Intermittent Timeout Failures in Unit Test with NetMQ Dealer and Router Pattern

Question

I've encountered a perplexing issue where my xUnit tests using the NetMQ library sometimes pass and sometimes fail due to timeouts. The inconsistency has made it challenging to pinpoint the root cause.

The Scenario: I am using the Dealer and Router pattern in NetMQ.

The Problem: When running the test after a prolonged period, it succeeds. However, if I immediately re-run it, it often times out. This behavior persists for a few minutes before it successfully runs again. After a successful run, running netstat shows the following:

127.0.0.1:8080         kubernetes:1742        TIME_WAIT

Here's a succinct version of the problematic code:

Unit Test:

public class SocketPulseReceiverTest
{
    private readonly ServiceCollection _serviceCollection = new();
    private readonly IServiceProvider _serviceProvider;

    public SocketPulseReceiverTest()
    {
        _serviceCollection.AddSocketPulseReceiver(new List<Assembly> { typeof(TestAction).Assembly });
        _serviceProvider = _serviceCollection.BuildServiceProvider();
    }

    [Fact]
    public void InvalidRequest_ReturnsErrorReply()
    {
        var service = _serviceProvider.GetService<ISocketPulseReceiver>();

        service?.Start("tcp://localhost:8080");
        using var dealer = new DealerSocket("tcp://localhost:8080");
        try
        {
            dealer.SendFrame("invalid data");
            var received = dealer.TryReceiveFrameString(TimeSpan.FromSeconds(2), out var replyStr);
            Assert.True(received, "Did not receive a reply in the expected time");
            var reply = JsonConvert.DeserializeObject<Reply>(replyStr!);
            Assert.Equal(State.Error, reply?.State);
        }
        finally
        {
            service?.Stop();
            dealer.Close();
            NetMQConfig.Cleanup();
        }
    }
}

Function using Router socket:

private void Worker(string address)
{
    using var routerSocket = new RouterSocket();
    routerSocket.Bind(address);

    while (_isRunning)
    {
        var msg = new NetMQMessage();
        try
        {
            routerSocket.TryReceiveMultipartMessage(TimeSpan.FromMilliseconds(100), ref msg, 2);
        }
        catch (NetMQException) { /* NetMQ internal exception handling */ }

        if (msg == null || msg.FrameCount == 0) continue;

        if (msg.FrameCount != 2)
            throw new InvalidOperationException("Unexpected msg received...");

        var identity = msg.Pop().ConvertToString();
        var content = msg.Pop().ConvertToString();

        Reply result;
        try
        {
            result = HandleMessage(content);
        }
        catch (Exception e)
        {
            result = new Reply { State = State.Error, Content = e.ToString() };
        }

        routerSocket.SendMoreFrame(identity).SendFrame(JsonConvert.SerializeObject(result));
    }
}

Upon debugging, I observed that the Router Socket consistently receives the frame sent by the client. However, the reply doesn't seem to reach the Dealer Socket.

Does anyone have insights or suggestions on why the test is behaving inconsistently, and how I can ensure consistent results?

Thank you in advance!

Answer 1

I managed to pinpoint and resolve the issue.

The root cause of the problem was found in the following line:

var identity = msg.Pop().ConvertToString();

In this line, I converted a bytecode identity into a string and then attempted to send that string back to the DEALER. Naturally, this would result in the DEALER not recognizing it, especially if the converted string had invalid characters or question marks. This inconsistency explains why the test occasionally passed—only when, by chance, the string conversion retained valid characters without any unexpected symbols.

To address this issue, I modified the code as follows:

NetMQMessage reply = new();
reply.Append(identity);
reply.Append(JsonConvert.SerializeObject(result));
routerSocket.TrySendMultipartMessage(reply);

This replaced the previously faulty:

routerSocket.SendMoreFrame(identity).SendFrame(JsonConvert.SerializeObject(result));

By retaining the original identity format and using the Append method, I ensured the identity remains intact, allowing for consistent message routing back to the DEALER.

While this might seem like an oversight that many might not encounter, I'm sharing it in hopes that it might assist someone else who finds themselves in a similar situation.