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From Leif Hedstrom <zw...@apache.org>
Subject [DISCUSS] TS-1919: Eliminate CacheLookupHttpConfig
Date Sun, 03 Nov 2013 20:27:32 GMT
Hi all,

I’d like to get some closure on the TS-1919 Jira issue. There have been concerns raised
about this patch, and I will try to explain the pros and the cons of this patch. Note that
this is committed on the v5.0.x branch (it’s an incompatible change), and I really want
to decide the future of this commit such that we can either agree to keep it, or back it out
right now.


Explanation of the patch.

TS-1919 eliminates a helper class named CacheLookupHttpConfig. This is used for the Cache
Clustering feature to transport 4 configurations between the cluster members, on every response.
This is to assure that both members of a negotiated transaction use the same value for those
4 configurations. The configurations passed along are:

	proxy.config.http.cache.enable_default_vary_headers
	proxy.config.http.cache.vary_default_text
	proxy.config.http.cache.vary_default_images
	proxy.config.http.cache.vary_default_other

Instead of passing along the CacheLookupHttpConfig class while processing a request, we now
simply pass along the normal HTTP configuration (HttpConfigParams); this holds all configurations,
including the overridable configurations.

In addition to this, the helper class also encapsulated 5 additional configurations which
are necessary by the cache code, but the changes in TS-1919 doesn’t affect this; those configs
are still passed along, as will be explained further. However, *only* the four configs above
are transferred as part of the clustering messages. In fact, TS-1919 would make it possible
to make those 5 configurations overridable. In any case, these additional configs are

	proxy.config.http.global_user_agent_header
	proxy.config.http.cache.ignore_accept_mismatch
	proxy.config.http.cache.ignore_accept_language_mismatch
	proxy.config.http.cache.ignore_accept_encoding_mismatch
	proxy.config.http.cache.ignore_accept_charset_mismatch


Why I thought this was a useful patch?

There are a number of reasons why I thought this was necessary to change:

It much more cleanly separates the HTTP SM from the cache core. For example, this allows us
to eliminate the ifdef’s around HTTP_CACHE. The patch uses opaque void*’s when passing
module boundaries.
Most importantly, it allows us to pass along the full, overridable Http txn config. This means
that some (but definitely not all) cache specific records.config parameters can now be made
overridable per remap rule or per transaction via APIs. This is a fairly frequently requested
RFE.
It simplifies the cluster protocol, and reduces overhead around allocations, copying, marshaling
and unmarshalling the 4 configs above, as well as the other 5 configs that it has to copy
as part of the cache dependency.


What are the downsides and concerns? Well, the one that was brought up was that being able
to pass along these 4 configurations across the clustering communications was mission critical.
I can’t speak for that view, since I don’t have that use case. The other major downside
is that this breaks compatibility, such that you can not mix a v4.x box with a 5.x box in
a cache cluster (they will split).

I feel that cluster configurations (broadcasted) solves most of the concerns in that if you
change e.g. Vary: default_text, it’ll get distributed within a few seconds. Also, I feel
that neither of the 4 settings are something you would change particularly frequently, so
in a very worst case scenario, you’d take the cluster out of rotation while pushing that
config change to all members.

Please discuss and voice your thoughts here. I’d really like to get a closure on this, such
that I can start implementing per transaction overridable cache configurations for v5.0.x
if we decide to keep TS-1919 as it is. In particular, if you have a strong “-1” opinion
on this commit as it is, please speak up now.

Cheers,

— Leif


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