriteLine($"Result: {result}");
}
catch (Exception ex)
{
    Console.WriteLine($"An error occurred: {ex.Message}");
}

we can also include a (PROPOSAL) IsServiceSuccess() extension to have an exception free route. wdyt?

[Tiebe-Vercoutter]

@mtmk thank you for the quick response and explanation of how I can retrieve the error via the headers!
I think both of those suggestions would be handy to have (similar like this for reference: https://learn.microsoft.com/en-us/dotnet/api/System.Net.Http.HttpResponseMessage.IsSuccessStatusCode?view=net-8.0).