Open source

Ramping arrival rate

With the ramping-arrival-rate executor, k6 starts iterations at a variable rate. It is an open-model executor, meaning iterations start independently of system response (for details, read Open and Closed models).

This executor has stages that configure target number of iterations and the time k6 takes to reach or stay at this target. Unlike the ramping VUs executor, which configures VUs, this executor dynamically changes the number of iterations to start, and starts these iterations as long as the test has enough allocated VUs. To learn how allocation works, read Arrival-rate VU allocation.


Iteration starts are spaced fractionally. Iterations do not start at exactly the same time. At a rate of 10 with a timeUnit of 1s, each iteration starts about every tenth of a second (that is, each 100ms).


Besides the common configuration options, this executor has the following options:

stages(required)arrayArray of objects that specify the target number of iterations to ramp up or down to.[]
preAllocatedVUs(required)integerNumber of VUs to pre-allocate before test start to preserve runtime resources.-
startRateintegerNumber of iterations to execute each timeUnit period at test start.0
timeUnitstringPeriod of time to apply the startRate to the stagestarget value. Its value is constant for the whole duration of the scenario, it is not possible to change it for a specific stage."1s"
maxVUsintegerMaximum number of VUs to allow during the test run.If unset, same as preAllocatedVUs

When to use

If you need start iterations independent of system-under-test performance, and want to ramp the number of iterations up or down during specific periods of time.


Don’t put sleep at the end of an iteration. The arrival-rate executors already pace the iteration rate through the rate and timeUnit properties. It’s unnecessary to use a sleep() function at the end of the VU code.


This is an example of a four-stage test.

It starts at the defined startRate, 300 iterations per minute over a one minute period. After one minute, the iteration rate ramps to 600 iterations started per minute over the next two minutes, and stays at this rate for four more minutes. In the last two minutes, it ramps down to a target of 60 iterations per minute.

import http from 'k6/http';

export const options = {
  discardResponseBodies: true,

  scenarios: {
    contacts: {
      executor: 'ramping-arrival-rate',

      // Start iterations per `timeUnit`
      startRate: 300,

      // Start `startRate` iterations per minute
      timeUnit: '1m',

      // Pre-allocate necessary VUs.
      preAllocatedVUs: 50,

      stages: [
        // Start 300 iterations per `timeUnit` for the first minute.
        { target: 300, duration: '1m' },

        // Linearly ramp-up to starting 600 iterations per `timeUnit` over the following two minutes.
        { target: 600, duration: '2m' },

        // Continue starting 600 iterations per `timeUnit` for the following four minutes.
        { target: 600, duration: '4m' },

        // Linearly ramp-down to starting 60 iterations per `timeUnit` over the last two minutes.
        { target: 60, duration: '2m' },

export default function () {


The following graph depicts the performance of the example script:

Ramping Arrival Rate

Based upon our test scenario inputs and results:

  • The configuration defines 4 stages for a total test duration of 9 minutes.
  • Stage 1 maintains the startRate iteration rate at 300 iterations started per minute for 1 minute.
  • Stage 2 ramps up the iteration rate linearly from the stage 1 of 300 iterations started per minute, to the target of 600 iterations started per minute over a 2-minute duration.
  • Stage 3 maintains the stage 2 iteration rate at 600 iterations started per minute over a 4-minute duration.
  • Stage 4 ramps down the iteration rate linearly to the target rate of 60 iterations started per minute over the last two minutes duration.
  • Changes to the iteration rate are performed by k6, adjusting the number of VUs
  • The script waits for a period of time (defined by the gracefulStop option) for iterations to finish. It doesn’t start new iterations during the gracefulStop period.
  • Our example performed, 4020 iterations over the course of the test.

Get the stage index

To get the current running stage index, use the getCurrentStageIndex helper function from the k6-jslib-utils library. It returns a zero-based number equal to the position in the shortcut stages array or in the executor’s stages array.

import { getCurrentStageIndex } from '';

export const options = {
  stages: [
    { target: 10, duration: '30s' },
    { target: 50, duration: '1m' },
    { target: 10, duration: '30s' },

export default function () {
  if (getCurrentStageIndex() === 1) {
    console.log('Running the second stage where the expected target is 50');

Using this feature, it is possible to automatically tag using the current running stage. Check the Tagging stages section for more details.