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CDN Integration

PRISM works well with CDNs for edge caching of rendered pages. This page covers cache keying, Vary headers, and provider-specific configuration.

Cache Keying with X-Prism-Variant​

When viewport-aware rendering is enabled, PRISM returns an X-Prism-Variant header (mobile or desktop). Your CDN must use this header to maintain separate caches per device class. Otherwise, a mobile-rendered page may be served to desktop bots or vice versa.

Vary Header​

The simplest approach is to add X-Prism-Variant to the Vary response header. Most CDNs respect Vary for cache keying:

Vary: X-Prism-Variant

Your reverse proxy should add this header. See the Reverse Proxy page for examples.

PRISM Cache Headers​

PRISM includes these headers on rendered responses:

HeaderDescription
X-Prism-CacheCache status: HIT, MISS, STALE, or BYPASS
X-Prism-Render-TimeRender duration in ms (on MISS only)
X-Prism-VariantDevice class: mobile or desktop
Cache-ControlSynthesized from configured TTL if origin did not provide one

Cloudflare​

Cache Rules​

Create a Cache Rule to vary by the X-Prism-Variant header:

  1. Go to Caching > Cache Rules.
  2. Create a rule matching your rendered paths (e.g., hostname equals example.com).
  3. Under Cache key, add Header > X-Prism-Variant.

Page Rules (Legacy)​

If using legacy Page Rules, Cloudflare does not support custom Vary headers. Use a Cache Rule or a Worker instead.

Worker-Based Cache Key​

For full control, use a Cloudflare Worker to include the variant in the cache key:

addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request));
});

async function handleRequest(request) {
  const variant = request.headers.get('X-Prism-Variant') || 'desktop';
  const cacheKey = new Request(request.url + '?_variant=' + variant, request);
  return fetch(cacheKey);
}

Fastly​

VCL Configuration​

In vcl_recv, add the variant to the hash:

sub vcl_hash {
    set req.hash += req.http.X-Prism-Variant;
}

Or use the Vary header approach -- Fastly respects Vary by default.

Surrogate Keys​

For targeted purging, PRISM's response URLs can be used as surrogate keys. Configure Fastly to purge by URL pattern when content changes.

Akamai​

Property Manager​

  1. Add a Modify Outgoing Response Header behavior to include Vary: X-Prism-Variant.
  2. Or use a Cache ID Modification behavior to add the X-Prism-Variant header value to the cache key.

Cache Key​

In your property configuration:

Cache Key: URL + X-Prism-Variant header

Bot-Only Mode CDN Considerations​

In bot-only mode, only bot requests are rendered. This means:

  • Bot traffic hits PRISM, gets rendered HTML, and the CDN caches it.
  • Human traffic bypasses PRISM and gets the SPA shell directly from the origin.

If your CDN caches both bot and human responses at the same URL, you need to ensure they are cached separately. Options:

1. Separate Bot Traffic at the CDN Edge​

Route bot traffic to PRISM and human traffic directly to the origin at the CDN level. Most CDNs can match User-Agent patterns in routing rules.

2. Don't CDN-Cache PRISM Responses​

If bot traffic volume is low, skip CDN caching for rendered pages and let PRISM's built-in cache handle it:

Cache-Control: private, no-store

Adding Vary: User-Agent causes the CDN to create a separate cache entry for every unique User-Agent string, which effectively disables caching. Avoid this approach.

Cache Purging​

When content changes on your origin, purge both PRISM's internal cache and your CDN cache:

  1. PRISM cache: Send a POST to the admin API:

    curl -X POST http://localhost:9090/cache/purge -d '{"pattern": "/products/*"}'

  2. CDN cache: Use your CDN provider's purge API. If using viewport-aware rendering, remember to purge both mobile and desktop variants.