# CI Pipeline and Burn-In Strategy

## Principle

CI pipelines must execute tests reliably, quickly, and provide clear feedback. Burn-in testing (running changed tests multiple times) flushes out flakiness before merge. Stage jobs strategically: install/cache once, run changed specs first for fast feedback, then shard full suites with fail-fast disabled to preserve evidence.

## Rationale

CI is the quality gate for production. A poorly configured pipeline either wastes developer time (slow feedback, false positives) or ships broken code (false negatives, insufficient coverage). Burn-in testing ensures reliability by stress-testing changed code, while parallel execution and intelligent test selection optimize speed without sacrificing thoroughness.

## Pattern Examples

### Example 1: GitHub Actions Workflow with Parallel Execution

**Context**: Production-ready CI/CD pipeline for E2E tests with caching, parallelization, and burn-in testing.

**Implementation**:

```yaml
# .github/workflows/e2e-tests.yml
name: E2E Tests
on:
  pull_request:
  push:
    branches: [main, develop]

env:
  NODE_VERSION_FILE: '.nvmrc'
  CACHE_KEY: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}

jobs:
  install-dependencies:
    name: Install & Cache Dependencies
    runs-on: ubuntu-latest
    timeout-minutes: 10
    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version-file: ${{ env.NODE_VERSION_FILE }}
          cache: 'npm'

      - name: Cache node modules
        uses: actions/cache@v4
        id: npm-cache
        with:
          path: |
            ~/.npm
            node_modules
            ~/.cache/Cypress
            ~/.cache/ms-playwright
          key: ${{ env.CACHE_KEY }}
          restore-keys: |
            ${{ runner.os }}-node-

      - name: Install dependencies
        if: steps.npm-cache.outputs.cache-hit != 'true'
        run: npm ci --prefer-offline --no-audit

      - name: Install Playwright browsers
        if: steps.npm-cache.outputs.cache-hit != 'true'
        run: npx playwright install --with-deps chromium

  test-changed-specs:
    name: Test Changed Specs First (Burn-In)
    needs: install-dependencies
    runs-on: ubuntu-latest
    timeout-minutes: 15
    steps:
      - name: Checkout code
        uses: actions/checkout@v4
        with:
          fetch-depth: 0 # Full history for accurate diff

      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version-file: ${{ env.NODE_VERSION_FILE }}
          cache: 'npm'

      - name: Restore dependencies
        uses: actions/cache@v4
        with:
          path: |
            ~/.npm
            node_modules
            ~/.cache/ms-playwright
          key: ${{ env.CACHE_KEY }}

      - name: Detect changed test files
        id: changed-tests
        run: |
          CHANGED_SPECS=$(git diff --name-only origin/main...HEAD | grep -E '\.(spec|test)\.(ts|js|tsx|jsx)$' || echo "")
          echo "changed_specs=${CHANGED_SPECS}" >> $GITHUB_OUTPUT
          echo "Changed specs: ${CHANGED_SPECS}"

      - name: Run burn-in on changed specs (10 iterations)
        if: steps.changed-tests.outputs.changed_specs != ''
        run: |
          SPECS="${{ steps.changed-tests.outputs.changed_specs }}"
          echo "Running burn-in: 10 iterations on changed specs"
          for i in {1..10}; do
            echo "Burn-in iteration $i/10"
            npm run test -- $SPECS || {
              echo "❌ Burn-in failed on iteration $i"
              exit 1
            }
          done
          echo "✅ Burn-in passed - 10/10 successful runs"

      - name: Upload artifacts on failure
        if: failure()
        uses: actions/upload-artifact@v4
        with:
          name: burn-in-failure-artifacts
          path: |
            test-results/
            playwright-report/
            screenshots/
          retention-days: 7

  test-e2e-sharded:
    name: E2E Tests (Shard ${{ matrix.shard }}/${{ strategy.job-total }})
    needs: [install-dependencies, test-changed-specs]
    runs-on: ubuntu-latest
    timeout-minutes: 30
    strategy:
      fail-fast: false # Run all shards even if one fails
      matrix:
        shard: [1, 2, 3, 4]
    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Setup Node.js
        uses: actions/setup-node@v4
        with:
          node-version-file: ${{ env.NODE_VERSION_FILE }}
          cache: 'npm'

      - name: Restore dependencies
        uses: actions/cache@v4
        with:
          path: |
            ~/.npm
            node_modules
            ~/.cache/ms-playwright
          key: ${{ env.CACHE_KEY }}

      - name: Run E2E tests (shard ${{ matrix.shard }})
        run: npm run test:e2e -- --shard=${{ matrix.shard }}/4
        env:
          TEST_ENV: staging
          CI: true

      - name: Upload test results
        if: always()
        uses: actions/upload-artifact@v4
        with:
          name: test-results-shard-${{ matrix.shard }}
          path: |
            test-results/
            playwright-report/
          retention-days: 30

      - name: Upload JUnit report
        if: always()
        uses: actions/upload-artifact@v4
        with:
          name: junit-results-shard-${{ matrix.shard }}
          path: test-results/junit.xml
          retention-days: 30

  merge-test-results:
    name: Merge Test Results & Generate Report
    needs: test-e2e-sharded
    runs-on: ubuntu-latest
    if: always()
    steps:
      - name: Download all shard results
        uses: actions/download-artifact@v4
        with:
          pattern: test-results-shard-*
          path: all-results/

      - name: Merge HTML reports
        run: |
          npx playwright merge-reports --reporter=html all-results/
          echo "Merged report available in playwright-report/"

      - name: Upload merged report
        uses: actions/upload-artifact@v4
        with:
          name: merged-playwright-report
          path: playwright-report/
          retention-days: 30

      - name: Comment PR with results
        if: github.event_name == 'pull_request'
        uses: daun/playwright-report-comment@v3
        with:
          report-path: playwright-report/
```

**Key Points**:

- **Install once, reuse everywhere**: Dependencies cached across all jobs
- **Burn-in first**: Changed specs run 10x before full suite
- **Fail-fast disabled**: All shards run to completion for full evidence
- **Parallel execution**: 4 shards cut execution time by ~75%
- **Artifact retention**: 30 days for reports, 7 days for failure debugging

---

### Example 2: Burn-In Loop Pattern (Standalone Script)

**Context**: Reusable bash script for burn-in testing changed specs locally or in CI.

**Implementation**:

```bash
#!/bin/bash
# scripts/burn-in-changed.sh
# Usage: ./scripts/burn-in-changed.sh [iterations] [base-branch]

set -e  # Exit on error

# Configuration
ITERATIONS=${1:-10}
BASE_BRANCH=${2:-main}
SPEC_PATTERN='\.(spec|test)\.(ts|js|tsx|jsx)$'

echo "🔥 Burn-In Test Runner"
echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
echo "Iterations: $ITERATIONS"
echo "Base branch: $BASE_BRANCH"
echo ""

# Detect changed test files
echo "📋 Detecting changed test files..."
CHANGED_SPECS=$(git diff --name-only $BASE_BRANCH...HEAD | grep -E "$SPEC_PATTERN" || echo "")

if [ -z "$CHANGED_SPECS" ]; then
  echo "✅ No test files changed. Skipping burn-in."
  exit 0
fi

echo "Changed test files:"
echo "$CHANGED_SPECS" | sed 's/^/  - /'
echo ""

# Count specs
SPEC_COUNT=$(echo "$CHANGED_SPECS" | wc -l | xargs)
echo "Running burn-in on $SPEC_COUNT test file(s)..."
echo ""

# Burn-in loop
FAILURES=()
for i in $(seq 1 $ITERATIONS); do
  echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
  echo "🔄 Iteration $i/$ITERATIONS"
  echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"

  # Run tests with explicit file list
  if npm run test -- $CHANGED_SPECS 2>&1 | tee "burn-in-log-$i.txt"; then
    echo "✅ Iteration $i passed"
  else
    echo "❌ Iteration $i failed"
    FAILURES+=($i)

    # Save failure artifacts
    mkdir -p burn-in-failures/iteration-$i
    cp -r test-results/ burn-in-failures/iteration-$i/ 2>/dev/null || true
    cp -r screenshots/ burn-in-failures/iteration-$i/ 2>/dev/null || true

    echo ""
    echo "🛑 BURN-IN FAILED on iteration $i"
    echo "Failure artifacts saved to: burn-in-failures/iteration-$i/"
    echo "Logs saved to: burn-in-log-$i.txt"
    echo ""
    exit 1
  fi

  echo ""
done

# Success summary
echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
echo "🎉 BURN-IN PASSED"
echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
echo "All $ITERATIONS iterations passed for $SPEC_COUNT test file(s)"
echo "Changed specs are stable and ready to merge."
echo ""

# Cleanup logs
rm -f burn-in-log-*.txt

exit 0
```

**Usage**:

```bash
# Run locally with default settings (10 iterations, compare to main)
./scripts/burn-in-changed.sh

# Custom iterations and base branch
./scripts/burn-in-changed.sh 20 develop

# Add to package.json
{
  "scripts": {
    "test:burn-in": "bash scripts/burn-in-changed.sh",
    "test:burn-in:strict": "bash scripts/burn-in-changed.sh 20"
  }
}
```

**Key Points**:

- **Exit on first failure**: Flaky tests caught immediately
- **Failure artifacts**: Saved per-iteration for debugging
- **Flexible configuration**: Iterations and base branch customizable
- **CI/local parity**: Same script runs in both environments
- **Clear output**: Visual feedback on progress and results

---

### Example 3: Shard Orchestration with Result Aggregation

**Context**: Advanced sharding strategy for large test suites with intelligent result merging.

**Implementation**:

```javascript
// scripts/run-sharded-tests.js
const { spawn } = require('child_process');
const fs = require('fs');
const path = require('path');

/**
 * Run tests across multiple shards and aggregate results
 * Usage: node scripts/run-sharded-tests.js --shards=4 --env=staging
 */

const SHARD_COUNT = parseInt(process.env.SHARD_COUNT || '4');
const TEST_ENV = process.env.TEST_ENV || 'local';
const RESULTS_DIR = path.join(__dirname, '../test-results');

console.log(`🚀 Running tests across ${SHARD_COUNT} shards`);
console.log(`Environment: ${TEST_ENV}`);
console.log('━'.repeat(50));

// Ensure results directory exists
if (!fs.existsSync(RESULTS_DIR)) {
  fs.mkdirSync(RESULTS_DIR, { recursive: true });
}

/**
 * Run a single shard
 */
function runShard(shardIndex) {
  return new Promise((resolve, reject) => {
    const shardId = `${shardIndex}/${SHARD_COUNT}`;
    console.log(`\n📦 Starting shard ${shardId}...`);

    const child = spawn('npx', ['playwright', 'test', `--shard=${shardId}`, '--reporter=json'], {
      env: { ...process.env, TEST_ENV, SHARD_INDEX: shardIndex },
      stdio: 'pipe',
    });

    let stdout = '';
    let stderr = '';

    child.stdout.on('data', (data) => {
      stdout += data.toString();
      process.stdout.write(data);
    });

    child.stderr.on('data', (data) => {
      stderr += data.toString();
      process.stderr.write(data);
    });

    child.on('close', (code) => {
      // Save shard results
      const resultFile = path.join(RESULTS_DIR, `shard-${shardIndex}.json`);
      try {
        const result = JSON.parse(stdout);
        fs.writeFileSync(resultFile, JSON.stringify(result, null, 2));
        console.log(`✅ Shard ${shardId} completed (exit code: ${code})`);
        resolve({ shardIndex, code, result });
      } catch (error) {
        console.error(`❌ Shard ${shardId} failed to parse results:`, error.message);
        reject({ shardIndex, code, error });
      }
    });

    child.on('error', (error) => {
      console.error(`❌ Shard ${shardId} process error:`, error.message);
      reject({ shardIndex, error });
    });
  });
}

/**
 * Aggregate results from all shards
 */
function aggregateResults() {
  console.log('\n📊 Aggregating results from all shards...');

  const shardResults = [];
  let totalTests = 0;
  let totalPassed = 0;
  let totalFailed = 0;
  let totalSkipped = 0;
  let totalFlaky = 0;

  for (let i = 1; i <= SHARD_COUNT; i++) {
    const resultFile = path.join(RESULTS_DIR, `shard-${i}.json`);
    if (fs.existsSync(resultFile)) {
      const result = JSON.parse(fs.readFileSync(resultFile, 'utf8'));
      shardResults.push(result);

      // Aggregate stats
      totalTests += result.stats?.expected || 0;
      totalPassed += result.stats?.expected || 0;
      totalFailed += result.stats?.unexpected || 0;
      totalSkipped += result.stats?.skipped || 0;
      totalFlaky += result.stats?.flaky || 0;
    }
  }

  const summary = {
    totalShards: SHARD_COUNT,
    environment: TEST_ENV,
    totalTests,
    passed: totalPassed,
    failed: totalFailed,
    skipped: totalSkipped,
    flaky: totalFlaky,
    duration: shardResults.reduce((acc, r) => acc + (r.duration || 0), 0),
    timestamp: new Date().toISOString(),
  };

  // Save aggregated summary
  fs.writeFileSync(path.join(RESULTS_DIR, 'summary.json'), JSON.stringify(summary, null, 2));

  console.log('\n━'.repeat(50));
  console.log('📈 Test Results Summary');
  console.log('━'.repeat(50));
  console.log(`Total tests:    ${totalTests}`);
  console.log(`✅ Passed:      ${totalPassed}`);
  console.log(`❌ Failed:      ${totalFailed}`);
  console.log(`⏭️  Skipped:     ${totalSkipped}`);
  console.log(`⚠️  Flaky:       ${totalFlaky}`);
  console.log(`⏱️  Duration:    ${(summary.duration / 1000).toFixed(2)}s`);
  console.log('━'.repeat(50));

  return summary;
}

/**
 * Main execution
 */
async function main() {
  const startTime = Date.now();
  const shardPromises = [];

  // Run all shards in parallel
  for (let i = 1; i <= SHARD_COUNT; i++) {
    shardPromises.push(runShard(i));
  }

  try {
    await Promise.allSettled(shardPromises);
  } catch (error) {
    console.error('❌ One or more shards failed:', error);
  }

  // Aggregate results
  const summary = aggregateResults();

  const totalTime = ((Date.now() - startTime) / 1000).toFixed(2);
  console.log(`\n⏱️  Total execution time: ${totalTime}s`);

  // Exit with failure if any tests failed
  if (summary.failed > 0) {
    console.error('\n❌ Test suite failed');
    process.exit(1);
  }

  console.log('\n✅ All tests passed');
  process.exit(0);
}

main().catch((error) => {
  console.error('Fatal error:', error);
  process.exit(1);
});
```

**package.json integration**:

```json
{
  "scripts": {
    "test:sharded": "node scripts/run-sharded-tests.js",
    "test:sharded:ci": "SHARD_COUNT=8 TEST_ENV=staging node scripts/run-sharded-tests.js"
  }
}
```

**Key Points**:

- **Parallel shard execution**: All shards run simultaneously
- **Result aggregation**: Unified summary across shards
- **Failure detection**: Exit code reflects overall test status
- **Artifact preservation**: Individual shard results saved for debugging
- **CI/local compatibility**: Same script works in both environments

---

### Example 4: Selective Test Execution (Changed Files + Tags)

**Context**: Optimize CI by running only relevant tests based on file changes and tags.

**Implementation**:

```bash
#!/bin/bash
# scripts/selective-test-runner.sh
# Intelligent test selection based on changed files and test tags

set -e

BASE_BRANCH=${BASE_BRANCH:-main}
TEST_ENV=${TEST_ENV:-local}

echo "🎯 Selective Test Runner"
echo "━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━"
echo "Base branch: $BASE_BRANCH"
echo "Environment: $TEST_ENV"
echo ""

# Detect changed files (all types, not just tests)
CHANGED_FILES=$(git diff --name-only $BASE_BRANCH...HEAD)

if [ -z "$CHANGED_FILES" ]; then
  echo "✅ No files changed. Skipping tests."
  exit 0
fi

echo "Changed files:"
echo "$CHANGED_FILES" | sed 's/^/  - /'
echo ""

# Determine test strategy based on changes
run_smoke_only=false
run_all_tests=false
affected_specs=""

# Critical files = run all tests
if echo "$CHANGED_FILES" | grep -qE '(package\.json|package-lock\.json|playwright\.config|cypress\.config|\.github/workflows)'; then
  echo "⚠️  Critical configuration files changed. Running ALL tests."
  run_all_tests=true

# Auth/security changes = run all auth + smoke tests
elif echo "$CHANGED_FILES" | grep -qE '(auth|login|signup|security)'; then
  echo "🔒 Auth/security files changed. Running auth + smoke tests."
  npm run test -- --grep "@auth|@smoke"
  exit $?

# API changes = run integration + smoke tests
elif echo "$CHANGED_FILES" | grep -qE '(api|service|controller)'; then
  echo "🔌 API files changed. Running integration + smoke tests."
  npm run test -- --grep "@integration|@smoke"
  exit $?

# UI component changes = run related component tests
elif echo "$CHANGED_FILES" | grep -qE '\.(tsx|jsx|vue)$'; then
  echo "🎨 UI components changed. Running component + smoke tests."

  # Extract component names and find related tests
  components=$(echo "$CHANGED_FILES" | grep -E '\.(tsx|jsx|vue)$' | xargs -I {} basename {} | sed 's/\.[^.]*$//')
  for component in $components; do
    # Find tests matching component name
    affected_specs+=$(find tests -name "*${component}*" -type f) || true
  done

  if [ -n "$affected_specs" ]; then
    echo "Running tests for: $affected_specs"
    npm run test -- $affected_specs --grep "@smoke"
  else
    echo "No specific tests found. Running smoke tests only."
    npm run test -- --grep "@smoke"
  fi
  exit $?

# Documentation/config only = run smoke tests
elif echo "$CHANGED_FILES" | grep -qE '\.(md|txt|json|yml|yaml)$'; then
  echo "📝 Documentation/config files changed. Running smoke tests only."
  run_smoke_only=true
else
  echo "⚙️  Other files changed. Running smoke tests."
  run_smoke_only=true
fi

# Execute selected strategy
if [ "$run_all_tests" = true ]; then
  echo ""
  echo "Running full test suite..."
  npm run test
elif [ "$run_smoke_only" = true ]; then
  echo ""
  echo "Running smoke tests..."
  npm run test -- --grep "@smoke"
fi
```

**Usage in GitHub Actions**:

```yaml
# .github/workflows/selective-tests.yml
name: Selective Tests
on: pull_request

jobs:
  selective-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0

      - name: Run selective tests
        run: bash scripts/selective-test-runner.sh
        env:
          BASE_BRANCH: ${{ github.base_ref }}
          TEST_ENV: staging
```

**Key Points**:

- **Intelligent routing**: Tests selected based on changed file types
- **Tag-based filtering**: Use @smoke, @auth, @integration tags
- **Fast feedback**: Only relevant tests run on most PRs
- **Safety net**: Critical changes trigger full suite
- **Component mapping**: UI changes run related component tests

---

## CI Configuration Checklist

Before deploying your CI pipeline, verify:

- [ ] **Caching strategy**: node_modules, npm cache, browser binaries cached
- [ ] **Timeout budgets**: Each job has reasonable timeout (10-30 min)
- [ ] **Artifact retention**: 30 days for reports, 7 days for failure artifacts
- [ ] **Parallelization**: Matrix strategy uses fail-fast: false
- [ ] **Burn-in enabled**: Changed specs run 5-10x before merge
- [ ] **wait-on app startup**: CI waits for app (wait-on: 'http://localhost:3000')
- [ ] **Secrets documented**: README lists required secrets (API keys, tokens)
- [ ] **Local parity**: CI scripts runnable locally (npm run test:ci)

## Integration Points

- Used in workflows: `*ci` (CI/CD pipeline setup)
- Related fragments: `selective-testing.md`, `playwright-config.md`, `test-quality.md`
- CI tools: GitHub Actions, GitLab CI, CircleCI, Jenkins

_Source: Murat CI/CD strategy blog, Playwright/Cypress workflow examples, SEON production pipelines_