Architecting a Global Edge Cache with Cloudflare Workers and the Cache API

The Problem: Origin Strain and Latency in Dynamic Content

Standard CDN caching often relies on static rules that struggle with dynamic content—data that changes frequently but is identical for all users. When the origin server handles every request for these assets, latency increases and server costs scale linearly with traffic. The goal is to move the caching logic to the edge, allowing the application to programmatically decide what to cache, for how long, and how to handle stale data without hitting the origin.

The Minimal Edge Cache Design

The most efficient implementation uses a Cloudflare Worker as an interceptor between the client and the origin. Instead of relying on default browser or CDN caching, the Worker utilizes the caches.default API to manage a programmatic cache store.

The logic flow follows a strict sequence: Intercept → Lookup → Fetch/Store → Respond. If a valid response exists in the cache, it is returned immediately. If not, the Worker fetches the resource from the origin, stores the result for future requests, and then serves the client.

// Run this in a Cloudflare Worker environment (v8+ runtime)
export default {
  async fetch(request, env, ctx) {
    const cache = caches.default;
    // Create a cache key based on the request URL
    const cacheKey = new Request(request.url, request);

    let response = await cache.match(cacheKey);

    if (!response) {
      console.log('Cache Miss: Fetching from origin');
      response = await fetch(request);

      // Only cache successful GET requests
      if (request.method === 'GET' && response.ok) {
        // We use ctx.waitUntil to avoid delaying the response to the user
        ctx.waitUntil(cache.put(cacheKey, response.clone()));
      }
    } else {
      console.log('Cache Hit: Serving from edge');
    }

    return response;
  }
};

Trust and Data Boundaries

When implementing edge caching, the Worker acts as the primary trust boundary. Because the cache is shared across users at a specific Point of Presence (PoP), you must strictly define what constitutes a cacheable request.

• Identity Isolation: Never cache responses that contain Set-Cookie headers or Authorization tokens. If the response is personalized, the cache key must include a unique identifier (like a user ID) or the request must bypass the cache entirely.
• Header Validation: The Worker should validate Cache-Control headers from the origin. If the origin specifies private or no-store, the Worker must respect these to prevent data leakage across sessions.
• Request Sanitization: Strip unnecessary query parameters from the cache key to increase the hit rate (e.g., removing tracking IDs like utm_source).

Operational Checks and Verification

To verify that the cache is functioning and not serving stale or incorrect data, use curl to inspect the response headers. You can add a custom header in the Worker (e.g., X-Edge-Cache: HIT) to make debugging easier.

Verification Command:
Run this from your local terminal (replace example.com with your worker route):

curl -I https://example.com/api/data

Expected Results:

1. First Request: Should show a miss (or your custom X-Edge-Cache: MISS header) and a response time reflecting the origin latency.
2. Second Request: Should show a hit (X-Edge-Cache: HIT) with significantly lower latency (typically <50ms).

Failure Modes and Constraints

Edge caching is not a replacement for a database. You must design for the following failure modes:

Failure Mode	Impact	Mitigation Strategy
PoP Isolation	A hit in New York does not mean a hit in London.	Accept regional variance in latency.
Cache Eviction	Cloudflare may evict items based on popularity/space.	Never use the Cache API for permanent storage.
Origin Timeout	Origin is down during a cache miss.	Implement stale-while-revalidate logic to serve old data while fetching new.

When to Change the Architecture

The Cache API is suitable for eventual consistency. If your application requirements evolve, you may need to move away from this design:

• Strong Consistency: If you need a global "purge" that happens instantly across all PoPs or a single source of truth for a counter, migrate to Durable Objects.
• Large Datasets: If you are caching gigabytes of assets that must persist regardless of access frequency, move to Cloudflare R2.
• Complex Invalidation: If you need to invalidate cache based on complex event triggers (e.g., a database update), implement a custom purging mechanism via the Cloudflare API.