Important: This documentation is about an older version. It's relevant only to the release noted, many of the features and functions have been updated or replaced. Please view the current version.

Using k6

Execution context variables

Open source

Execution context variables

In some cases, it’s really useful to have information about the script’s current test-execution state. For example, you might want to

Have different VUs run different test logic
Use different data for each VU and iteration
Figure out the stage that a test is in

To solve these issues, you can use execution context variables.

k6/execution

The k6/execution module exposes details about the current execution state, such as the name of the currently executed scenario, how many VUs are currently active, and more. The module provides test-execution information via three properties:

Property	Meta-information and execution details about
instance	The current running k6 instance
scenario	The current running scenario
vu	The current VU and iteration

k6 v0.34.0 introduced the k6/execution module. If you are using a version k6 that does not have this module, refer to the __VU and __ITER section.

Example: log all context variables

If you want to experiment with what each context variable looks like as a test runs, you can copy this template literal into one of your test scripts.

Note that this omits the abort variable, since that function would abort the test.

import exec from 'k6/execution';

export default function () {
  console.log(`Execution context

Instance info
-------------
Vus active: ${exec.instance.vusActive}
Iterations completed: ${exec.instance.iterationsCompleted}
Iterations interrupted:  ${exec.instance.iterationsInterrupted}
Iterations completed:  ${exec.instance.iterationsCompleted}
Iterations active:  ${exec.instance.vusActive}
Initialized vus:  ${exec.instance.vusInitialized}
Time passed from start of run(ms):  ${exec.instance.currentTestRunDuration}

Scenario info
-------------
Name of the running scenario: ${exec.scenario.name}
Executor type: ${exec.scenario.executor}
Scenario start timestamp: ${exec.scenario.startTime}
Percenatage complete: ${exec.scenario.progress}
Iteration in instance: ${exec.scenario.iterationInInstance}
Iteration in test: ${exec.scenario.iterationInTest}

Test info
---------
All test options: ${exec.test.options}

VU info
-------
Iteration id: ${exec.vu.iterationInInstance}
Iteration in scenario: ${exec.vu.iterationInScenario}
VU ID in instance: ${exec.vu.idInInstance}
VU ID in test: ${exec.vu.idInTest}
VU tags: ${exec.vu.tags}`);
}

For detailed reference, refer to the k6/execution module.

Examples and use cases

⚠️ __VU and __ITER are both global variables with execution-context information that k6 makes available to the test script.

__ITER

A numeric counter with the current iteration number for a specific VU. Zero-based.

__VU

Current VU number in use. k6 assigns the value incrementally for each new VU instance, starting from one. The variable is 0 when executing the setup and teardown functions.

Running in k6 Cloud

When you run tests in k6 Cloud, the __VU value is per server/load generator. Read the details in the cloud docs.

Examples

import http from 'k6/http';
import { sleep } from 'k6';
export default function () {
  http.get('http://test.k6.io');
  console.log(`VU: ${__VU}  -  ITER: ${__ITER}`);
  sleep(1);
}

You can use execution-context variables to configure different test behaviors and parameterizations. A typical use case is a load test that simulates different users performing a login flow.

import http from 'k6/http';
import { sleep } from 'k6';
export default function () {
  const email = `user+${__VU}@mail.com`;
  const payload = JSON.stringify({ email: email, password: 'test' });
  const params = { headers: { 'Content-Type': 'application/json' } };
  http.post('http://test.k6.io/login', payload, params);
  console.log(email);
  // .. continue the user flow
  sleep(1);
}

Grafana Cloud k6 environment variables

If you run tests in k6 Cloud, you have additional environment variables that tell you the server, load zone, and distribution of the currently running test. Read about cloud environment variables.