On 2014-08-23 12:36, Graham Leggett wrote:
On 23 Aug 2014, at 3:40 PM, Mark Montague <m...@catseye.org> wrote:
[root@sky ~]# httpd -t
AH00526: Syntax error on line 148 of /etc/httpd/conf/dev.catseye.org.conf:
CacheEnable cannot occur within <If> section
[root@sky ~]#
The solution here is to lift the restriction above. Having a generic mechanism
to handle conditional behaviour, and then having a special case to handle the
same behaviour in a different way is wrong way to go.
I've looked into allowing CacheEnable directives within <If> sections.
This can be done by removing the NOT_IN_FILES flag from the call to
ap_check_cmd_context() in modules/cache/mod_cache.c:add_cache_enable()
The problem is that <If> sections are currently walked only during
mod_cache's normal handler phase, not during the quick handler phase.
It looks easy enough to add a call to ap_if_walk() to
cache_quick_handler(), but this would add significant extra processing
to the quick handler phase, as all <If> expressions for the enclosing
context would be evaluated, and I think that at the end we'd have to
discard the results that ap_if_walk() caches in the request record so
that they can be recomputed later during normal request processing after
all information about the request is available. Is this acceptable?
Also, the proof of concept patch that I sent yesterday, which adds an
expr= clause to the CacheEnable directive, records cache bypasses in the
request notes (for logging), in the X-Cache and X-Cache-Detail headers,
in the cache-status subprocess environment variable, and adds a new
subprocess environment variable named cache-bypass. If we enable using
CacheEnable in <If> sections, conditional cache bypasses will no longer
be called out explicitly to the server administrator; they will need to
infer a bypass from comparing the URL path to their configuration. I do
not see this as a large problem, but I thought I would mention it for
consideration.
Given these things, what thoughts does the developer community have?
Would a patch to allow CacheEnable within <If> sections have a better
chance of being accepted than one that adds a expr= clause to the
CacheEnable directive?
Or should mod_cache not allow cache bypassing at all? "Use NGINX (
http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_bypass
) if you want that" or "use Varnish (
https://www.varnish-cache.org/docs/4.0/users-guide/increasing-your-hitrate.html#cookies
) if you want that" are answers I'm fine with, if there's no interest in
this feature for httpd's mod_cache.
--
Mark Montague
m...@catseye.org
