How to disable buffering with apache2 and mod_prox

Question

I'm using mod_proxy_fcgi with apache 2.4 on a debian Jessie with my C++ application which does ServerSentEvents with libfcgipp.

My problem is, that apache still buffers my response data. I confirmed that it isn't buffered by the libfcgipp library by using wireshark: After starting the fcgi application via spawn-fcgi, the data gets send to the apache web server as soon as possible. But in my browser (which I use for testing, later there will be a C++ client) it only shows up after I "killed"/closed the sending request in the server application.

So I assume I need to disable buffering for either apache or mod_proxy_fcgi (or both). But I cannot find the appropriate documentation on how to do this.

Answer 1

A few notes, since I just spent the past few hours experimenting to find the answer to this question:

1. It's not possible to entirely disable output buffering when using mod_proxy/mod_proxy_fcgi, however, you can still have responses streamed in chunks.
2. It seems, based on my experimentation, that chunks have to be at least 4096 bytes before the output will be flushed to the browser.
3. You can disable output buffering with the mod_fastcgi or mod_fcgi module, but those mods aren't as popular/widely used with Apache 2.4.
4. If you have mod_deflate enabled and don't set SetEnv no-gzip 1 for the virtualhost/directory/etc. that's streaming data, then gzip will not allow the buffer to flush until the request is complete.

I was testing things out to see how to best use Drupal 8's new BigPipe functionality for streaming requests to the client, and I posted some more notes in this GitHub issue.