Monorepo Testing Strategies

// affected-detector.js
const { execSync } = require('child_process');
const fs = require('fs');
const path = require('path');

class AffectedDetector {
  constructor(workspaceRoot) {
    this.workspaceRoot = workspaceRoot;
    this.projectGraph = this.buildProjectGraph();
  }

  buildProjectGraph() {
    // Build dependency graph of all projects
    const packages = this.discoverPackages();
    const graph = new Map();

    for (const pkg of packages) {
      const deps = this.getPackageDependencies(pkg);
      graph.set(pkg.name, {
        path: pkg.path,
        dependencies: deps,
        dependents: []
      });
    }

    // Build reverse dependencies (dependents)
    for (const [name, data] of graph.entries()) {
      for (const dep of data.dependencies) {
        if (graph.has(dep)) {
          graph.get(dep).dependents.push(name);
        }
      }
    }

    return graph;
  }

  getAffectedProjects(baseBranch = 'main') {
    // Get changed files
    const changedFiles = execSync(
      `git diff --name-only ${baseBranch}...HEAD`,
      { encoding: 'utf-8' }
    ).trim().split('\n');

    // Determine which projects are affected
    const affected = new Set();

    for (const file of changedFiles) {
      const project = this.getProjectForFile(file);
      if (project) {
        affected.add(project);

        // Add all dependent projects
        this.addDependents(project, affected);
      }
    }

    return Array.from(affected);
  }

  getProjectForFile(filePath) {
    // Find which project owns this file
    for (const [name, data] of this.projectGraph.entries()) {
      if (filePath.startsWith(data.path)) {
        return name;
      }
    }
    return null;
  }

  addDependents(projectName, affected) {
    const project = this.projectGraph.get(projectName);
    if (!project) return;

    for (const dependent of project.dependents) {
      if (!affected.has(dependent)) {
        affected.add(dependent);
        // Recursively add dependents
        this.addDependents(dependent, affected);
      }
    }
  }

  shouldRunE2ETests(affectedProjects) {
    // Run E2E if core projects are affected
    const coreProjects = ['api', 'web-app', 'auth'];
    return affectedProjects.some(p => coreProjects.includes(p));
  }
}

module.exports = { AffectedDetector };

# .github/workflows/monorepo-test.yml
name: Monorepo Tests

on: [push, pull_request]

jobs:
  detect-affected:
    runs-on: ubuntu-latest
    outputs:
      affected: ${{ steps.affected.outputs.projects }}
      matrix: ${{ steps.affected.outputs.matrix }}

    steps:

      - uses: actions/checkout@v4
        with:
          fetch-depth: 0  # Full history for accurate diff

      - name: Detect affected projects
        id: affected
        run: |
          node scripts/detect-affected.js \
            --base=origin/${{ github.base_ref || 'main' }} \
            --output=json > affected.json

          echo "projects=$(cat affected.json | jq -c '.projects')" >> $GITHUB_OUTPUT
          echo "matrix=$(cat affected.json | jq -c '.matrix')" >> $GITHUB_OUTPUT

      - name: Upload affected list
        uses: actions/upload-artifact@v3
        with:
          name: affected-projects
          path: affected.json

  test:
    needs: detect-affected
    if: needs.detect-affected.outputs.affected != '[]'
    runs-on: ubuntu-latest

    strategy:
      matrix: ${{ fromJson(needs.detect-affected.outputs.matrix) }}

    steps:

      - uses: actions/checkout@v4

      - name: Restore test cache
        uses: actions/cache@v3
        with:
          path: |
            node_modules
            .test-cache
          key: test-${{ matrix.project }}-${{ hashFiles(format('packages/{0}/**', matrix.project)) }}

      - name: Run tests for ${{ matrix.project }}
        run: |
          npm run test --workspace=packages/${{ matrix.project }}

      - name: Upload results
        uses: actions/upload-artifact@v3
        if: always()
        with:
          name: test-results-${{ matrix.project }}
          path: packages/${{ matrix.project }}/test-results/

// test-prioritizer.ts
interface TestPriority {
  name: string;
  priority: number;
  estimatedDuration: number;
  criticalPath: boolean;
}

class TestPrioritizer {
  private testHistory: Map<string, TestHistory> = new Map();

  prioritize(tests: string[]): TestPriority[] {
    return tests
      .map(test => ({
        name: test,
        priority: this.calculatePriority(test),
        estimatedDuration: this.estimateDuration(test),
        criticalPath: this.isCriticalPath(test)
      }))
      .sort((a, b) => {
        // Critical path tests first
        if (a.criticalPath !== b.criticalPath) {
          return a.criticalPath ? -1 : 1;
        }

        // Then by priority
        if (a.priority !== b.priority) {
          return b.priority - a.priority;
        }

        // Finally by estimated duration (fast tests first)
        return a.estimatedDuration - b.estimatedDuration;
      });
  }

  calculatePriority(testName: string): number {
    const history = this.testHistory.get(testName);
    if (!history) return 50;

    // Factors affecting priority:
    // 1. Failure rate (higher = higher priority)
    const failureRate = history.failures / history.runs;

    // 2. Recency of failures
    const daysSinceLastFailure = this.daysSince(history.lastFailure);
    const recencyScore = Math.max(0, 100 - daysSinceLastFailure * 2);

    // 3. Test flakiness (lower priority for flaky tests)
    const flakinessScore = history.flakiness * -50;

    return Math.min(100,
      (failureRate * 100 * 0.4) +
      (recencyScore * 0.4) +
      flakinessScore +
      20 // Base priority
    );
  }

  isCriticalPath(testName: string): boolean {
    const criticalPatterns = [
      /auth/i,
      /payment/i,
      /security/i,
      /core/i
    ];

    return criticalPatterns.some(pattern => pattern.test(testName));
  }

  estimateDuration(testName: string): number {
    const history = this.testHistory.get(testName);
    if (!history) return 5000; // Default 5 seconds

    // Use P95 duration for estimation
    return history.durationP95;
  }

  private daysSince(date: Date): number {
    const now = new Date();
    return (now.getTime() - date.getTime()) / (1000 * 60 * 60 * 24);
  }
}

# .github/workflows/distributed-tests.yml
name: Distributed Tests

jobs:
  generate-matrix:
    runs-on: ubuntu-latest
    outputs:
      matrix: ${{ steps.matrix.outputs.value }}

    steps:

      - uses: actions/checkout@v4

      - name: Generate test matrix
        id: matrix
        run: |
          # Intelligently distribute tests across runners
          node scripts/generate-test-matrix.js \
            --runners=20 \
            --strategy=balanced \
            --output=json > matrix.json

          echo "value=$(cat matrix.json)" >> $GITHUB_OUTPUT

  test:
    needs: generate-matrix
    runs-on: ubuntu-latest

    strategy:
      fail-fast: false
      matrix: ${{ fromJson(needs.generate-matrix.outputs.matrix) }}

    steps:

      - uses: actions/checkout@v4

      - name: Run test shard ${{ matrix.shard }}
        run: |
          # Each runner executes its assigned tests
          npm run test:shard -- \
            --shard=${{ matrix.shard }} \
            --total=${{ matrix.total }} \
            --tests="${{ matrix.tests }}"

      - name: Upload shard results
        uses: actions/upload-artifact@v3
        if: always()
        with:
          name: results-shard-${{ matrix.shard }}
          path: test-results/

  aggregate:
    needs: test
    runs-on: ubuntu-latest
    if: always()

    steps:

      - name: Download all results
        uses: actions/download-artifact@v3
        with:
          path: all-results/

      - name: Merge and report
        run: |
          node scripts/merge-test-results.js \
            --input=all-results/ \
            --output=final-report.html

          # Generate summary
          node scripts/summarize-results.js \
            --input=all-results/ \
            >> $GITHUB_STEP_SUMMARY

// build-cache-manager.ts
import crypto from 'crypto';
import fs from 'fs';
import path from 'path';

interface CacheKey {
  project: string;
  hash: string;
  dependencies: string[];
}

class BuildCacheManager {
  private cacheDir: string;

  constructor(cacheDir: string) {
    this.cacheDir = cacheDir;
  }

  computeCacheKey(project: string): CacheKey {
    // Hash includes:
    // 1. Project source files
    const sourceHash = this.hashDirectory(`packages/${project}/src`);

    // 2. Dependencies
    const deps = this.getProjectDependencies(project);
    const depsHash = this.hashDependencies(deps);

    // 3. Configuration files
    const configHash = this.hashFiles([
      `packages/${project}/package.json`,
      `packages/${project}/tsconfig.json`,
      '.eslintrc.json',
      'jest.config.js'
    ]);

    const combinedHash = crypto
      .createHash('sha256')
      .update(sourceHash + depsHash + configHash)
      .digest('hex')
      .substring(0, 16);

    return {
      project,
      hash: combinedHash,
      dependencies: deps
    };
  }

  async getCached(key: CacheKey): Promise<Buffer | null> {
    const cachePath = path.join(
      this.cacheDir,
      key.project,
      `${key.hash}.tar.gz`
    );

    if (fs.existsSync(cachePath)) {
      return fs.readFileSync(cachePath);
    }

    return null;
  }

  async setCached(key: CacheKey, data: Buffer): Promise<void> {
    const cachePath = path.join(
      this.cacheDir,
      key.project,
      `${key.hash}.tar.gz`
    );

    fs.mkdirSync(path.dirname(cachePath), { recursive: true });
    fs.writeFileSync(cachePath, data);

    // Clean old cache entries
    await this.cleanOldCaches(key.project, 10); // Keep last 10
  }

  private hashDirectory(dir: string): string {
    const hash = crypto.createHash('sha256');

    const files = this.getAllFiles(dir);
    for (const file of files.sort()) {
      const content = fs.readFileSync(file);
      hash.update(content);
    }

    return hash.digest('hex');
  }

  private getAllFiles(dir: string): string[] {
    if (!fs.existsSync(dir)) return [];

    const files: string[] = [];
    const entries = fs.readdirSync(dir, { withFileTypes: true });

    for (const entry of entries) {
      const fullPath = path.join(dir, entry.name);
      if (entry.isDirectory()) {
        files.push(...this.getAllFiles(fullPath));
      } else {
        files.push(fullPath);
      }
    }

    return files;
  }
}

# test_impact_analyzer.py
from sklearn.ensemble import RandomForestClassifier
import numpy as np

class TestImpactAnalyzer:
    def __init__(self):
        self.model = RandomForestClassifier(n_estimators=100)
        self.trained = False

    def train(self, historical_data):
        """Train model on historical test failures"""
        features = []
        labels = []

        for record in historical_data:
            feature_vector = self.extract_features(record)
            features.append(feature_vector)
            labels.append(1 if record['failed'] else 0)

        X = np.array(features)
        y = np.array(labels)

        self.model.fit(X, y)
        self.trained = True

    def extract_features(self, record):
        """Extract features from a test record"""
        return [
            len(record['changed_files']),
            record['lines_changed'],
            1 if any('test' in f for f in record['changed_files']) else 0,
            1 if any('core' in f for f in record['changed_files']) else 0,
            record['time_since_last_change'],
            record['author_test_failure_rate'],
            record['time_of_day'],  # Flaky tests often fail at certain times
            record['concurrent_builds']  # Resource contention indicator
        ]

    def predict_failures(self, current_change):
        """Predict which tests are likely to fail"""
        if not self.trained:
            raise Exception("Model not trained")

        all_tests = self.get_all_tests()
        predictions = []

        for test in all_tests:
            features = self.extract_features({
                **current_change,
                'test_name': test
            })

            probability = self.model.predict_proba([features])[0][1]

            predictions.append({
                'test': test,
                'failure_probability': probability
            })

        # Return tests sorted by failure probability
        return sorted(
            predictions,
            key=lambda x: x['failure_probability'],
            reverse=True
        )

stages:

  - name: Fast Tests
    tests: [lint, unit]
    timeout: 5min
    on_failure: block_merge

  - name: Integration Tests
    tests: [integration]
    timeout: 15min
    on_failure: block_merge
    requires: Fast Tests

  - name: E2E Tests
    tests: [e2e]
    timeout: 30min
    on_failure: notify
    requires: Integration Tests
    run_if: affected_projects.includes('api', 'web-app')

interface TestHealthMetrics {
  totalTests: number;
  averageDuration: number;
  flakyTestCount: number;
  cacheHitRate: number;
  parallelizationEfficiency: number;
}

function calculateHealth(metrics: TestHealthMetrics): number {
  const weights = {
    flakiness: 0.3,      // Lower is better
    duration: 0.2,       // Lower is better
    cacheHit: 0.25,      // Higher is better
    parallelization: 0.25 // Higher is better
  };

  const flakinessScore = Math.max(0, 100 - metrics.flakyTestCount * 10);
  const durationScore = Math.max(0, 100 - (metrics.averageDuration / 60));
  const cacheScore = metrics.cacheHitRate * 100;
  const parallelScore = metrics.parallelizationEfficiency * 100;

  return (
    flakinessScore * weights.flakiness +
    durationScore * weights.duration +
    cacheScore * weights.cacheHit +
    parallelScore * weights.parallelization
  );
}