WebSocketException: Unable to connect to the remote server

[danyromero-ebet]

I am not sure if this an issue of NBomber, that's why I posted here. I am following this document Initialize and populating Client Pool. I am trying to create 600 websockets connections previous to run any scenario:

            .WithInit(async ctx =>
            {
                // load from local JSON file
                var members = Data.LoadJson<MemberData[]>("./k6/json/data-badgenumber.json");

                int i = 0;

                foreach (var member in members)
                {
                    member.GmiSerialNumber = Guid.NewGuid().ToString().Replace("-", string.Empty)[17..];

                    var websocket = new WebSocket(new WebSocketConfig());
                    await websocket.Connect("wss://localhost:7201/freeway-hub");

                    ctx.Logger.Debug($"Connected to WebSocket for member: {member.BadgeNumber} ({i++}/{members.Length})");

                    var handshakePayload = new { protocol = "json", version = 1 };
                    await websocket.Send($"{JsonSerializer.Serialize(handshakePayload)}\x1e");

                    clientPool.AddClient(websocket);
                }

                memberFeed = DataFeed.Circular(members);
            })
            .WithClean(ctx =>
            {
                clientPool.DisposeClients(client => client.Dispose());
                return Task.CompletedTask;
            });

After a couple of seconds NBomber throws an exception:

NBomber logs show the following:

2025-05-26 12:08:36.199 +10:00 [DBG] [ThreadId:18] Connected to WebSocket for member: 2453 (498/600)
2025-05-26 12:08:36.213 +10:00 [DBG] [ThreadId:18] Connected to WebSocket for member: 2450 (499/600)
2025-05-26 12:08:36.228 +10:00 [DBG] [ThreadId:7] Connected to WebSocket for member: 2449 (500/600)
2025-05-26 12:08:36.245 +10:00 [DBG] [ThreadId:7] Connected to WebSocket for member: 2448 (501/600)
2025-05-26 12:08:36.261 +10:00 [DBG] [ThreadId:7] Connected to WebSocket for member: 2447 (502/600)
2025-05-26 12:08:36.276 +10:00 [DBG] [ThreadId:7] Connected to WebSocket for member: 2446 (503/600)
2025-05-26 12:08:36.291 +10:00 [DBG] [ThreadId:7] Connected to WebSocket for member: 2445 (504/600)
2025-05-26 12:08:36.307 +10:00 [DBG] [ThreadId:7] Connected to WebSocket for member: 2444 (505/600)
2025-05-26 12:08:36.322 +10:00 [DBG] [ThreadId:18] Connected to WebSocket for member: 2443 (506/600)
2025-05-26 12:08:36.337 +10:00 [DBG] [ThreadId:12] Connected to WebSocket for member: 2442 (507/600)
2025-05-26 12:08:36.368 +10:00 [INF] [ThreadId:18] Stopping scenarios...
2025-05-26 12:08:36.449 +10:00 [ERR] [ThreadId:18] Init scenario error: 'System.Net.WebSockets.WebSocketException (0x80004005): Unable to connect to the remote server
 ---> System.Net.Http.HttpRequestException: The SSL connection could not be established, see inner exception.
 ---> System.IO.IOException: Received an unexpected EOF or 0 bytes from the transport stream.
   at System.Net.Security.SslStream.ReceiveHandshakeFrameAsync[TIOAdapter](CancellationToken cancellationToken)
   at System.Net.Security.SslStream.ForceAuthenticationAsync[TIOAdapter](Boolean receiveFirst, Byte[] reAuthenticationData, CancellationToken cancellationToken)
   at System.Net.Http.ConnectHelper.EstablishSslConnectionAsync(SslClientAuthenticationOptions sslOptions, HttpRequestMessage request, Boolean async, Stream stream, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at System.Net.Http.ConnectHelper.EstablishSslConnectionAsync(SslClientAuthenticationOptions sslOptions, HttpRequestMessage request, Boolean async, Stream stream, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.ConnectAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.CreateHttp11ConnectionAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.AddHttp11ConnectionAsync(QueueItem queueItem)
   at System.Threading.Tasks.TaskCompletionSourceWithCancellation`1.WaitWithCancellationAsync(CancellationToken cancellationToken)
   at System.Net.Http.HttpConnectionPool.SendWithVersionDetectionAndRetryAsync(HttpRequestMessage request, Boolean async, Boolean doRequestAuth, CancellationToken cancellationToken)
   at System.Net.Http.RedirectHandler.SendAsync(HttpRequestMessage request, Boolean async, CancellationToken cancellationToken)
   at System.Net.WebSockets.WebSocketHandle.ConnectAsync(Uri uri, HttpMessageInvoker invoker, CancellationToken cancellationToken, ClientWebSocketOptions options)
   at System.Net.WebSockets.WebSocketHandle.ConnectAsync(Uri uri, HttpMessageInvoker invoker, CancellationToken cancellationToken, ClientWebSocketOptions options)
   at System.Net.WebSockets.ClientWebSocket.ConnectAsyncCore(Uri uri, HttpMessageInvoker invoker, CancellationToken cancellationToken)

As you can see after 507 connections issues appear, at this point the API gets unresponsive. The API is an ASP.NET application (.NET 8) using NATS and SignalR.

When I try with 200 websockets, all good no problem.

Similar test I have run with K6, just to test if my API is the cause of this issue however K6 can create 600 connections without issues.

Can you please assist on this? am I doing something wrong?

[AntyaDev]

Hi @danyromero-ebet
Can you please try to add a short delay after adding a connection to the pool?
await Task.Delay(10);
https://github.com/PragmaticFlow/NBomber/blob/dev/examples/Demo/MQTT/ClientPool/ClientPoolMqttExample.cs#L76

@danyromero-ebet can you also provide your example with K6 just for comparison?

[danyromero-ebet]

Thanks for your reply @AntyaDev. I forgot to mention, I am using a local environment, API running in a docker container on Windows 11.

I added the short delay and still not working, same error and the API gets unresponsive. Without any additional change I try to open, in a browser, the OpenApi spec doc (swagger) and I get "can't reach this page". Funny thing, K6 still can run the test script and I can see it working when monitoring the container logs. I am puzzled. Somehow, I believe this is a local env issue rather than NBomber so I tried with my Dev Server now, away from local env, and I could create 600 websockets, NBomber could run my test, no dramas, great!

I will continue investigating why locally I have issues, NBomber looks great. Also, still want to understand how K6 can do this before this issue and after.

Just in case this is the K6 script I have been running:

import http from 'k6/http';
import { randomString, randomIntBetween, randomItem } from "https://jslib.k6.io/k6-utils/1.4.0/index.js";
import { WebSocket } from 'k6/experimental/websockets';

const sessionDuration = randomIntBetween(10000, 30000);
const serviceType = ['bla1', 'bla2'];

const urlHttpBase = 'https://localhost:7201/freeway-hub';
const urlWebSocketBase = 'wss://localhost:7201/freeway-hub';
const url = 'https://localhost:7201/endpoint';

const data = JSON.parse(open("./json/data-badgenumber.json"));

export default function () {
    for (let i = 0; i < 601; i++) {
        startWSWorker(i);
    }
}

function startWSWorker(id) {
    var res = http.post(`${urlHttpBase}/negotiate`, null, { "responseType": "text" });
    var connectionId = res.json()["connectionId"];
    let user = data[id];
    var payload = {
        "connectionId": `${connectionId}`,
        "prop1": user.prop1,
        "prop2": randomString(12),
        "prop3": randomItem(serviceType)
    };

    var urlWebSocket = `${urlWebSocketBase}?id=${connectionId}`;

    // create a new websocket connection
    const ws = new WebSocket(urlWebSocket);
    ws.binaryType = 'arraybuffer';

    ws.addEventListener('open', () => {
        ws.send(JSON.stringify({ protocol: 'json', version: 1 }) + '\x1e');

        var response = http.post(url, JSON.stringify(payload), { headers: { 'Content-Type': 'application/json' } });
        console.log(`Response status (${id}): ${response.status} - CorrelationID: ${JSON.stringify(response.headers["Correlation-Id"])}`);

        // after a sessionDuration stop sending messages and leave the room
        const timeout1id = setTimeout(function () {
            console.log(`VU ${__VU}:${id}: ${sessionDuration}ms passed, leaving`);
            ws.close();
        }, sessionDuration);

        // when connection is closing, clean up the previously created timers
        ws.addEventListener('close', () => {
            clearTimeout(timeout1id);
            console.log(`VU ${__VU}:${id}: disconnected`);
        });
    });
}

[AntyaDev]

Hi @danyromero-ebet
Thank you for the information.
Regarding Task.Delay — I found that it can be useful in scenarios where multiple clients are trying to connect to a single server simultaneously. It throttles connection creation.

Regarding k6 — it's interesting to know what the max number of clients you can connect using your local server.
Regarding this exception:
The SSL connection could not be established, see inner exception.
I would consider checking the settings of WebSocketClient, (NBomber uses this client under the hood).

Also, could you implement a simple retry mechanism for reconnection? If this exception occurs, wait for 1-3 seconds and then try to reconnect once more.

To me, it seems that the standard .NET WebSocket client might be handling SSL or Keep-Alive differently, which could be causing some kind of unavailability.

[danyromero-ebet]

Thanks @AntyaDev I will try your suggestions, thanks again.