Subject: [CDNi] Framework & Synchronous CDNI Metadata Acquisition approaches

From: Francois Le Faucheur <[email-masked]>
Date: Fri, 18 Nov 2011 15:42:26 +0800

Bruce, Larry, and all,

(as an individual)

Kent, Ben, David, Grant and I had a chat in Taipei about how to transport the subset of metadata needed at request routing time and concluded that it made sense to use the Metadata Interface to convey the metadata needed at request-routing time, as opposed to attempt to piggy-back those in the Request-Routing interface/redirection message. 

I just looked back at the framework and found that this is in line with the text of section 3.9 that provides a good discussion of the scenario of Synchronous CDNI Metadata Acquisition. Specifically the text in steps 3-5 describes:
	* that metadata can be acquired via a Metadata Interface request
	* does not mention the conceptual alternative to piggy-back inside the "Request-Routing Interface/Redirection" request the subset of metadata needed at request routing time.

But while this is clear in our heads, it may be worth recording in the framework the rationale for that design decision so we don't have to reopen it later. The key points that came up in our discussion are:
	* there can be a very high number of RRI redirection message for the same content, so it is much more efficient to pull the metadata once for all
	* the request routing decision can potentially be influenced by quite a few different metadata object, so you might end retrying to convey a large subset of the whole metadata
	* the whole set of metadata is likely needed for subsequent delivery, so may as well pull it upfront.
	* if metadata objects are added in the future and some of those may influence request routing, they would be immediately available to the dCDN request router if you use the metadata interface, while (depending on how they are encoded) you may have to rev up the Request-Routing Interface/Redirection spec.
The only counter-argument was an extra inter-CDN RTT at request routing time, but that does outweigh the benefits.
Would you be willing to document the rationale (possibly combining the above with yours)?

The text in step 1 does mention:
	* that seed metadata can be acquired via a Metadata Interface request
	* mentions a potential alternative ways to convey the metadata seed information via piggy-backing on the "Request-Routing Interface/Redirection" trigger.
Since "seed" metadata may be extremely small (eg identification of uCDN), it may be worth keeping in step 1 the statement about the alternative for now.

Cheers

Francois