[Libevent-users] Re: responses delayed while using evhttp

[Julian Bui <julianbui@xxxxxxxxx>]

I went ahead and implemented the three callback methods related to threads and used them and everything is working as expected now.  The responses are being sent to the browser immediately when I call the method.  There is no longer a problem here.

I still would like to know a little about how the underlying thread architecture works, if anybody has time to explain.  And why there would be a 10s delay.

On Wed, Jun 20, 2012 at 9:09 AM, Julian Bui <julianbui@xxxxxxxxx> wrote:

Hi all,

I am having trouble sending HTTP replies/responses in a timely manner using evhttp.  

PROBLEM:

If I call evhttp_send_reply in the gencb (callback that handles all requests), then my requests are sent out in a timely manner and everything is fine.  However, I do some processing that will take a while so I don't want do the processing in this callback because it will hold up the dispatch loop and I think I won't be able to accept any connections while I'm doing this slow processing.  Instead, when I get a request, I store the evhttp_request* off, and then hand it to a thread, and then return from the gencb.  Later, when the processing is done, the original request's evhttp_request* is retrieved and used to send the response.  However, the browser doesn't receive the response until exactly 10.0s later after the evhttp_send_reply was invoked, everytime...unless I call evhttp_send_reply from the gencb in the dispatch loop thread, in which case the browser gets the response immediately.

PSEUDOCODE:

*Dispatch loop thread*

::onGeneralRequest { 1. Store the evhttp_request, 2. Hand off the request to a worker thread, 3. return }

*Worker thread*

      // do work that might take a few seconds

      cout << "about to send reply" << endl;

      evbuffer* resp_buf = evbuffer_new();

      evbuffer_add_printf(resp_buf, "This is the body");

      evhttp_send_reply(response.conn, 200, "bogus reason", resp_buf); // use the original evhttp_request* (response.conn), send a success message

      cout << "sent reply" << endl; // at this point the browser still doesn't get the response, 10s in the future it will get it

QUESTIONS:

• Do I need to use evthread.h?  I am using QThreads, part of the Qt library, so it looks like I need to implement and call three methods: 1) evthread_set_condition_callbacks, 2) evthread_set_id_callbacks, and 3) evthread_set_lock_callbacks.  Do I just implement and call these methods and then everything will be fine?  I don't need to tell the library to specifically use my custom threading library as long as I call these three methods?  There's no other setup?  After I set these methods all the threading/locking is transparent to me?
• Why does it seem like I can't send the reply from a different thread?  The variables I'm using are local to the particular request and are not shared, so I don't see why there should be any problem.  In other words, I don't expect that there would be any race conditions since the requests are isolated from each other (?)
• Any idea what this 10s delay is when I send the reply from a different thread?
  
  

Any feedback would be greatly appreciated.

Thanks,

-Julian