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Test lifecycle

In the lifecycle of a k6 test, a script always runs through these stages in the same order:

  1. Code in the init context prepares the script, loading files, importing modules, and defining the test lifecycle functions. Required.
  2. The setup function runs, setting up the test environment and generating data. Optional.
  3. VU code runs in the default or scenario function, running for as long and as many times as the options define. Required.
  4. The teardown function runs, postprocessing data and closing the test environment. Optional.

Lifecycle functions

Except for init code, each stage occurs in a lifecycle function, a function called in a specific sequence in the k6 runtime.

The four lifecycle stages
1// 1. init code
3export function setup() {
4 // 2. setup code
7export default function (data) {
8 // 3. VU code
11export function teardown(data) {
12 // 4. teardown code

Overview of the lifecycle stages

For examples and implementation details of each stage, refer to the subsequent sections.

Test stagePurposeExampleCalled
1. initLoad local files, import modules, declare lifecycle functionsOpen JSON file, Import moduleOnce per VU*
2. SetupSet up data for processing, share data among VUsCall API to start test environmentOnce
3. VU codeRun the test function, usually defaultMake https requests, validate responsesOnce per iteration, as many times as the test options require
4. TeardownProcess result of setup code, stop test environmentValidate that setup had a certain result, send webhook notifying that test has finishedOnce **

* In cloud scripts, init code might be called more often.

** If the Setup function ends abnormally (e.g throws an error), the teardown() function isn't called. Consider adding logic to the setup() function to handle errors and ensure proper cleanup.

The init stage

The init stage is required. Before the test runs, k6 needs to initialize the test conditions. To prepare the test, the code in the init context:

  • Imports modules
  • Loads files from the local file system
  • Configures the test for all options
  • Defines lifecycle functions for the VU, setup, and teardown stages (and for custom or handleSummary() functions, too).

All code that is outside of a lifecycle function is code in the init context. Code in the init context always executes first.

Examples of init code
1// init context: importing modules
2import http from 'k6/http';
3import { Trend } from 'k6/metrics';
5// init context: define k6 options
6export const options = {
7 vus: 10,
8 duration: '30s',
11// init context: global variables
12const customTrend = new Trend('oneCustomMetric');
14// init context: define custom function
15function myCustomFunction() {
16 // ...

Separating the init stage from the VU stage removes irrelevant computation from VU code, which improves k6 performance and makes test results more reliable. One limitation of init code is that it cannot make HTTP requests. This limitation ensures that the init stage is reproducible across tests (the response from protocol requests is dynamic and unpredictable)

The VU stage

Scripts must contain, at least, a scenario function that defines the logic of the VUs. The code inside this function is VU code. Typically, VU code is inside the default function, but it can also be inside the function defined by a scenario (see subsequent section for an example).

Default/Main function
1export default function () {
2 // do things here...

VU code runs over and over through the test duration. VU code can make HTTP requests, emit metrics, and generally do everything you'd expect a load test to do. The only exceptions are the jobs that happen in the init context.

  • VU code does not load files from your local filesystem.
  • VU code does not import any other modules.

Again, instead of VU code, init code does these jobs.

The default function life-cycle

A VU executes the default() function from start to end in sequence. Once the VU reaches the end of the function, it loops back to the start and executes the code all over.

As part of this "restart" process, k6 resets the VU. Cookies are cleared, and TCP connections might be torn down (depending on your test configuration options).

Setup and teardown stages

Like default, setup and teardown functions must be exported functions. But unlike the default function, k6 calls setup and teardown only once per test.

  • setup is called at the beginning of the test, after the init stage but before the VU stage.
  • teardown is called at the end of a test, after the VU stage (default function).

You can call the full k6 API in the setup and teardown stages, unlike the init stage. For example, you can make HTTP requests:

Setup/Teardown with HTTP request
1import http from 'k6/http';
3export function setup() {
4 const res = http.get('');
5 return { data: res.json() };
8export function teardown(data) {
9 console.log(JSON.stringify(data));
12export default function (data) {
13 console.log(JSON.stringify(data));

Skip setup and teardown execution

You can skip the execution of setup and teardown stages using the options --no-setup and --no-teardown.

Skipping setup/teardown execution
1$ k6 run --no-setup --no-teardown ...

Use data from setup in default and init

Again, let's have a look at the basic structure of a k6 test:

1// 1. init code
3export function setup() {
4 // 2. setup code
7export default function (data) {
8 // 3. VU code
11export function teardown(data) {
12 // 4. teardown code

You might have noticed the function signatures of the default() and teardown() functions take an argument, referred to here as data.

Here's an example of passing some data from the setup code to the VU and teardown stages:

1export function setup() {
2 return { v: 1 };
5export default function (data) {
6 console.log(JSON.stringify(data));
9export function teardown(data) {
10 if (data.v != 1) {
11 throw new Error('incorrect data: ' + JSON.stringify(data));
12 }

For example, with the data returned by the setup() function, you can:

  • Give each VU access to an identical copy of the data
  • Postprocess the data in teardown code

However, there are some restrictions.

  • You can pass only data (i.e. JSON) between setup and the other stages. You cannot pass functions.
  • If the data returned by the setup() function is large, it will consume more memory.
  • You cannot manipulate data in the default() function, then pass it to the teardown() function.

It's best to think that each stage and each VU has access to a fresh "copy" of whatever data the setup() function returns.

Diagram showing data getting returned by setup, then used (separately) by default and teardown functions

It would be extremely complicated and computationally intensive to pass mutable data between all VUs and then to teardown, especially in distributed setups. This would go against a core k6 goal: the same script should be executable in multiple modes.

Additional lifecycle functions

k6 has a few additional ways to use lifecycle functions:

  • handleSummary(). If you want to make a custom summary, k6 calls one more lifecycle function at the very end of the test.

    For details, refer to Custom summary.

  • Scenario functions. Instead of the default function, you can also run VU code in scenario functions.

    import http from 'k6/http';
    import { sleep } from 'k6';
    export const options = {
    scenarios: {
    my_web_test: {
    // the function this scenario will execute
    exec: 'webtest',
    executor: 'constant-vus',
    vus: 50,
    duration: '1m',
    export function webtest() {
    sleep(Math.random() * 2);