Cache Warming: Pre-Populating Edge Caches for Launch Day

Cache warming is the practice of proactively populating CDN edge caches with content before real users arrive. When a CDN cache is cold — after a deployment, a cache purge, or a new PoP activation — the first visitors to each edge location experience the full latency of an origin fetch instead of an instant cached response. Cache warming eliminates this cold start penalty by sending synthetic requests that force edge servers to fetch and store content in advance. For product launches, marketing campaigns, and post-deployment recoveries, warming is the difference between a flawless user experience and a slow, origin-hammering first few minutes. For a comprehensive overview, see our guide on CDN Caching Strategies: Pull, Push, and Hybrid — FlyFish NWA.

Why Does the Cold Cache Problem Matter?

A cold cache is a CDN edge server that has no cached content for a given resource. Every request to a cold cache triggers a cache miss, forcing the edge to fetch from the next tier in the cache hierarchy or directly from the origin server. This adds 200-800 milliseconds of latency compared to a warm cache hit that responds in 5-30 milliseconds.

The cold cache problem is most severe in these scenarios:

After cache invalidation. When you perform a cache purge — whether by URL, tag, or wildcard — the purged content must be refetched from origin on the next request. For sites with hundreds of pages and 200+ CDN PoPs, the first wave of traffic generates thousands of simultaneous origin requests.

New deployments. Deploying a new version of your site often includes purging cached assets. The combination of new filenames (from content hashing) and purged HTML pages means the entire cache is effectively cold.

Product and campaign launches. A coordinated launch — a new product page, a marketing email blast, a social media announcement — drives a spike of traffic to URLs that have never been cached. Without warming, every visitor at every PoP experiences origin latency on their first request.

New CDN PoP activation. When a CDN provider brings a new edge location online, its cache is entirely empty. Users routed to the new PoP experience cold cache performance until organic traffic populates it.

The performance difference is substantial. A warm cache hit typically takes 5-30 milliseconds. A cold cache miss with origin fetch takes 200-800 milliseconds. For a page with 20 resources (HTML, CSS, JS, images), a cold cache adds 1-4 seconds of cumulative load time, directly impacting Core Web Vitals and user experience.

How Does Manual Cache Warming Work?

The simplest form of cache warming uses HTTP requests to trigger cache population. You request every URL you want cached, and the CDN fetches it from origin, stores it at the edge, and marks it as fresh according to the TTL in the response headers.

A basic warming script using curl:

# warm-cache.sh - Simple cache warming script
URLS_FILE="critical-urls.txt"
CDN_DOMAIN="cdn.example.com"

while IFS= read -r path; do
  curl -s -o /dev/null -w "%{http_code} %{time_total}s %{url}\n" \
    "https://${CDN_DOMAIN}${path}"
done < "$URLS_FILE"

This approach has limitations: it only warms the cache at the PoP closest to the machine running the script. To warm multiple PoPs, you need to either run the script from multiple geographic locations or use DNS resolution techniques to target specific edge servers.

For warming specific PoPs, use curl's --resolve flag to direct requests to known edge server IPs:

# Warm specific PoP by resolving to its IP
curl -s --resolve "cdn.example.com:443:198.51.100.1" \
  "https://cdn.example.com/page.html"

This sends the request directly to the edge server at 198.51.100.1 while maintaining the correct Host header and TLS certificate validation.

What Are API-Based Cache Warming Methods?

Some CDN providers offer dedicated APIs or features for cache warming that are more efficient than synthetic HTTP requests:

Cloudflare Prefetch. Cloudflare's Enterprise plan includes a prefetch feature that warms URLs across the Cloudflare network. You submit a list of URLs through the API, and Cloudflare distributes the warming across its PoPs. This is more efficient than external warming because the requests originate from within the CDN network.

Fastly Soft Purge + SWR. Fastly's approach combines soft purge with stale-while-revalidate. Instead of hard-purging cached content, you soft-purge it (mark as stale), which triggers background revalidation on the next request. Users continue receiving the stale version while the cache updates. This effectively warms the cache without a cold period.

CloudFront Origin Shield. While not a warming API per se, CloudFront's Origin Shield ensures that cache misses across all Regional Edge Caches consolidate through a single location. This means you only need to warm the Origin Shield region, and all edge locations benefit through the cache hierarchy.

Third-party warming services. Services like CDN warming platforms use distributed infrastructure across multiple cloud regions to send warming requests from geographic locations corresponding to major CDN PoPs. They automate the process of targeting specific edge locations.

How Should You Structure a Cache Warming Strategy?

Effective cache warming requires prioritization. You cannot warm every URL at every PoP — a large site with 100,000 pages and 200 PoPs would require 20 million warming requests. Instead, focus warming efforts where they have the most impact.

Identify critical URLs. Your warming URL list should include:

• Homepage and top landing pages (from analytics data).
• Product category and popular product pages.
• Critical CSS, JavaScript, and font files.
• Shared images (logos, hero images, icons).
• API endpoints that serve initial page data.

Most sites can identify 100-500 critical URLs that account for 80% or more of traffic.

Prioritize key geographic regions. Analyze your traffic by geography and identify the top 10-20 CDN PoPs that serve your user base. For a US-focused site, warming PoPs in New York, Los Angeles, Chicago, Dallas, and Atlanta covers the majority of traffic. European visitors route to London, Frankfurt, Amsterdam, and Paris. Warming low-traffic PoPs in regions with minimal users wastes resources.

Time warming correctly. Cache warming must complete before traffic arrives. For a marketing launch at 9:00 AM, start warming at 8:30 AM. For post-deployment warming, integrate it into the deployment pipeline immediately after cache purge. Account for warming duration — 500 URLs across 20 PoPs at a rate of 50 requests per second takes approximately 3-4 minutes.

How Do You Integrate Cache Warming into CI/CD Pipelines?

Automating cache warming as part of your deployment pipeline ensures that every deployment includes cache recovery. The warming step should execute after the cache purge and before the deployment is marked as complete.

A typical CI/CD pipeline with cache warming:

# Pipeline stages
1. Build application
2. Run tests
3. Deploy to origin servers
4. Purge CDN cache (targeted URLs or tags)
5. Warm CDN cache (critical URLs)
6. Verify warming (check X-Cache headers)
7. Mark deployment complete

The warming step reads from a maintained list of critical URLs, usually stored in the repository alongside the application code. When new pages are added, the warming list is updated as part of the development workflow.

A GitHub Actions example for the warming step:

# .github/workflows/deploy.yml (warming step)
- name: Warm CDN cache
  run: |
    while IFS= read -r url; do
      status=$(curl -s -o /dev/null -w "%{http_code}" "$url")
      echo "Warmed: $url (HTTP $status)"
    done < ./config/critical-urls.txt

For verification, check that warmed URLs return X-Cache: HIT headers on subsequent requests. If a URL returns MISS after warming, investigate whether the response includes Cache-Control: no-store or other headers that prevent caching. Proper Cache-Control configuration is a prerequisite for successful warming.

What Is the Difference Between Cache Warming and Push CDN?

Cache warming and push CDN both solve the cold cache problem but use fundamentally different mechanisms:

Pull CDN with cache warming is the standard approach for most web applications because it supports dynamic content, requires no changes to the content delivery workflow, and integrates naturally with existing deployment pipelines. Push CDN is best suited for large static asset libraries (video, software downloads) where content is stable and origin bandwidth is expensive.

How Do You Monitor Cache Warming Completion?

Verification ensures that warming actually populated the caches and that the content will be served to real users from cache. Without verification, a misconfigured cache key or a no-store header could silently cause warming to fail.

Response header verification. After warming, send follow-up requests to verify cache hits:

# Verify cache hit on warmed URL
curl -s -I "https://cdn.example.com/page.html" | grep -i "x-cache"
# Expected: X-Cache: HIT
# Or: cf-cache-status: HIT (Cloudflare)
# Or: X-Cache: Hit from cloudfront (CloudFront)

Age header monitoring. The Age header indicates how many seconds a response has been cached. Immediately after warming, the Age value should be small (seconds). If Age is zero on a second request, the content is not being cached.

CDN analytics dashboards. Most CDN providers offer real-time cache hit ratio metrics. Monitor the hit ratio during and after warming. A successful warming campaign shows the hit ratio climbing from near zero to 80-95% over the warming period.

Synthetic monitoring. Set up synthetic checks from multiple geographic locations using services like Pingdom, Datadog Synthetics, or Catchpoint. These services send requests from various regions and report response times and cache header values. Post-warming, all synthetic checks should show cache hits with response times under 50 milliseconds.

What Are Best Practices for Production Cache Warming?

Based on production experience across high-traffic CDN deployments, these practices maximize warming effectiveness while minimizing risk:

• Rate-limit warming requests. Sending thousands of requests per second overwhelms the origin and may trigger CDN rate limiting or WAF rules. Limit to 20-100 requests per second, adjusting based on origin capacity.
• Use conditional requests. Include If-None-Match or If-Modified-Since headers when re-warming content that may not have changed. The origin responds with 304 Not Modified, saving bandwidth.
• Warm during off-peak hours. When possible, schedule major cache invalidations and re-warming during low-traffic periods. The origin handles warming requests with less contention.
• Separate warming from purging. Do not purge and warm in the same request cycle. Purge first, wait for propagation (15-60 seconds depending on provider), then warm. This ensures the warmed content is not immediately evicted by a delayed purge.
• Maintain the URL list. Treat the critical URL list as infrastructure code. Review it quarterly, update it when site architecture changes, and validate it against analytics to ensure it covers your actual top pages.
• Consider stale-while-revalidate as an alternative. For routine TTL expirations (not purges or deployments), SWR eliminates the cold cache penalty without proactive warming. Use warming for events where SWR cannot help — fresh deployments, mass purges, and launch-day traffic.

Cache warming bridges the gap between cache invalidation and real user traffic. By proactively populating edge caches with critical content, you ensure that the first real user at every PoP receives the same fast, cached response as the millionth user. Combined with proper TTL configuration and stampede prevention, cache warming completes the toolkit for maintaining consistently high cache performance across deployments, launches, and traffic spikes.