Making subrequests from handler

[agentzh]

On Fri, Nov 27, 2009 at 10:01 AM, Peter A Leonov <gojpeg at gmail.com> wrote:
> I'm just wondering if it is possible to perform any subrequests this way.

I think it makes little sense to say "subrequests can only be issued
from output filters rather than content handlers". Why? Because output
filters are just plain C functions called by content handlers
themselves, usually in this form:

     ngx_int_t
     my_content_handler(ngx_http_request *r) {
         ...
         rc = ngx_http_send_header(r);
         ...
         return ngx_http_output_filter(r, cl);
     }

The output filter chain is a plain C function call stack. So if an
output filter can issue subrequests, then logically so does a content
handler :) There's no real difference in C's context here and no magic
here.

> The code of SSI module tells me than the filter can do so with ease. But
> what about handlers? All about the latest nginx 0.8.*.
>

The "echo" module and the "fancyindex" module both issue subrequests
from content handler. There's also a workable case in this thread
(although shaun eventually gave up the subrequest model due to an
issue of canceling pending subrequets, well, you don't need
cancellling subrequests in your scenario anyway):

    http://www.ruby-forum.com/topic/199392

> The thing I'm trying to implemet is a simple JavaScript method
> subrequest(uri, callback). Any number of subrequests may be waited at the
> same time, and when finishef be combined in random order.

Given the "postponed chain model"  currently implemented in Nginx, if
one does not buffer and rearrange the outputs of the subrequests using
a custom output filter himself, then the order of the subrequest
outputs will be exactly the *same* as the order they are originally
issued even though they're actually run in parallel.

I must say the basic idea of subrequests called by a scripting
language like JS is definitely great. Why not scripting all the Nginx
internal infrastructure directly for our apps? It's really a pity to
just use Nginx as a boring proxy to the bloated FastCGI apps or even
to an Apache monster ;)

We're also going to turn coco lua to a first-class citizen in Nginx
module development just like C. Transparent non-blocking I/O is also a
plus here when compared to plain C coding. It's interesting to do the
same with other interpreters if the C-level coroutine support is in
place.

> Could you please help me figure it out or just point to a source code?
>

Please see the source code of the ngx_http_subrequest function in the
core and the standard postpone filter for more details. The "echo"
module may help you experimenting various aspects of both parallel and
sequential subrequests from a content handler.

Good luck!
-agentzh