Server Errors

The NATS server sends -ERR protocol messages to a client when something goes wrong on the server side: a permission denial, an authentication problem, a client-level limit, and so on. The ServerError event on NatsConnection raises one of these messages every time the server emits one, so an application can observe them without scraping logs.

Subscribing to the Event

Hook the event the same way as any other connection event. The raw error text is on args.Error:

await using NatsConnection nc = new NatsConnection();

nc.ServerError += (sender, args) =>
{
    Console.WriteLine($"Server error: {args.Error}");
    return default;
};

Classifying the Error

args.Kind is parsed from the raw text and exposes the common server errors as a NatsServerErrorKind enum value. Anything the parser does not recognise stays as NatsServerErrorKind.Unknown; args.Error always carries the original text.

await using NatsConnection nc = new NatsConnection();

nc.ServerError += (sender, args) =>
{
    switch (args.Kind)
    {
    case NatsServerErrorKind.PermissionsViolation:
        Console.WriteLine($"Permission denied: {args.Error}");
        break;
    case NatsServerErrorKind.AuthorizationViolation:
    case NatsServerErrorKind.AuthenticationExpired:
    case NatsServerErrorKind.AuthenticationRevoked:
    case NatsServerErrorKind.AccountAuthenticationExpired:
        Console.WriteLine($"Auth problem: {args.Error}");
        break;
    default:
        Console.WriteLine($"Server error ({args.Kind}): {args.Error}");
        break;
    }

    return default;
};

The recognised kinds are:

• PermissionsViolation - subscribe or publish denied for the connected user
• AuthorizationViolation - authorization failure during connect
• AuthenticationExpired - user authentication expired
• AuthenticationRevoked - user authentication revoked
• AccountAuthenticationExpired - account authentication expired
• StaleConnection - server timed out a stalled connection
• MaximumConnectionsExceeded - server-wide connection limit reached
• MaximumAccountConnectionsExceeded - per-account connection limit reached
• MaximumSubscriptionsExceeded - per-connection subscription limit reached

The event fires for every -ERR line, including messages whose Kind is Unknown. Match on args.Error if the application needs to handle a server error not covered by the enum.