CDN & Edge Computing

Cache-Control headers for optimal CDN caching in TypeScript

import { Response } from 'express';

// Static assets with content hash in filename: cache forever
// /static/app.a3f4b2c.js (webpack contenthash)
export function staticAssetHeaders(res: Response): void {
  res.setHeader('Cache-Control', 'public, max-age=31536000, immutable');
  // immutable tells the browser: the content will never change, do not revalidate
}

// API responses: cache at CDN for 60s, but allow stale content during refresh
export function apiCacheHeaders(res: Response, maxAgeSeconds = 60): void {
  res.setHeader(
    'Cache-Control',
    'public, s-maxage=' + maxAgeSeconds + ', stale-while-revalidate=300'
  );
  // s-maxage: CDN TTL (overrides max-age for CDNs)
  // stale-while-revalidate: serve stale for up to 300s while fetching fresh
}

// User-specific content: CDN must not cache; browser may cache in private store
export function privateHeaders(res: Response): void {
  res.setHeader('Cache-Control', 'private, no-store');
}

// Add Surrogate-Key for targeted purging (Fastly) or Cache-Tag (Cloudflare)
export function addCacheTags(res: Response, tags: string[]): void {
  res.setHeader('Surrogate-Key', tags.join(' '));
  // Purge: fastly.purgeKey('product-42') removes all responses with this tag
}

immutable combined with a content-hash filename means the browser never revalidates the file; the new hash on the next deploy causes the browser to fetch the new file. stale-while-revalidate enables background refresh so users never see latency on cache miss.

Cloudflare Cache-Tag purging via API in Ruby

require 'net/http'
require 'json'

class CloudflareCacheService
  CF_API = 'https://api.cloudflare.com/client/v4'

  def initialize
    @zone_id = ENV.fetch('CF_ZONE_ID')
    @api_token = ENV.fetch('CF_API_TOKEN')
  end

  # Purge by URL: specific resources
  def purge_urls(urls)
    post('/purge_cache', { files: urls })
  end

  # Purge by Cache-Tag: all responses with a specific tag
  # Requires Cloudflare Enterprise or Cache Rules with tagging
  def purge_by_tag(tags)
    post('/purge_cache', { tags: Array(tags) })
  end

  # Purge everything (use sparingly)
  def purge_all
    post('/purge_cache', { purge_everything: true })
  end

  private

  def post(path, body)
    uri = URI("#{CF_API}/zones/#{@zone_id}#{path}")
    http = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true
    req = Net::HTTP::Post.new(uri.path, {
      'Content-Type' => 'application/json',
      'Authorization' => "Bearer #{@api_token}"
    })
    req.body = body.to_json
    response = http.request(req)
    JSON.parse(response.body)
  end
end

# After updating product #42: purge all cached responses tagged with 'product-42'
cdn = CloudflareCacheService.new
cdn.purge_by_tag('product-42')

Cache-Tag-based purging allows surgically invalidating all CDN edges serving a specific product without purging unrelated content. This is superior to URL-based purging because a product page may be at many URLs (pagination, sorting, filtering combinations).