Edge Computing: Processing at the Network Periphery

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// A production-ready Cloudflare Worker handling personalized content
export default {
  async fetch(request, env, ctx) {
    const url = new URL(request.url);
    
    // Geographic personalization using Cloudflare's request properties
    const country = request.cf?.country || 'US';
    const city = request.cf?.city || 'Unknown';
    const colo = request.cf?.colo; // Edge location code (e.g., 'DFW', 'LHR')
    
    // Check for authentication token
    const authToken = request.headers.get('Authorization');
    if (url.pathname.startsWith('/api/') && !authToken) {
      return new Response('Unauthorized', { status: 401 });
    }
    
    // Rate limiting using Workers KV
    const clientIP = request.headers.get('CF-Connecting-IP');
    const rateLimitKey = `ratelimit:${clientIP}:${Math.floor(Date.now() / 60000)}`;
    const requestCount = parseInt(await env.RATE_LIMIT_KV.get(rateLimitKey)) || 0;
    
    if (requestCount > 100) {
      return new Response('Rate limit exceeded', { 
        status: 429,
        headers: { 'Retry-After': '60' }
      });
    }
    
    // Increment counter asynchronously (non-blocking)
    ctx.waitUntil(env.RATE_LIMIT_KV.put(rateLimitKey, String(requestCount + 1), {
      expirationTtl: 120
    }));
    
    // A/B test assignment based on consistent hashing
    const abGroup = hashToGroup(clientIP, ['control', 'variant-a', 'variant-b']);
    
    // Fetch from origin with modified headers
    const originRequest = new Request(request);
    originRequest.headers.set('X-AB-Group', abGroup);
    originRequest.headers.set('X-User-Country', country);
    originRequest.headers.set('X-Edge-Location', colo);
    
    const response = await fetch(originRequest);
    
    // Clone and modify response
    const modifiedResponse = new Response(response.body, response);
    modifiedResponse.headers.set('X-Served-From', colo);
    modifiedResponse.headers.set('X-AB-Group', abGroup);
    
    return modifiedResponse;
  }
};
 
function hashToGroup(input, groups) {
  let hash = 0;
  for (let i = 0; i < input.length; i++) {
    hash = ((hash << 5) - hash) + input.charCodeAt(i);
    hash = hash & hash;
  }
  return groups[Math.abs(hash) % groups.length];
}

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// Lambda@Edge: Viewer Request for A/B testing and authentication
exports.handler = async (event) => {
  const request = event.Records[0].cf.request;
  const headers = request.headers;
  
  // Authentication check
  const authHeader = headers['authorization'];
  if (!authHeader || !authHeader[0]) {
    // Check for protected paths
    if (request.uri.startsWith('/api/private/')) {
      return {
        status: '401',
        statusDescription: 'Unauthorized',
        headers: {
          'content-type': [{ value: 'application/json' }],
          'www-authenticate': [{ value: 'Bearer realm="api"' }]
        },
        body: JSON.stringify({ error: 'Authentication required' })
      };
    }
  }
  
  // A/B test assignment using consistent hashing
  const viewerIp = headers['x-forwarded-for'] 
    ? headers['x-forwarded-for'][0].value.split(',')[0] 
    : 'unknown';
  
  const experimentGroup = assignGroup(viewerIp, {
    'control': 50,
    'variant-new-ui': 30,
    'variant-fast-checkout': 20
  });
  
  // Add experiment header for origin processing
  request.headers['x-experiment-group'] = [{ value: experimentGroup }];
  
  // URL rewriting based on experiment
  if (experimentGroup === 'variant-new-ui' && request.uri === '/') {
    request.uri = '/experiments/new-ui/index.html';
  }
  
  // Geo-based content selection
  const country = headers['cloudfront-viewer-country'];
  if (country && country[0]) {
    request.headers['x-viewer-country'] = [{ value: country[0].value }];
    
    // EU visitors get GDPR-compliant version
    const euCountries = ['DE', 'FR', 'IT', 'ES', 'NL', 'BE', 'AT', 'PL'];
    if (euCountries.includes(country[0].value)) {
      request.headers['x-gdpr-mode'] = [{ value: 'true' }];
    }
  }
  
  return request;
};
 
// Consistent hashing for A/B assignment
function assignGroup(identifier, weights) {
  // Simple hash function
  let hash = 0;
  for (let i = 0; i < identifier.length; i++) {
    hash = ((hash << 5) - hash) + identifier.charCodeAt(i);
    hash = hash & hash;
  }
  
  // Normalize to 0-100
  const bucket = Math.abs(hash) % 100;
  
  // Assign to group based on weights
  let cumulative = 0;
  for (const [group, weight] of Object.entries(weights)) {
    cumulative += weight;
    if (bucket < cumulative) {
      return group;
    }
  }
  
  return Object.keys(weights)[0]; // Fallback
}

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// Cloudflare Worker implementing best practices
 
// Global scope: executes once per isolate, not per request
const CONFIG = {
  TIMEOUT_MS: 5000,
  MAX_SUBREQUESTS: 5,
  CACHE_TTL_SECONDS: 300,
};
 
// Pre-compile regex patterns at module scope
const PATH_PATTERNS = {
  api: /^\/api\/v[1-3]\//,
  static: /\.(js|css|png|jpg|webp|woff2)$/,
};
 
export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
    const url = new URL(request.url);
    
    // BEST PRACTICE: Early exit for simple cases
    if (PATH_PATTERNS.static.test(url.pathname)) {
      // Static assets: pass through to origin/cache
      return fetch(request);
    }
    
    // BEST PRACTICE: Wrap main logic in try-catch with fallback
    try {
      return await handleRequest(request, env, ctx);
    } catch (error) {
      // BEST PRACTICE: Fail open - fall back to origin
      console.error('Edge function error:', error);
      return fetch(request);
    }
  }
};
 
async function handleRequest(
  request: Request, 
  env: Env, 
  ctx: ExecutionContext
): Promise<Response> {
  // BEST PRACTICE: Validate inputs early
  const authResult = validateAuth(request, env);
  if (!authResult.valid) {
    return new Response(JSON.stringify({ error: authResult.error }), {
      status: 401,
      headers: { 'Content-Type': 'application/json' }
    });
  }
  
  // BEST PRACTICE: Check cache before expensive operations
  const cacheKey = request.url;
  const cache = caches.default;
  const cachedResponse = await cache.match(cacheKey);
  
  if (cachedResponse) {
    return cachedResponse;
  }
  
  // BEST PRACTICE: Timeout wrapper for origin fetches
  const response = await fetchWithTimeout(request, CONFIG.TIMEOUT_MS);
  
  // BEST PRACTICE: Clone before caching (response can only be consumed once)
  const responseToCache = response.clone();
  
  // BEST PRACTICE: Non-blocking cache write
  ctx.waitUntil(
    cache.put(cacheKey, responseToCache)
  );
  
  return response;
}
 
// BEST PRACTICE: Reusable timeout wrapper
async function fetchWithTimeout(
  request: Request, 
  timeoutMs: number
): Promise<Response> {
  const controller = new AbortController();
  const timeoutId = setTimeout(() => controller.abort(), timeoutMs);
  
  try {
    const response = await fetch(request, { signal: controller.signal });
    return response;
  } finally {
    clearTimeout(timeoutId);
  }
}
 
// BEST PRACTICE: Centralized auth validation
function validateAuth(
  request: Request, 
  env: Env
): { valid: boolean; error?: string } {
  const authHeader = request.headers.get('Authorization');
  
  if (!authHeader) {
    return { valid: false, error: 'Missing authorization header' };
  }
  
  // Token validation logic...
  return { valid: true };
}
 
interface Env {
  API_KEY: string;
  CACHE_KV: KVNamespace;
}