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Date: Wed, 4 May 2011 12:39:19 +0200
From: rm@tuxteam.de
To: modules-dev@httpd.apache.org
Subject: Re: using mod_proxy for subrequests
Message-ID: <20110504103919.GC20933@seid-online.de>
References: <20110504093446.GB20933@seid-online.de>
 <BANLkTikpdDHwFmdwi-F60Y4xoDBcxPa_AA@mail.gmail.com>
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On Wed, May 04, 2011 at 11:36:35AM +0200, Sorin Manolache wrote:
> 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]

Hmm, I don't seem to get what's you do different compared with my
approach:


> request_rec *subr = ap_sub_req_method_uri("GET", "/some_name", r, NULL);

Same as my (1)
Here, "/some_name" is still an arbitrary URI and _not_ the proxy URI I
want to query. BTW, this does clutter the URL namespace, a big no-no in
my usecase ...

> ap_add_output_filter(post_processing_filter_name, filter_context,
> subr, subr->connection);

Same as my (3)

> 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
> }

And you still need to _run_ the subrequest to get at the restponse
status etc. 

> 
> 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

But that means loosing all request context in the subrequest! One of
tthe main reasons to use mod_proxy instead of
some-arbitrary-webclient-lib is the fact that mod_proxy passes all
incomming header to the backend server. A must in my case.

> I'm
> using this trick but I do not know all its consequences.

Hmmm - bold. The costs of server downtime might easily exeed my
monthly income in this case :-)


cheers, RalfD


> 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
> >
> >