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From Sorin Manolache <sor...@gmail.com>
Subject Re: using mod_proxy for subrequests
Date Wed, 04 May 2011 09:36:35 GMT
On Wed, May 4, 2011 at 11:34,  <rm@tuxteam.de> wrote:
> Hello list,
>
> as the subject line says, I'm trying to run a subrequest through
> mod_proxy and need to post-process the subrequests response data.
> Looking at older posts on this list it seems as if the only way to
> accomplish this is:
>
> (1)  create a subrequest with ap_sub_req_lookup_uri(...)
>
> (2) modify parts of the created subrequest (filename, handler, proxyreq
> etc.)
>
> (3) Install a filter that captures the response data
>
> (4) run that subrequest

Play it in conjunction to RewriteRules:

RewriteCond     %{IS_SUBREQ}            true
RewriteRule     ^/some_name$
http://backend.host.net/path?query_string [P]

request_rec *subr = ap_sub_req_method_uri("GET", "/some_name", r, NULL);
ap_add_output_filter(post_processing_filter_name, filter_context,
subr, subr->connection);
int status = ap_run_subreq(subr);
int http_status = subr->status;
// optional: subr->main = r;
if (ap_is_HTTP_ERROR(status) || ap_is_HTTP_ERROR(http_status))
   // some error handling
}

There are some subtleties here:

1. The rewrite rules are ran in the translate_name hook. If you want
to use %{ENV:request_note_name} in your rewrite rule, you have to copy
them somehow (for example in another translate_name callback that is
run before the mod_rewrite callbacks) from the main request notes to
the subrequest notes.

2. Subrequests are not kept alive. In order to keep them alive, you
could try to hook APR_OPTIONAL_HOOK(proxy, fixups, &proxy_fixups,
NULL, NULL, APR_HOOK_MIDDLE). In the proxy_fixups callback, you can
set subr->main = NULL; Then, after ap_run_subreq, you can re-set
subr->main = r (the "optional" line in the code example above). I'm
using this trick but I do not know all its consequences.

Sorin


>
> Now, (1) seems unelegant since it does need a valid URI which has
> nothing to do with the final proxy request. Hence the value of the
> subrequest's status has no meaning -- but isn't this exactly the purpose
> of subrequests? To quote Nick Kew '....to run a fast partial request, to
> gather information: what would happen if we ran thos request?'
> Is there really no way to create a subrequest directly aiming at
> mod_proxy.
> It would be utterly nice to be able to access a (proxied) subrequests
> metadata (content-type, etag etc.) before running the filter.
>
> Any ideas? Mabe a nice API extension for Apache or mod_proxy?
>
> TIA Ralf Mattes
>
>

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