WebSocket load testing tools: six options compared

The WebSocket load testing tools with native support are k6, Artillery, Gatling and Tsung, while JMeter needs the WebSocket Samplers plugin and Locust needs client code you write or its experimental Socket.IO user. Choose by the language your team writes, then by how closely you must check the messages the server pushes back.

We have not run these tools against a WebSocket server for this article. The facts come from each project’s own documentation and repository, checked on 9 October 2026, and the “choose instead” notes are our judgement.

What makes WebSocket load different from HTTP load?

A WebSocket user opens one connection and keeps it for the whole session, and the server can send messages whenever it likes. A test therefore has two dials, open connections and messages per second, and each loads the server in its own way.

k6’s WebSocket guide describes full-duplex traffic over one TCP connection and explains that a virtual user stops looping over the main function and runs an event loop while the socket is open. Gatling’s WebSocket reference adds that traffic flows both ways, so the HTTP request and response model no longer fits.

Plan three shapes of test. Many idle connections show how many sockets your servers can hold. A smaller crowd of chatty clients tests message handling. A broadcast, where one message fans out to every subscriber, tests the push path.

What should you measure in a WebSocket load test?

Measure connect time, message latency and dropped connections, alongside how many connections stay open. These show what users of a real-time app feel.

k6’s built-in metrics include ws_connecting, ws_ping, ws_session_duration, ws_sessions and message counts. None of them times an application message, so the delay between a client’s message and the server’s reply needs a custom Trend metric. Artillery’s ws engine reports message counts and rates, and its Socket.IO engine adds socketio.response_time, from emitting a message to receiving the response.

For dropped connections, count close and error events your script did not cause, and compare sessions started with sessions still open at the end of each stage.

When is k6 the right WebSocket tool?

When your team writes JavaScript and wants each virtual user to hold several sockets at once. k6 has WebSocket support built in, with no extension to install.

The k6/websockets module follows the browser WebSocket API and uses a global event loop, which lets one virtual user keep multiple connections open. The older k6/ws module is still documented, but k6 recommends k6/websockets for new tests, and k6/experimental/websockets is deprecated.

import { WebSocket } from 'k6/websockets';

export default function () {
  const ws = new WebSocket('wss://example.com/ws');
  ws.onopen = () => ws.send('hello');
  ws.onmessage = (e) => {
    console.log(e.data);
    ws.close();
  };
}

Thresholds work on custom metrics too, so a missed latency goal gives CI a non-zero exit code. Choose Artillery instead when your server speaks Socket.IO and you want emits and acknowledgements checked without writing client code. Our k6 vs Artillery comparison covers the wider differences.

When does Artillery fit?

When you want short YAML scenarios, or when your server uses Socket.IO. Both engines ship with Artillery itself.

A scenario on the WebSocket engine is built from five actions: connect, send, think, loop and function. The Socket.IO engine goes further: an emit can declare the response it expects, validate an acknowledgement with a JSONPath match, and share a scenario with HTTP requests, which suits apps that log in over HTTP before opening a socket.

Choose k6 or Gatling instead when a test must check what a plain WebSocket server pushes back, because the ws engine’s documentation describes no step that waits for or matches a reply.

Who should use Gatling for WebSockets?

Teams on the JVM, or on Gatling’s JavaScript SDK, who want message checks written into the scenario. WebSocket support is part of Gatling’s HTTP SDK, so HTTP and WebSocket steps share one simulation.

The reference covers connect, sendText, sendBytes and close, plus checks that block until the expected message arrives or a timeout passes. Protocol settings can reconnect a socket the server closed, reply automatically to chosen text messages such as a ping, and buffer messages no check matched. Gatling’s repository lists WebSocket in the Apache 2.0 Community Edition, with no limits.

Choose k6 instead when you prefer to react to pushed messages in event handlers over blocking checks.

Can JMeter load test WebSockets?

Yes, with a plugin. JMeter itself has no WebSocket sampler, and the usual add-on is WebSocket Samplers by Peter Doornbosch.

JMeter’s component reference does not mention WebSocket at all. The plugin’s repository describes six samplers: open connection, request-response, ping-pong, single read, single write and close. They run on the thread group’s own thread, support wss and binary frames, and can filter out frames a test does not care about. You install it through the Plugins Manager or as a jar in lib/ext; version 1.3.2 was tagged in January 2026 (checked on 9 October 2026).

Choose another tool when you would rather not maintain a third-party plugin on every load generator.

What about Locust and Tsung?

Locust reaches raw WebSockets only through client code you add, and Tsung supports them in XML configuration but releases rarely. Both can work, with more effort.

Locust’s documentation says its built-in support covers only HTTP and HTTPS; other protocols are added by wrapping a client library and firing a request event after each call. The library must be one gevent can patch, or it blocks the whole process. For Socket.IO there is an experimental SocketIOUser. Choose Locust when your team writes Python and is happy to own that client code.

Tsung is an Erlang project that calls itself a multi-protocol distributed load testing tool. Its session documentation defines WebSocket connect, message and close requests, with an ack attribute to wait for a response. Its latest tagged release, 1.8.0, dates from March 2023.

Where does a hosted runner like LoadFocus fit?

Holding many open sockets from your own machines takes work, and a hosted runner takes over the load generation. It fits best when your tests are already k6 scripts.

LoadFocus, the publisher of this site, runs k6 scripts from 25 cloud regions, and its k6 page lists WebSocket connections among the supported k6 features. It also offers a WebSocket ramp-up and ramp-down template. We have not checked whether it runs JMeter plugins such as WebSocket Samplers, so we make no claim there.

How do you choose between them?

Start with the language your team writes, then ask whether you test raw WebSockets or Socket.IO and how strictly you must check pushed messages. That usually leaves one or two candidates.

Write the same short connect, send and receive test in both and compare connect time and message latency at low load. Our guide on how to run a load test covers the steps, and the open source load testing tools roundup compares these projects beyond WebSockets.

FAQ

Which WebSocket load testing tool should I start with? If your team writes JavaScript, start with k6, which has WebSocket support built in. For Socket.IO apps, Artillery’s engine needs the least code, and JVM teams will feel at home in Gatling.

Can JMeter test WebSockets without a plugin? Its component reference lists no WebSocket sampler, so the practical route is the WebSocket Samplers plugin, installed through the Plugins Manager or as a jar in lib/ext.

Should I use k6/ws or k6/websockets? Use k6/websockets for new tests, as k6’s documentation recommends. The legacy k6/ws module is still documented for existing scripts.

Related reading

• Open source load testing tools: six options reviewed
• How to load test a GraphQL API: a practical guide
• HTTP benchmarking tools: wrk, hey, oha, ab and more
• gRPC load testing: tools, setup and what to watch
• Artillery load testing: a review of the Node.js tool
• Tsung load testing: a review of the Erlang tool

The technical claims in this article have not been independently reviewed.