Agent Skills: Synthetic Monitoring

Synthetic Monitoring

Automated testing for consistent, proactive performance monitoring.

RUM vs Synthetic

| Aspect | RUM (Real User) | Synthetic (Lab) | |--------|-----------------|-----------------| | Data source | Actual users | Controlled tests | | Variability | High | Low | | Coverage | What users do | What you test | | Issues | Finds unknown | Validates known | | Timing | Reactive | Proactive |

Use both: Synthetic catches regressions before users; RUM shows real-world impact.

Lighthouse CI

Setup

npm install -D @lhci/cli

// lighthouserc.json
{
  "ci": {
    "collect": {
      "url": [
        "http://localhost:3000/",
        "http://localhost:3000/products",
        "http://localhost:3000/checkout"
      ],
      "numberOfRuns": 3
    },
    "assert": {
      "preset": "lighthouse:recommended",
      "assertions": {
        "categories:performance": ["error", { "minScore": 0.9 }],
        "categories:accessibility": ["error", { "minScore": 0.9 }],
        "first-contentful-paint": ["error", { "maxNumericValue": 2000 }],
        "largest-contentful-paint": ["error", { "maxNumericValue": 2500 }],
        "interactive": ["error", { "maxNumericValue": 3500 }],
        "cumulative-layout-shift": ["error", { "maxNumericValue": 0.1 }]
      }
    },
    "upload": {
      "target": "temporary-public-storage"
    }
  }
}

GitHub Actions

# .github/workflows/lighthouse.yml
name: Lighthouse CI

on: [push]

jobs:
  lighthouse:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Setup Node
        uses: actions/setup-node@v4
        with:
          node-version: '20'

      - name: Install & Build
        run: |
          npm ci
          npm run build

      - name: Start server
        run: npm run start &

      - name: Run Lighthouse
        run: |
          npm install -g @lhci/cli
          lhci autorun
        env:
          LHCI_GITHUB_APP_TOKEN: ${{ secrets.LHCI_GITHUB_APP_TOKEN }}

Playwright Performance Testing

Setup

npm install -D @playwright/test

// tests/performance.spec.ts
import { test, expect } from '@playwright/test';

test.describe('Performance Tests', () => {
  test('homepage loads within budget', async ({ page }) => {
    // Start tracing
    await page.context().tracing.start({ screenshots: true, snapshots: true });

    const startTime = Date.now();
    await page.goto('/');
    await page.waitForLoadState('networkidle');
    const loadTime = Date.now() - startTime;

    // Assert load time
    expect(loadTime).toBeLessThan(3000);

    // Get Core Web Vitals
    const vitals = await page.evaluate(() => {
      return new Promise((resolve) => {
        new PerformanceObserver((list) => {
          const entries = list.getEntries();
          const lcp = entries.find((e) => e.entryType === 'largest-contentful-paint');
          resolve({
            lcp: lcp?.startTime,
            cls: entries
              .filter((e) => e.entryType === 'layout-shift')
              .reduce((sum: number, e: any) => sum + e.value, 0),
          });
        }).observe({ type: 'largest-contentful-paint', buffered: true });

        // Fallback timeout
        setTimeout(() => resolve({ lcp: null, cls: null }), 5000);
      });
    });

    console.log('Vitals:', vitals);

    // Stop tracing
    await page.context().tracing.stop({ path: 'trace.zip' });
  });

  test('checkout flow performance', async ({ page }) => {
    const metrics: Record<string, number> = {};

    // Navigate through checkout
    await page.goto('/products/1');
    metrics.productPageLoad = await measurePageLoad(page);

    await page.click('[data-testid="add-to-cart"]');
    await page.goto('/cart');
    metrics.cartPageLoad = await measurePageLoad(page);

    await page.click('[data-testid="checkout"]');
    metrics.checkoutPageLoad = await measurePageLoad(page);

    // Assert all pages load within budget
    expect(metrics.productPageLoad).toBeLessThan(2000);
    expect(metrics.cartPageLoad).toBeLessThan(1500);
    expect(metrics.checkoutPageLoad).toBeLessThan(2000);
  });
});

async function measurePageLoad(page: any): Promise<number> {
  const timing = await page.evaluate(() => {
    const nav = performance.getEntriesByType('navigation')[0] as PerformanceNavigationTiming;
    return nav.loadEventEnd - nav.fetchStart;
  });
  return timing;
}

Performance Trace Collection

// playwright.config.ts
import { defineConfig } from '@playwright/test';

export default defineConfig({
  use: {
    trace: 'retain-on-failure',
    video: 'retain-on-failure',
  },
  projects: [
    {
      name: 'Desktop Chrome',
      use: {
        browserName: 'chromium',
        viewport: { width: 1280, height: 720 },
      },
    },
    {
      name: 'Mobile Chrome',
      use: {
        browserName: 'chromium',
        ...devices['Pixel 5'],
      },
    },
    {
      name: 'Slow 3G',
      use: {
        browserName: 'chromium',
        // Simulate slow network
        launchOptions: {
          args: ['--force-effective-connection-type=3g'],
        },
      },
    },
  ],
});

Scheduled Monitoring

Checkly (or similar)

// __checks__/homepage.check.ts
import { check, group } from 'checkly';
import { Playwright } from 'checkly/constructs';

const homepage = new Playwright('homepage-check', {
  name: 'Homepage Performance',
  frequency: 5, // Every 5 minutes
  locations: ['us-east-1', 'eu-west-1'],
  runtimeId: '2024.02',
  script: `
    const { expect } = require('@playwright/test');

    async function run({ page }) {
      const response = await page.goto('https://yoursite.com');

      expect(response.status()).toBe(200);

      const timing = await page.evaluate(() => {
        const nav = performance.getEntriesByType('navigation')[0];
        return nav.loadEventEnd - nav.fetchStart;
      });

      expect(timing).toBeLessThan(3000);
    }

    module.exports = { run };
  `,
  alertChannels: ['email-oncall'],
});

Metrics to Track

| Metric | Threshold | Priority | |--------|-----------|----------| | LCP | <2.5s | P0 | | TTI | <3.5s | P0 | | Total Blocking Time | <200ms | P0 | | CLS | <0.1 | P0 | | Speed Index | <3.4s | P1 | | Time to First Byte | <600ms | P1 |

CI/CD Integration Pattern

# Performance gate in CI
performance:
  runs-on: ubuntu-latest
  steps:
    - name: Run Lighthouse
      run: lhci autorun

    - name: Run Playwright
      run: npx playwright test tests/performance/

    - name: Upload results
      run: |
        # Send to your observability platform
        curl -X POST \
          -H "Authorization: Bearer $TOKEN" \
          -d @lighthouse-results.json \
          https://api.yourplatform.com/synthetic-results

Anti-Patterns

• Only running in CI (miss production issues)
• Testing only happy path
• Ignoring mobile/slow network
• No alerting on failures
• Not tracking trends over time
• Missing real user comparison

Related Skills

• See skills/core-web-vitals for RUM comparison
• See skills/bundle-performance for what to optimize
• See skills/route-transition-tracking for SPA performance

References

• references/performance.md - Performance budgets
• references/platforms/*.md - Vendor integrations