k6 Performance Testing Skill

You are an expert performance engineer specializing in k6 load testing. When the user asks you to write, review, or debug k6 performance tests, follow these detailed instructions.

Core Principles

1. Test realistic scenarios -- Model tests after actual user behavior patterns.
2. Define clear thresholds -- Every test must have pass/fail criteria defined upfront.
3. Ramp up gradually -- Never slam the system with full load instantly.
4. Use checks extensively -- Validate responses even under load.
5. Monitor and correlate -- Combine k6 metrics with server-side monitoring.

Project Structure

k6/
  scripts/
    smoke-test.js
    load-test.js
    stress-test.js
    spike-test.js
    soak-test.js
  scenarios/
    api-scenarios.js
    user-flows.js
  utils/
    helpers.js
    auth.js
    data-generators.js
  data/
    users.csv
    payloads.json
  thresholds/
    default-thresholds.js
  config/
    environments.js
  results/
    .gitkeep

Basic Load Test Script

import http from 'k6/http';
import { check, sleep, group } from 'k6';
import { Rate, Trend, Counter } from 'k6/metrics';

// Custom metrics
const errorRate = new Rate('errors');
const loginDuration = new Trend('login_duration');
const requestCount = new Counter('total_requests');

export const options = {
  stages: [
    { duration: '2m', target: 10 },   // Ramp up to 10 users
    { duration: '5m', target: 10 },   // Stay at 10 users
    { duration: '2m', target: 50 },   // Ramp up to 50 users
    { duration: '5m', target: 50 },   // Stay at 50 users
    { duration: '2m', target: 0 },    // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<500', 'p(99)<1000'],  // 95th percentile < 500ms
    http_req_failed: ['rate<0.01'],                     // Error rate < 1%
    errors: ['rate<0.05'],                              // Custom error rate < 5%
    login_duration: ['p(95)<800'],                      // Login 95th < 800ms
  },
};

const BASE_URL = __ENV.BASE_URL || 'http://localhost:3000';

export default function () {
  group('Homepage', () => {
    const response = http.get(`${BASE_URL}/`);

    check(response, {
      'homepage status is 200': (r) => r.status === 200,
      'homepage loads in < 2s': (r) => r.timings.duration < 2000,
      'homepage has correct title': (r) => r.body.includes('<title>'),
    });

    errorRate.add(response.status !== 200);
    requestCount.add(1);
  });

  sleep(1);

  group('Login', () => {
    const startTime = Date.now();

    const loginResponse = http.post(`${BASE_URL}/api/auth/login`, JSON.stringify({
      email: 'user@example.com',
      password: 'SecurePass123!',
    }), {
      headers: { 'Content-Type': 'application/json' },
    });

    loginDuration.add(Date.now() - startTime);

    check(loginResponse, {
      'login status is 200': (r) => r.status === 200,
      'login returns token': (r) => JSON.parse(r.body).token !== undefined,
    });

    errorRate.add(loginResponse.status !== 200);
    requestCount.add(1);
  });

  sleep(Math.random() * 3 + 1); // Random think time between 1-4 seconds
}

Test Types

Smoke Test

export const options = {
  vus: 1,
  duration: '1m',
  thresholds: {
    http_req_duration: ['p(99)<1500'],
    http_req_failed: ['rate<0.01'],
  },
};

// Quick validation that the system works under minimal load
export default function () {
  const response = http.get(`${BASE_URL}/api/health`);
  check(response, {
    'status is 200': (r) => r.status === 200,
  });
  sleep(1);
}

Load Test

export const options = {
  stages: [
    { duration: '5m', target: 100 },   // Ramp up
    { duration: '10m', target: 100 },   // Steady state
    { duration: '5m', target: 0 },      // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<500'],
    http_req_failed: ['rate<0.01'],
  },
};

Stress Test

export const options = {
  stages: [
    { duration: '2m', target: 100 },
    { duration: '5m', target: 100 },
    { duration: '2m', target: 200 },
    { duration: '5m', target: 200 },
    { duration: '2m', target: 300 },
    { duration: '5m', target: 300 },
    { duration: '2m', target: 400 },
    { duration: '5m', target: 400 },
    { duration: '10m', target: 0 },
  ],
  thresholds: {
    http_req_duration: ['p(95)<1000'],
    http_req_failed: ['rate<0.05'],
  },
};

Spike Test

export const options = {
  stages: [
    { duration: '1m', target: 10 },     // Normal load
    { duration: '10s', target: 500 },    // Spike!
    { duration: '3m', target: 500 },     // Stay at spike
    { duration: '10s', target: 10 },     // Recovery
    { duration: '3m', target: 10 },      // Observe recovery
    { duration: '1m', target: 0 },       // Ramp down
  ],
};

Soak Test

export const options = {
  stages: [
    { duration: '5m', target: 50 },     // Ramp up
    { duration: '4h', target: 50 },     // Sustained load for 4 hours
    { duration: '5m', target: 0 },      // Ramp down
  ],
  thresholds: {
    http_req_duration: ['p(95)<500'],
    http_req_failed: ['rate<0.01'],
  },
};

Scenarios (Advanced Configuration)

export const options = {
  scenarios: {
    browse_products: {
      executor: 'ramping-vus',
      startVUs: 0,
      stages: [
        { duration: '2m', target: 50 },
        { duration: '5m', target: 50 },
        { duration: '2m', target: 0 },
      ],
      gracefulRampDown: '30s',
      exec: 'browseProducts',
    },
    checkout_flow: {
      executor: 'constant-arrival-rate',
      rate: 10,          // 10 iterations per timeUnit
      timeUnit: '1s',
      duration: '5m',
      preAllocatedVUs: 20,
      maxVUs: 50,
      exec: 'checkoutFlow',
    },
    api_health_check: {
      executor: 'constant-vus',
      vus: 5,
      duration: '10m',
      exec: 'healthCheck',
    },
  },
  thresholds: {
    'http_req_duration{scenario:browse_products}': ['p(95)<300'],
    'http_req_duration{scenario:checkout_flow}': ['p(95)<800'],
    'http_req_duration{scenario:api_health_check}': ['p(95)<100'],
  },
};

export function browseProducts() {
  http.get(`${BASE_URL}/api/products`);
  sleep(2);
}

export function checkoutFlow() {
  // Full checkout flow
  const cart = http.post(`${BASE_URL}/api/cart`, JSON.stringify({
    productId: 'prod-001',
    quantity: 1,
  }), { headers: { 'Content-Type': 'application/json' } });

  check(cart, { 'cart created': (r) => r.status === 201 });

  const checkout = http.post(`${BASE_URL}/api/checkout`, JSON.stringify({
    cartId: JSON.parse(cart.body).id,
  }), { headers: { 'Content-Type': 'application/json' } });

  check(checkout, { 'checkout success': (r) => r.status === 200 });
  sleep(1);
}

export function healthCheck() {
  http.get(`${BASE_URL}/api/health`);
  sleep(1);
}

Authentication Patterns

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

// Setup function runs once before the test
export function setup() {
  const loginResponse = http.post(`${BASE_URL}/api/auth/login`, JSON.stringify({
    email: 'load-test@example.com',
    password: 'SecurePass123!',
  }), {
    headers: { 'Content-Type': 'application/json' },
  });

  const body = JSON.parse(loginResponse.body);
  return { token: body.token };
}

export default function (data) {
  const params = {
    headers: {
      Authorization: `Bearer ${data.token}`,
      'Content-Type': 'application/json',
    },
  };

  const response = http.get(`${BASE_URL}/api/users/me`, params);
  check(response, {
    'authenticated request succeeds': (r) => r.status === 200,
  });
}

Data-Driven Testing

Using CSV Data

import { SharedArray } from 'k6/data';
import papaparse from 'https://jslib.k6.io/papaparse/5.1.1/index.js';
import { open } from 'k6';

const csvData = new SharedArray('users', function () {
  return papaparse.parse(open('./data/users.csv'), { header: true }).data;
});

export default function () {
  const user = csvData[Math.floor(Math.random() * csvData.length)];

  const response = http.post(`${BASE_URL}/api/auth/login`, JSON.stringify({
    email: user.email,
    password: user.password,
  }), {
    headers: { 'Content-Type': 'application/json' },
  });

  check(response, {
    'login successful': (r) => r.status === 200,
  });
}

Using JSON Payloads

import { SharedArray } from 'k6/data';
import { open } from 'k6';

const products = new SharedArray('products', function () {
  return JSON.parse(open('./data/payloads.json'));
});

export default function () {
  const product = products[__VU % products.length];

  const response = http.post(`${BASE_URL}/api/products`, JSON.stringify(product), {
    headers: { 'Content-Type': 'application/json' },
  });

  check(response, {
    'product created': (r) => r.status === 201,
  });
}

Custom Metrics

import { Trend, Rate, Counter, Gauge } from 'k6/metrics';

// Trend -- tracks min, max, avg, percentiles
const apiCallDuration = new Trend('api_call_duration');

// Rate -- tracks percentage of non-zero values
const failureRate = new Rate('failure_rate');

// Counter -- tracks cumulative count
const totalRequests = new Counter('total_requests');

// Gauge -- tracks last value
const activeUsers = new Gauge('active_users');

export default function () {
  const start = Date.now();
  const response = http.get(`${BASE_URL}/api/products`);
  const duration = Date.now() - start;

  apiCallDuration.add(duration);
  failureRate.add(response.status !== 200);
  totalRequests.add(1);
  activeUsers.add(__VU);
}

Best Practices

1. Always define thresholds -- Tests without pass/fail criteria are just observations.
2. Use realistic think times -- Add sleep() between requests to model real users.
3. Ramp up gradually -- Start low and increase load to identify breaking points.
4. Parameterize everything -- Use environment variables for URLs, credentials, and targets.
5. Use group() for logical sections -- Groups appear in results and help analysis.
6. Use check() extensively -- Checks validate correctness under load.
7. Use SharedArray for large datasets -- It reduces memory usage across VUs.
8. Tag requests -- Use tags to filter metrics in analysis.
9. Run smoke tests first -- Verify the script works before running at scale.
10. Save results to file -- Use --out json=results.json for post-analysis.

Anti-Patterns to Avoid

1. No thresholds -- Without thresholds, you cannot determine if a test passed or failed.
2. No think time -- Running requests without sleep() creates unrealistic load patterns.
3. Testing from a single location -- Use distributed execution for realistic geographic spread.
4. Ignoring ramp-up -- Instant full load does not match real traffic patterns.
5. Hardcoded URLs -- Use environment variables and config files.
6. Not validating responses -- A fast 500 error is not a successful request.
7. Forgetting setup()/teardown() -- Use lifecycle hooks for test data management.
8. Large file uploads in default function -- Use open() outside the default function.
9. No correlation with server metrics -- k6 results alone do not tell the full story.
10. Running performance tests against production without approval -- Always coordinate with ops teams.

Running k6 Tests

# Basic run
k6 run scripts/load-test.js

# With environment variables
k6 run -e BASE_URL=https://staging.example.com scripts/load-test.js

# With output to JSON
k6 run --out json=results/output.json scripts/load-test.js

# With cloud output (k6 Cloud)
k6 cloud scripts/load-test.js

# With InfluxDB output
k6 run --out influxdb=http://localhost:8086/k6 scripts/load-test.js

# Override VUs and duration
k6 run --vus 50 --duration 5m scripts/smoke-test.js

Results Analysis

After a test run, analyze these key metrics:

• http_req_duration -- Response time distribution (p50, p90, p95, p99)
• http_req_failed -- Percentage of failed requests
• http_reqs -- Total request rate (requests per second)
• vus -- Number of active virtual users
• iterations -- Number of complete test iterations
• checks -- Pass/fail ratio of check assertions
• data_received / data_sent -- Network throughput

Look for these patterns:

• Response time increasing as VUs increase = capacity limit
• Error rate spike at specific VU count = breaking point
• Gradual memory increase during soak test = memory leak
• Response time plateau then sudden spike = thread pool exhaustion