Re: New Version Notification for draft-nottingham-cache-header-00.txt

[Alex Rousskov <[e-mail]>]

On 09/07/2018 12:44 AM, Mark Nottingham wrote:
> FYI; IMO it's past time to standardise x-cache and have a real spec for it. 
> 
> This is a straw-man, based on a bit of research on existing implementations.
> 
> Pretty version at:
>   https://mnot.github.io/I-D/cache-header/
> 
> Comments? I think the primary audience here is proxy cache and CDN
> vendors, and their users.


If we are to standardize X-Cache, then we should be mindful of the trap
created by early implementations that thought a single action value can
adequately relay what has happened at the proxy:

1. The obvious HIT and MISS actions came first.

2. Then we quickly discovered that a "we found it in the cache" HIT
action is often followed by a REFRESH action (and a resulting MISS from
the proxy-server traffic point of view). HIT_REFRESH... concatenations
were used to combat this.

3. Realizing that even fresh cache HITs may become misses due to various
protocol requirements and internal problems, some added X-Cache-Lookup
that was dedicated to the first (and often critical for triage) cache
lookup, depicting what happened to that HIT later in X-Cache.

4. Then flash crowds and collapsed forwarding solutions came along. Now,
a single client HIT or MISS request could cause even more transactions,
some of which could be cache hits, some could be revalidations or even
errors. Some added COLLAPSED_HIT_REFRESH... and/or other variants. This
concatenation approach still loses a lot of info, but it highlights the
fact that a lot of info could be lost by including the COLLAPSED tag.

If the goal here is to standardize the current status quo, then the
document should cover Cache-Lookup and COLLAPSED tags (and/or their
variations).

If the goal here is to standardize a scalable solution based on the
current experience, then we should combine Cache-Lookup and Cache while
enumerating individual cache actions as separate same-node entries
(instead of trying to merge them into a single FOO_BAR_BAZ super-action
summary for one node).


Cheers,

Alex.