CDN - Optimizely CMS 12

Exam Area: Area 2 – Installation, Operation & Configuration (20%) Reference: https://docs.developers.optimizely.com/digital-experience-platform/docs/cdn

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1. What is a CDN?

A Content Delivery Network (CDN) is a geographically distributed network of edge servers that cache content close to end users, reducing latency and origin server load.

CMS 12 uses CDN for:

• Static assets (JS, CSS, images, fonts)
• Media files from Blob Storage
• Optionally: full-page HTML caching

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2. CDN in DXP

The Optimizely DXP includes a CDN (based on Akamai) as part of the platform:

User (any country)
    ↓
CDN Edge Node (nearest location)
    ↓ (cache hit: serve from edge)
    ↓ (cache miss: forward to origin)
Origin: Azure App Service (your CMS)

Cache hit ratio for a well-configured site: 80-95%

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3. What the CDN Caches

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4. Cache Headers

// Set cache headers in ASP.NET Core
// Option 1: Output caching attribute
[ResponseCache(Duration = 3600, VaryByHeader = "Accept-Language")]
public IActionResult Index() { ... }

// Option 2: Programmatic
Response.Headers["Cache-Control"] = "public, max-age=3600, s-maxage=86400";
Response.Headers["Vary"] = "Accept-Encoding, Accept-Language";

// Option 3: Output Cache Middleware (ASP.NET Core 7+)
builder.Services.AddOutputCache(options =>
{
    options.AddBasePolicy(builder =>
        builder.Expire(TimeSpan.FromMinutes(60)));
});
app.UseOutputCache();

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5. Cache Invalidation

When content is published, the CDN cache for affected URLs should be purged:

// CMS publishes content → trigger CDN purge

// Option 1: Custom event handler
[ServiceConfiguration]
public class CdnPurgeHandler : IInitializableModule
{
    public void Initialize(InitializationEngine context)
    {
        var events = context.Locate.Advanced.GetInstance<IContentEvents>();
        events.PublishedContent += OnPublished;
    }

    private void OnPublished(object sender, ContentEventArgs e)
    {
        var url = _urlResolver.GetUrl(e.ContentLink);
        _cdnClient.Purge(url);
    }

    public void Uninitialize(InitializationEngine context) { }
}

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6. CDN Purge via DXP Portal

DXP Portal → CDN → Cache Management
  → Purge by URL:    /articles/my-page
  → Purge by tag:    content-id-42
  → Purge all:       Full cache clear (use sparingly — causes traffic spike)

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7. CDN Configuration Best Practices

✅ Use content-hash-named static files (Webpack/Vite)
   → e.g. main.a1b2c3d4.js  (1-year cache TTL)
   
✅ Separate CDN URL for Blob Storage media
   → cdn.mysite.com/media/... → origin: mystorage.blob.core.windows.net/...
   
✅ Exclude /episerver/* from CDN caching
   → CMS Edit UI must not be cached

✅ Set Vary: Accept-Encoding on all cacheable responses
   
✅ Use Cache-Control: private for personalised/user-specific content

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8. Vary Header

The Vary header tells the CDN to cache separate versions per header value:

Vary: Accept-Language → separate cache per language
Vary: Accept-Encoding → separate cache per compression (gzip/br)
Vary: Cookie         → separate cache per session (avoid for CDN)

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9. Visitor Groups and Personalisation

Important: If you use Visitor Groups (personalisation), ensure personalised content is not cached by the CDN — it must be served per-user:

// For personalised responses
Response.Headers["Cache-Control"] = "private, no-store";

Or use Edge Side Includes (ESI) to cache the non-personalised parts and ESI-expand personalised regions dynamically.

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Review Questions

1. What is the primary benefit of using a CDN? (Reduced latency and lower origin server load — content served from edge nodes near the user)
2. Should the CMS Edit UI (/episerver) be cached by the CDN? (No — it must always be served fresh from origin)
3. What header controls how long a CDN caches a response? (Cache-Control with max-age or s-maxage)
4. Why must personalised content not be CDN-cached? (Each user gets different content — caching it would serve wrong content to other users)
5. What does the Vary header tell the CDN? (To maintain separate cache entries for different values of the specified request header)