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Documentation Grafana k6 Using k6 Test lifecycle

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.

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// 1. init code

export function setup() {
  // 2. setup code
}

export default function (data) {
  // 3. VU code
}

export function teardown(data) {
  // 4. teardown code
}

Overview of the lifecycle stages

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

* 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, code in the init context runs once per VU.

Some operations that might happen in init include the following:

• Import modules
• Load files from the local file system
• Configure the test for all options
• Define 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.

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// init context: importing modules
import http from 'k6/http';
import { Trend } from 'k6/metrics';

// init context: define k6 options
export const options = {
  vus: 10,
  duration: '30s',
};

// init context: global variables
const customTrend = new Trend('oneCustomMetric');

// init context: define custom function
function myCustomFunction() {
  // ...
}

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).

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export default function () {
  // 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:

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import http from 'k6/http';

export function setup() {
  const res = http.get('https://httpbin.test.k6.io/get');
  return { data: res.json() };
}

export function teardown(data) {
  console.log(JSON.stringify(data));
}

export default function (data) {
  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.

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$ k6 run --no-setup --no-teardown ...

Use data from setup in default and teardown

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

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// 1. init code

export function setup() {
  // 2. setup code
}

export default function (data) {
  // 3. VU code
}

export function teardown(data) {
  // 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:

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export function setup() {
  return { v: 1 };
}

export default function (data) {
  console.log(JSON.stringify(data));
}

export function teardown(data) {
  if (data.v != 1) {
    throw new Error('incorrect data: ' + JSON.stringify(data));
  }
}

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.

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.

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  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() {
    http.get('https://test.k6.io/contacts.php');
    sleep(Math.random() * 2);
  }