websockets on Grafana Labs

Blob

Mon, 21 Sep 2026 15:16:28 +0000

Blob

Blob is an interface that represents a blob, which is a file-like object of immutable, raw data; they can be read as text or binary data, or converted into a ReadableStream.

It’s the type of the data received on WebSocket.onmessage when WebSocket.binaryType is set to "blob".

A Blob instance has the following methods/properties:

Class/Property	Description
Blob.size	The number of bytes of data contained within the `Blob`.
Blob.type	A string containing the MIME type, or an empty string if the type could not be determined.
Blob.arrayBuffer()	Returns a `Promise` that resolves with the contents of the blob as binary data contained in an `ArrayBuffer`.
Blob.bytes()	Returns a `Promise` that resolves with a `Uint8Array` containing the contents of the blob as an array of bytes.
Blob.slice()	Returns a new `Blob` object which contains data from a subset of the blob on which it’s called.
Blob.stream()	Returns a ReadableStream which upon reading returns the data contained within the `Blob`.
Blob.text()	Returns a `Promise` that resolves with a string containing the contents of the blob, interpreted as `UTF-8`.

WebSocket

Mon, 21 Sep 2026 15:16:28 +0000

WebSocket

Creates a WebSocket instance for connection to a remote host.

The following events can close the connection:

remote host close event.
WebSocket.close().
k6 VU interruption based on test configuration or CLI commands.

Parameter	Type	Description
url	string	The URL to which to connect (e.g. “ws://localhost:10000”).
protocols	array	Not yet implemented, reserved for the future use.
params	object	Params object containing additional request parameters.

Returns

Type	Description
WebSocket	An instance of WebSocket object.

Ready-state constants

Compare an instance’s readyState with these constants to check the connection state:

Constant	Value	Description
`WebSocket.CONNECTING`	`0`	The connection is not yet open.
`WebSocket.OPEN`	`1`	The connection is open and ready to communicate.
`WebSocket.CLOSING`	`2`	The connection is closing.
`WebSocket.CLOSED`	`3`	The connection is closed or could not be opened.

The constants are read-only and also available on each instance, for example, ws.OPEN.

Example

A k6 script that initiates a WebSocket connection.

import { WebSocket } from 'k6/websockets';

export default function () {
  const ws = new WebSocket('wss://quickpizza.grafana.com/ws');

  ws.onopen = () => {
    console.log(ws.readyState === WebSocket.OPEN); // true
    console.log(ws.OPEN === WebSocket.OPEN); // true
    ws.close();
  };

  ws.onclose = () => {
    console.log(ws.readyState === WebSocket.CLOSED); // true
  };
}

Params

Mon, 21 Sep 2026 15:16:28 +0000

Params

Params is an object used by the WebSocket constructor. The Params object contains request-specific options, such as headers that should be inserted into the connection initialization request.

Name	Type	Description
`Params.compression`	string	Compression algorithm to be used by the WebSocket connection. The only supported algorithm currently is `deflate`.
`Params.jar`	http.CookieJar	The cookie jar that will be used when making the initial HTTP request to establish the WebSocket connection. If empty, the default VU cookie jar will be used.
`Params.headers`	object	Custom HTTP headers in key-value pairs that will be added to the initial HTTP request to establish the WebSocket connection. Keys are header names and values are header values.
`Params.tags`	object	Custom metric tags in key-value pairs where the keys are names of tags and the values are tag values. The WebSocket connection will generate metrics samples with these tags attached, allowing users to filter the results data or set thresholds on sub-metrics.

Example of custom metadata headers and tags

A k6 script that makes a WebSocket request with a custom header and tags results data with a specific tag

import { WebSocket } from 'k6/websockets';

export default function () {
  const url = 'ws://localhost:10000';
  const params = {
    headers: { 'X-MyHeader': 'k6test' },
    tags: { k6test: 'yes' },
  };

  const ws = new WebSocket(url, null, params);

  ws.onopen = () => {
    console.log('WebSocket connection established!');
    ws.close();
  };
}

The preceding example uses a WebSocket echo server, which you can run with the following command:

docker run --detach --rm --name ws-echo-server -p 10000:8080 jmalloc/echo-server