> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/grafana/k6/llms.txt
> Use this file to discover all available pages before exploring further.

# VU Instance and Test Objects

> Global VU and test objects for runtime information.

k6 provides global `__VU` and `__ITER` variables for accessing VU and iteration information.

## \_\_VU

The `__VU` variable contains the current VU's unique ID.

<ResponseField name="__VU" type="number">
  Current VU ID (1-based, unique within the test)
</ResponseField>

```javascript theme={null}
export default function () {
  console.log(`VU ID: ${__VU}`);
}
```

### Usage

```javascript theme={null}
import http from 'k6/http';

export default function () {
  // Use VU ID for unique data
  const userId = 1000 + __VU;
  
  http.post('https://test.k6.io/api/users', JSON.stringify({
    id: userId,
    name: `user-${__VU}`,
  }));
}
```

## \_\_ITER

The `__ITER` variable contains the current iteration number for the VU.

<ResponseField name="__ITER" type="number">
  Current iteration number (0-based)
</ResponseField>

```javascript theme={null}
export default function () {
  console.log(`Iteration: ${__ITER}`);
}
```

### Usage

```javascript theme={null}
import { sleep } from 'k6';

export default function () {
  // Skip first iteration (warmup)
  if (__ITER === 0) {
    console.log('Warmup iteration');
    return;
  }
  
  // Normal test logic
  console.log(`Running iteration ${__ITER}`);
}
```

## Environment Variables

### \_\_ENV

Access environment variables passed to k6.

```javascript theme={null}
export default function () {
  const apiUrl = __ENV.API_URL || 'https://test.k6.io';
  console.log(`Using API: ${apiUrl}`);
}
```

Run with:

```bash theme={null}
k6 run -e API_URL=https://api.example.com script.js
```

## Examples

### Per-VU Data Assignment

```javascript theme={null}
import { SharedArray } from 'k6/data';

const users = new SharedArray('users', function () {
  return [
    { username: 'user1', password: 'pass1' },
    { username: 'user2', password: 'pass2' },
    { username: 'user3', password: 'pass3' },
  ];
});

export default function () {
  // Each VU gets a different user
  const user = users[(__VU - 1) % users.length];
  console.log(`VU ${__VU} using ${user.username}`);
}
```

### Iteration-based Pacing

```javascript theme={null}
import { sleep } from 'k6';
import http from 'k6/http';

export default function () {
  http.get('https://test.k6.io');
  
  // Increase sleep time with each iteration
  const sleepTime = 1 + (__ITER * 0.1);
  sleep(sleepTime);
}
```

### Conditional Execution

```javascript theme={null}
import http from 'k6/http';

export default function () {
  // Only even-numbered VUs hit the API
  if (__VU % 2 === 0) {
    http.get('https://test.k6.io/api');
  } else {
    http.get('https://test.k6.io/web');
  }
}
```

### Unique Session IDs

```javascript theme={null}
import http from 'k6/http';

export default function () {
  // Create unique session ID combining VU and iteration
  const sessionId = `session-${__VU}-${__ITER}`;
  
  http.get('https://test.k6.io', {
    headers: { 'X-Session-ID': sessionId },
  });
}
```

## Comparison with exec

The global variables are simpler but less feature-rich than the `k6/execution` module:

| Feature       | Global Vars | k6/execution                  |
| ------------- | ----------- | ----------------------------- |
| VU ID         | `__VU`      | `exec.vu.idInInstance`        |
| Iteration     | `__ITER`    | `exec.vu.iterationInInstance` |
| Scenario info | ❌           | ✅                             |
| Test control  | ❌           | ✅                             |
| Tags/Metadata | ❌           | ✅                             |

### When to use global variables:

* Simple VU identification
* Iteration counting
* Quick prototyping

### When to use k6/execution:

* Need scenario information
* Want to control test execution
* Need to set tags/metadata
* Require detailed execution state

```javascript theme={null}
// Simple case: use global vars
export default function () {
  console.log(`VU ${__VU}, Iteration ${__ITER}`);
}

// Complex case: use k6/execution
import exec from 'k6/execution';

export default function () {
  console.log(`Scenario: ${exec.scenario.name}`);
  console.log(`Progress: ${exec.scenario.progress}`);
  exec.vu.tags['iteration'] = exec.vu.iterationInInstance;
}
```

<Note>
  The global variables `__VU` and `__ITER` are available in both the init and VU contexts, but they are only meaningful in the VU context.
</Note>