Correlation & Dynamic Data

Learn how to extract dynamic values from responses and use them in subsequent requests - essential for testing realistic user flows.

What is Correlation?

Correlation means extracting values from one response and reusing them in later requests. This is critical for:

Session tokens and authentication
CSRF tokens and security nonces
Resource IDs from created objects
Dynamic form values (VIEWSTATE, hidden fields)
Transaction IDs and reference numbers

Recorded scripts capture session-specific data that expires quickly. You must correlate these values to create working tests.

Extracting from JSON Responses

Most modern APIs return JSON data. Extract values by parsing the response:

import http from 'k6/http';
import { check } from 'k6';

export default function () {
  // Make a request that returns JSON data
  const reqHeaders = {
    Authorization: 'Token abcdef0123456789',
  };
  const res = http.get('https://quickpizza.grafana.com/api/doughs', {
    headers: reqHeaders,
  });

  // Extract data from JSON by parsing and navigating the object
  const dough1 = res.json().doughs[0];
  check(dough1, {
    'dough 1 has correct name': (s) => s.name === 'Thin',
    'dough 1 has correct ID': (s) => s.ID === 1,
  });

  // Use the extracted data in subsequent requests
  const orderRes = http.post(
    'https://quickpizza.grafana.com/api/orders',
    JSON.stringify({ doughId: dough1.ID }),
    { headers: reqHeaders }
  );
}

Use res.json() to parse the entire response, or res.json('path.to.field') to extract a specific field directly.

Extracting from HTML Forms

Web applications often include hidden fields with dynamic values. Extract them before form submission:

Method 1: Using Response.submitForm()

The high-level API handles form extraction automatically:

import http from 'k6/http';
import { check } from 'k6';

export default function () {
  // Get the page with the form
  const res = http.get('https://example.com/form');
  
  // Submit the form, k6 extracts hidden fields automatically
  const submitRes = res.submitForm({
    fields: {
      username: 'testuser',
      password: 'testpass',
    },
  });
  
  check(submitRes, { 'login successful': (r) => r.status === 200 });
}

Method 2: Manual Extraction

For more control, extract specific fields manually:

import http from 'k6/http';
import { sleep } from 'k6';

export default function () {
  // Request the page containing a form
  const res = http.get('https://test.k6.io/my_messages.php', { responseType: 'text' });

  // Query the HTML for an input field named "redir"
  const elem = res.html().find('input[name=redir]');

  // Get the value of the attribute "value"
  const val = elem.attr('value');

  // Use the extracted value in subsequent requests
  console.log('The value of the hidden field redir is: ' + val);

  // Now make a request with the extracted value
  http.post('https://test.k6.io/submit', {
    redir: val,
    message: 'Hello from k6!',
  });

  sleep(1);
}

If you set discardResponseBodies: true in options, you must override it per request with {responseType: "text"} to access the response body.

Extracting .NET ViewState and CSRF Tokens

Common pattern for .NET applications:

import http from 'k6/http';
import { check } from 'k6';

export default function () {
  // Get the page with the form
  const res = http.get('https://example.com/login', { responseType: 'text' });
  
  // Extract multiple hidden fields
  const viewState = res.html().find('input[name=__VIEWSTATE]').attr('value');
  const viewStateGenerator = res.html().find('input[name=__VIEWSTATEGENERATOR]').attr('value');
  const eventValidation = res.html().find('input[name=__EVENTVALIDATION]').attr('value');
  const csrfToken = res.html().find('input[name=csrf_token]').attr('value');
  
  // Submit form with all extracted values
  const loginRes = http.post('https://example.com/login', {
    __VIEWSTATE: viewState,
    __VIEWSTATEGENERATOR: viewStateGenerator,
    __EVENTVALIDATION: eventValidation,
    csrf_token: csrfToken,
    username: 'testuser',
    password: 'testpass',
  });
  
  check(loginRes, { 'login successful': (r) => r.status === 200 });
}

Generic String Extraction

For responses that aren’t JSON or HTML, extract values using string boundaries:

import { findBetween } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';
import { check } from 'k6';
import http from 'k6/http';

export default function () {
  // This request returns XML
  const res = http.get('https://quickpizza.grafana.com/api/xml?color=green');

  // Use findBetween to extract the first <value> tag
  const color = findBetween(res.body, '<value>', '</value>');

  check(color, {
    'color is correct': (t) => t === 'green',
  });
  
  // Use extracted value in next request
  http.post('https://quickpizza.grafana.com/api/verify', 
    JSON.stringify({ color: color })
  );
}

findBetween()
Regular Expression
String Methods

Extract the first occurrence between boundaries:

import { findBetween } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

const token = findBetween(res.body, 'token="', '"');

Use regex for complex patterns:

const match = res.body.match(/token="([^"]+)"/);
const token = match ? match[1] : null;

Use built-in JavaScript string methods:

const start = res.body.indexOf('token="') + 7;
const end = res.body.indexOf('"', start);
const token = res.body.substring(start, end);

Complete Correlation Example

Realistic user flow with multiple correlations:

import http from 'k6/http';
import { check, sleep } from 'k6';
import { randomString } from 'https://jslib.k6.io/k6-utils/1.2.0/index.js';

export const options = {
  vus: 10,
  duration: '1m',
};

export default function () {
  // 1. Get login page and extract CSRF token
  const loginPage = http.get('https://example.com/login', { responseType: 'text' });
  const csrfToken = loginPage.html().find('input[name=csrf_token]').attr('value');
  
  check(csrfToken, { 'extracted CSRF token': (t) => t !== undefined });
  
  // 2. Login and get session cookie + auth token
  const loginRes = http.post('https://example.com/login', {
    csrf_token: csrfToken,
    username: 'testuser',
    password: 'testpass',
  });
  
  const authToken = loginRes.json('token');
  check(authToken, { 'received auth token': (t) => t !== undefined });
  
  // 3. Create a resource and get its ID
  const createRes = http.post(
    'https://example.com/api/items',
    JSON.stringify({
      name: `Item ${randomString(8)}`,
      description: 'Test item',
    }),
    {
      headers: {
        'Authorization': `Bearer ${authToken}`,
        'Content-Type': 'application/json',
      },
    }
  );
  
  const itemId = createRes.json('id');
  check(createRes, {
    'item created': (r) => r.status === 201,
    'has item ID': () => itemId !== undefined,
  });
  
  sleep(1);
  
  // 4. Update the resource using extracted ID
  const updateRes = http.put(
    `https://example.com/api/items/${itemId}`,
    JSON.stringify({ name: 'Updated Name' }),
    {
      headers: {
        'Authorization': `Bearer ${authToken}`,
        'Content-Type': 'application/json',
      },
    }
  );
  
  check(updateRes, { 'item updated': (r) => r.status === 200 });
  
  sleep(1);
  
  // 5. Delete the resource
  const deleteRes = http.del(
    `https://example.com/api/items/${itemId}`,
    null,
    {
      headers: { 'Authorization': `Bearer ${authToken}` },
    }
  );
  
  check(deleteRes, { 'item deleted': (r) => r.status === 204 });
}

Advanced Patterns

Extract data in setup, share with all VUs:

export function setup() {
  // Get data once for all VUs
  const res = http.get('https://api.example.com/config');
  return {
    apiKey: res.json('apiKey'),
    endpoints: res.json('endpoints'),
  };
}

export default function (data) {
  // All VUs use shared data
  http.get(data.endpoints.users, {
    headers: { 'X-API-Key': data.apiKey },
  });
}

Correlation with Multiple Matches

Extract multiple values from a response:

import http from 'k6/http';

export default function () {
  const res = http.get('https://api.example.com/products');
  
  // Get all product IDs
  const products = res.json('products');
  const productIds = products.map(p => p.id);
  
  // Use random product ID
  const randomId = productIds[Math.floor(Math.random() * productIds.length)];
  
  http.get(`https://api.example.com/products/${randomId}`);
}

Token Refresh Pattern

Handle token expiration and renewal:

import http from 'k6/http';

let accessToken = null;
let refreshToken = null;
let tokenExpiry = 0;

function getValidToken() {
  // Check if token needs refresh
  if (Date.now() > tokenExpiry) {
    const res = http.post(
      'https://api.example.com/auth/refresh',
      JSON.stringify({ refresh_token: refreshToken }),
      { headers: { 'Content-Type': 'application/json' } }
    );
    
    accessToken = res.json('access_token');
    refreshToken = res.json('refresh_token');
    tokenExpiry = Date.now() + (res.json('expires_in') * 1000);
  }
  
  return accessToken;
}

export function setup() {
  // Initial login
  const res = http.post(
    'https://api.example.com/auth/login',
    JSON.stringify({ username: 'user', password: 'pass' })
  );
  
  accessToken = res.json('access_token');
  refreshToken = res.json('refresh_token');
  tokenExpiry = Date.now() + (res.json('expires_in') * 1000);
}

export default function () {
  const token = getValidToken();
  
  http.get('https://api.example.com/data', {
    headers: { 'Authorization': `Bearer ${token}` },
  });
}

Common Extraction Scenarios

OAuth 2.0 Token Flow

export function setup() {
  // Get access token
  const tokenRes = http.post(
    'https://auth.example.com/oauth/token',
    {
      grant_type: 'client_credentials',
      client_id: __ENV.CLIENT_ID,
      client_secret: __ENV.CLIENT_SECRET,
    }
  );
  
  return {
    accessToken: tokenRes.json('access_token'),
    tokenType: tokenRes.json('token_type'),
  };
}

export default function (data) {
  http.get('https://api.example.com/resource', {
    headers: { 
      'Authorization': `${data.tokenType} ${data.accessToken}` 
    },
  });
}

WordPress Nonce

import http from 'k6/http';

export default function () {
  // Get WordPress page
  const page = http.get('https://example.com/wp-admin/', {
    responseType: 'text',
  });
  
  // Extract nonce
  const nonce = page.html().find('input[name=_wpnonce]').attr('value');
  
  // Submit form with nonce
  http.post('https://example.com/wp-admin/post.php', {
    _wpnonce: nonce,
    action: 'edit',
    post_title: 'Test Post',
  });
}

Pagination with Cursor

import http from 'k6/http';

export default function () {
  let cursor = null;
  let hasMore = true;
  
  while (hasMore) {
    const url = cursor 
      ? `https://api.example.com/items?cursor=${cursor}`
      : 'https://api.example.com/items';
    
    const res = http.get(url);
    const data = res.json();
    
    // Process items
    console.log(`Fetched ${data.items.length} items`);
    
    // Extract next cursor
    cursor = data.nextCursor;
    hasMore = data.hasMore;
  }
}

Best Practices

Always Validate Extracted Data

const token = res.json('token');

if (!token) {
  console.error('Failed to extract token:', res.body);
  return; // Exit early
}

// Use token safely
http.get(url, { headers: { 'Authorization': `Bearer ${token}` } });

Use Descriptive Variable Names

// Good
const csrfToken = res.html().find('input[name=csrf_token]').attr('value');
const userId = createUserRes.json('id');

// Bad
const val1 = res.html().find('input[name=csrf_token]').attr('value');
const id = createUserRes.json('id');

Handle Missing Values Gracefully

import { check } from 'k6';

const res = http.get('https://api.example.com/data');
const data = res.json();

if (!check(data, { 'has token': (d) => d.token !== undefined })) {
  console.error('Response missing token:', res.body);
  return;
}

const token = data.token;

Debugging Extraction

Troubleshoot extraction issues:

import http from 'k6/http';

export const options = {
  iterations: 1,
};

export default function () {
  const res = http.get('https://example.com/page');
  
  // Log full response
  console.log('Status:', res.status);
  console.log('Body:', res.body);
  
  // Try extraction
  const token = res.html().find('input[name=token]').attr('value');
  console.log('Extracted token:', token);
  
  // Log all matching elements
  const inputs = res.html().find('input');
  console.log('Found inputs:', inputs.size());
}

API CRUD Operations

See correlation in complete API workflows

HTTP Authentication

Extract and use authentication tokens

Test Lifecycle

Use setup() to share extracted data

API Reference: Response

Response methods for data extraction

Examples

Reference

Cloud & Studio

Release Notes

Correlation & Dynamic Data

What is Correlation?

Extracting from JSON Responses

Extracting from HTML Forms

Method 1: Using Response.submitForm()

Method 2: Manual Extraction

Extracting .NET ViewState and CSRF Tokens

Generic String Extraction

Complete Correlation Example

Advanced Patterns

Correlation with Multiple Matches

Token Refresh Pattern

Common Extraction Scenarios

Best Practices

Always Validate Extracted Data

Use Descriptive Variable Names

Handle Missing Values Gracefully

Debugging Extraction

API CRUD Operations

HTTP Authentication

Test Lifecycle

API Reference: Response

Build docs developers (and LLMs) love

Examples

Reference

Cloud & Studio

Release Notes

​What is Correlation?

​Extracting from JSON Responses

​Extracting from HTML Forms

​Method 1: Using Response.submitForm()

​Method 2: Manual Extraction

​Extracting .NET ViewState and CSRF Tokens

​Generic String Extraction

​Complete Correlation Example

​Advanced Patterns

​Sharing Data Between VUs

​Correlation with Multiple Matches

​Token Refresh Pattern

​Common Extraction Scenarios

​Best Practices

​Always Validate Extracted Data

​Use Descriptive Variable Names

​Handle Missing Values Gracefully

​Debugging Extraction

​Related Resources

API CRUD Operations

HTTP Authentication

Test Lifecycle

API Reference: Response

Build docs developers (and LLMs) love

What is Correlation?

Extracting from JSON Responses

Extracting from HTML Forms

Method 1: Using Response.submitForm()

Method 2: Manual Extraction

Extracting .NET ViewState and CSRF Tokens

Generic String Extraction

Complete Correlation Example

Advanced Patterns

Sharing Data Between VUs

Correlation with Multiple Matches

Token Refresh Pattern

Common Extraction Scenarios

Best Practices

Always Validate Extracted Data

Use Descriptive Variable Names

Handle Missing Values Gracefully

Debugging Extraction

Related Resources