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sleep( t )

Suspend VU execution for the specified duration.

ParameterTypeDescription
tnumberDuration, in seconds.

Warning

Don’t use sleep with async code, such as functions that return promises or event handlers. sleep blocks VU execution and prevents promises from resolving and event handlers from running. This is due to the ECMAScript specification, which requires the execution stack to be empty before the event loop can process events and run callbacks.

For async code, use setTimeout and related timer functions instead. They schedule callbacks without blocking the event loop.

Examples

Fetching two different pages with a 0-30 second random sleep in between:

JavaScript
JavaScript 
import { sleep } from 'k6';
import http from 'k6/http';

export default function () {
  http.get('https://k6.io');
  sleep(Math.random() * 30);
  http.get('https://k6.io/features');
}

Using the k6-utils library to specify a range between a minimum and maximum:

JavaScript
JavaScript 
import { sleep } from 'k6';
import http from 'k6/http';
import { randomIntBetween } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

export default function () {
  http.get('https://k6.io');
  sleep(randomIntBetween(20, 30));
  http.get('https://k6.io/features');
}

Avoid: Using sleep in a loop to poll for changes. This blocks the event loop, so the condition never updates:

JavaScript
JavaScript 
import { Client, Stream } from 'k6/net/grpc';
import { sleep } from 'k6';

const client = new Client();

export default () => {
  if (__ITER == 0) {
    client.connect('127.0.0.1:10000', { plaintext: true, reflect: true });
  }

  const stream = new Stream(client, 'main.FeatureExplorer/ListFeatures', null);
  let received = false;

  stream.on('data', () => {
    received = true;
  });

  stream.on('end', () => {
    client.close();
  });

  stream.write({
    lo: { latitude: 400000000, longitude: -750000000 },
    hi: { latitude: 420000000, longitude: -730000000 },
  });

  // BAD: sleep blocks the event loop - the 'data' handler never runs
  while (!received) {
    sleep(0.1);
  }
};

Prefer: Using setInterval to poll. The event loop stays free, so handlers run and the condition updates:

JavaScript
JavaScript 
import { Client, Stream } from 'k6/net/grpc';

const client = new Client();

export default () => {
  if (__ITER == 0) {
    client.connect('127.0.0.1:10000', { plaintext: true, reflect: true });
  }

  const stream = new Stream(client, 'main.FeatureExplorer/ListFeatures', null);
  let received = false;
  let cleanedUp = false;

  function cleanup() {
    if (cleanedUp) return;
    cleanedUp = true;
    clearInterval(intervalId);
    clearTimeout(timeoutId);
    stream.end();
    client.close();
  }

  stream.on('data', () => {
    received = true;
  });

  stream.on('end', cleanup);

  stream.write({
    lo: { latitude: 400000000, longitude: -750000000 },
    hi: { latitude: 420000000, longitude: -730000000 },
  });

  const intervalId = setInterval(() => {
    if (received) {
      cleanup();
      console.log('Received response');
    }
  }, 100);

  const timeoutId = setTimeout(cleanup, 5000);
};