Configuring Caching Rules in Azure CDN

1. The Foundation: How Caching is Communicated

Before we configure rules in Azure, it's helpful to understand the underlying mechanism that governs web caching: HTTP headers. When your origin server (e.g., an Azure App Service or Blob Storage) sends a file, it can include headers that instruct the CDN and browsers on how to handle it.

The video below explains the two most important concepts: duration and validation.

Azure CDN caching rules give you the power to enforce a caching policy at the CDN level, allowing you to override or supplement the headers sent by your origin server.

2. Implementing Caching Rules

Azure CDN (specifically the Standard Microsoft SKU) uses a rules engine to define caching logic. Rules are processed in order, and once a request matches the conditions of a rule, that rule is applied, and no subsequent rules are evaluated for that request.

You can set two types of rules:

• Global Caching Rules: A default policy that applies to all traffic to your endpoint.
• Custom Caching Rules: More specific rules that apply only to requests matching certain criteria, like a specific file path or file extension.

Global and Custom Rule Behavior

The primary action for a caching rule is to set the Cache-Behavior. This tells the CDN how to interact with the origin's Cache-Control headers.

A typical strategy looks like this:

1. Global Rule: Use SetIfMissing with a moderate duration (e.g., 1 day). This acts as a fallback.
2. Custom Rule for Static Assets: Create a rule that matches on file extensions (.css, .js, .woff2). Set Cache-Behavior to Override with a long duration (e.g., 30 days), as these files are often versioned.
3. Custom Rule for Images: Create a rule for image extensions (.jpg, .png, .svg). Use Override with a long duration (e.g., 90 days).
4. Custom Rule for HTML: Create a rule for .html files. Use Override with a very short duration (e.g., 1 hour) to ensure content updates are reflected relatively quickly.

3. Controlling Cache with Query Strings

By default, Azure CDN treats URLs with different query strings as completely separate, cacheable items.

• https://my-cdn-endpoint.azureedge.net/images/logo.png?v=1
• https://my-cdn-endpoint.azureedge.net/images/logo.png?v=2

These are cached as two distinct objects. This is the desired behavior for cache-busting, where you change the query string to force clients to download a new version of a file.

However, sometimes query strings are used for other purposes, like analytics tracking (?utm_source=...), and do not affect the content of the file. In these cases, caching each unique URL is inefficient and reduces your cache hit ratio.

Azure CDN gives you three options to control this behavior.

The choice depends on your application's design:

• If you use query strings for versioning/cache-busting, stick with the default "Cache every unique URL".
• If your query strings are only for tracking and don't change the file content, switch to "Ignore query strings" to improve your cache hit ratio.

You can also configure this using the Azure CLI. This is often faster for automation and scripting.

# Example: Set the endpoint to ignore query strings
az cdn endpoint update \
    --resource-group YourResourceGroup \
    --profile-name YourCdnProfileName \
    --name YourCdnEndpointName \
    --query-string-caching-behavior IgnoreQueryString

Conclusion

In this lesson, we moved from basic CDN setup to fine-grained control over caching behavior. By mastering caching rules and query string settings, you can build a highly efficient delivery pipeline that reduces load on your origin servers and provides a faster experience for your users.

Here are the key takeaways:

• CDN caching behavior is fundamentally driven by HTTP headers like Cache-Control and E-Tag from the origin.
• Caching rules in Azure CDN allow you to Override, SetIfMissing, or BypassCache based on global or custom criteria (like file path or extension).
• A robust strategy involves setting different cache durations for different content types: long for static assets, short for dynamic content like HTML.
• Query string caching behavior is critical for efficiency. You must choose whether to cache every unique URL, ignore query strings, or bypass caching for them based on how your application uses query parameters.

This completes our module on performance optimization with caching and CDN. You now have the skills to implement both application-level caching with Redis and edge-level caching with Azure CDN.

In our next lesson, we will begin a new module, "Managing APIs with Azure API Management," starting with an overview of its core components and tiers.