Planning for handling cache misses for CDN content

Available methods for handling cache misses

If a center or edge cache to which a client sends a request for CDN content does not have the requested content (that is, when a cache miss occurs), the cache fetches it from the CDN content servers directly. You must determine the method to use for changing (rewriting) the host name in client requests so that looping does not occur with cache misses for CDN content. When host name rewriting is set up, any upstream transparent caches that intercept the request will not redirect traffic back through the CDN. Therefore, looping is avoided. This looping would occur, for example, if a Web accelerator were deployed in front of an enterprise's origin server.

Determining the appropriate method

Available methods are

Method 1: DNS CNAME rewriting
Method 2: Configuration of NetCache accelerator rules

These methods are applicable for deployments using GRM DNS servers, GRM L7 servers, or both.

Use DNS CNAME rewriting if...	Configure NetCache accelerator rules on center and edge caches if...
You are redirecting URLs for a large number of Web sites. In this case, using DNS CNAME rewriting is more efficient than using NetCache accelerator rules.	You are redirecting URLs for a few Web sites only. In this case, it is simpler to use NetCache acceleration rules.
You are redirecting URLs for multiple customers.	You are redirecting requests for a single customer with one origin server.

Note
If the GRM DNS server takes over a content provider's existing DNS, do not use accelerator rules. If you do so, you will duplicate your effort.

About method 1: DNS CNAME rewriting

A CDN hoster can configure its DNS nameserver to return a canonical name (CNAME) with the associated IP address. Returning the CNAME along with the IP address avoids looping that might occur otherwise. The cache can then fetch content from the origin server by using the canonical name and store the content under the original name.

For example, assume that the CDN hoster asked a content provider to rewrite URLs to images.enterprise.cdnhoster.com. No origin server actually exists that can serve content for images.enterprise.cdnhoster.com if a cache must resolve a cache miss. Therefore, the CDN hoster adds to its DNS database a CNAME record, images.enterprise.com, that corresponds to the fully qualified domain name (FQDN) images.enterprise.cdnhoster.com. Subsequently, when a cache miss occurs, the cache uses the CNAME to rewrite the host header and fetches the content from the content provider's origin server (DNS CNAME rewriting).

About method 2: configuring accelerator rules on GRM center caches or edge caches

Using NetCache accelerator rules for host-name rewriting eliminates the need for the DNS nameserver that can rewrite the host name and configuring DNS CNAME rewriting. On each center or edge cache, you must configure standard NetCache accelerator rules with origin server addresses to force the cache to serve content from the origin servers only.

A disadvantage of using accelerator rules is that this method is not very scalable.

If you are using accelerator rules, Network Appliance recommends that you use the NetCache multiple-cache management feature. Multiple-cache management simplifies distribution of the accelerator rules among the caches.