Re: [server-timing] CDN use case

On Wed, Apr 29, 2015 at 7:17 PM, Nottingham, Mark <mnotting@akamai.com>
wrote:

> """
> For example, a CDN edge node may want to report which data center was
> being used, if the resource was available in cache, and how long it took to
> retrieve the response from cache or from the origin server. Further, the
> same process may be repeated by other proxies, thus allowing full
> end-to-end visibility into how the request was routed and where the time
> was spent.
> """
>
> I guess I'm kind of finding that hard to believe — the syntax of S-T is
> quite constrained, and it's not clear how this information would be
> visualised. Is this really a use case for S-T?
>

There are some limitations, but yes, I do believe you can deliver useful
data.. Further, the application can add its own smarts where needed:

- The UA can't rely on order of headers (or even the order of multiple
metrics within a single header) to sort them in any meaningful way.
- Since multiple servers may be emitting metrics, and each may have a
different clock, there is no meaningful startTime equivalent for ST metrics.

As a result, the UA receives the metrics and returns them in some arbitrary
order. From there, the application can apply own smarts to visualize it,
either as as a table, or as a sorted sequence if it can apply some smarts
to order the metrics. Concretely... let's say I get the following response.

< 200 OK
< Server-Timing: db=53, app=47.2
< Server-Timing: dc;atl, origin=141.2
< Server-Timing: squid;miss

Without any further knowledge the UA can simply output the table with
appropriate metric name, value, and description. But, as an owner of the
site I could sort this based on knowledge of my routing architecture:

1) request hit Atlanta data center (dc;atl), and total time to fetch from
origin was 141.2ms
2) My CDN routed to Squid cache at origin, which resulted in a cache miss
3) Squid forwarded request to app server, which took 53ms to fetch data
from the database, and 47.2ms to generate the response

ig