Postponing responses to requests

I’m trying to figure out how to get some code working in an NGINX module
we
are writing. The basic idea is that requests come in to our module, and
we
make callouts from the module to both some external code and to an
upstream
HTTP interface. We then want to send responses to the requests.

The bit that we are a little confused about is how to manage our
responses
correctly. Our code looks a bit like this.

  • Get request in our request handler
    • Do some work that will trigger an async callback later (from a
      library)
    • Return NGX_DONE to say “leave us alone for now”
  • Get async callback from our handler
    • Issue upstream request (using upstream module)
  • Get upstream response
    • Do some more processing before sending response (potentially much
      later after more async work)

The problem occurs in that last step; we can do our async processing and
manage the upstream requests OK, but as soon as we get our response from
the upstream module, we find that finalize_request is called and a
response is sent. However, we want to hold off until later sending the
response.

We’d like to be able to just say “don’t send a response on this request
just now, even though the upstream request has returned”. I think
perhaps the underlying issue is that we are just issuing an upstream
request not a subrequest, but I’m now quite confused, and it would be
very helpful if somebody could give me some helpful hints, or else point
me at some documentation of how to do this, or just at some sample code
we could read through for ideas.

Thanks,

Pete

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,64052,64052#msg-64052

Hello!

On Mon, Mar 15, 2010 at 12:23:37PM -0400, [email protected] wrote:

correctly. Our code looks a bit like this.
much later after more async work)
request just now, even though the upstream request has
returned". I think perhaps the underlying issue is that we are
just issuing an upstream request not a subrequest, but I’m now
quite confused, and it would be very helpful if somebody could
give me some helpful hints, or else point me at some
documentation of how to do this, or just at some sample code we
could read through for ideas.

Basically you have two options:

  1. Use subrequest with SUBREQUEST_WAITED and SUBREQUEST_IN_MEMORY
    flags. It works for small responses and only when appropriate
    protocol module supports it (proxy and memcached do).

  2. Install output filter and do anything you want with response.
    This works always, but it’s up to you to correctly implement it.

Maxim D.

Some fiddling around later, it seems that using a subrequest is much
simpler than what we were trying, and as an added bonus is working
(though with limited testing so far).

Thanks for the pointers.

Pete

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,64052,64673#msg-64673

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