Disabling Output Buffer with Apache and PHP-FPM via mod_proxy

I successfully disabled output buffering by rewriting your Proxy section (based on this answer):

<FilesMatch \.php$>
    SetHandler "proxy:unix:/run/php-fpm/php-fpm.sock|fcgi://localhost"
</FilesMatch>

<Proxy fcgi://localhost>
    ProxySet enablereuse=on flushpackets=on
</Proxy>

Reposting the answer I just posted to a very similar question here: How to disable buffering with apache2 and mod_proxy_fcgi?

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.

In my environment (Apache 2.4, php-fpm) it worked when turning off compression and padding the output to output_buffering, see script:

header('Content-Encoding: none;');
$padSize = ini_get('output_buffering');

for($i=0;$i<10;$i++) {
  echo str_pad("$i<br>", $padSize);
  flush();
  sleep(1);
}

https://www.php.net/manual/en/function.fastcgi-finish-request.php was what saved my sanity. I tried all kinds of hacks and techniques to get Apache and php-fpm (7.4) to display progress in a browser for a long-running process, including Server-Sent Events, writing progress to a text file and polling it with xhr, flush()ing like crazy, etc. Nothing worked until I did something like this (in my MVC action-controller)

public function longRunningProcessAction()
{

    $path = \realpath('./data/progress.sqlite');
    $db = new \PDO("sqlite:$path");
    $db->setAttribute(\PDO::ATTR_ERRMODE, \PDO::ERRMODE_EXCEPTION);
    $stmt = $db->prepare("UPDATE progress SET status = :status");
    $stmt->execute([':status' => "starting"]);
    header("content-type: application/json");
    echo json_encode(['status'=>'started']);
    // this here is critical ...
    session_write_close();
    fastcgi_finish_request();
    // otherwise it will NOT work
    for ($i = 0; $i <= 150; $i++) {
        usleep(250*1000);
        $stmt->execute([':status' => "$i of 150"]);
        // this also works
        file_put_contents('./data/progress.txt',"$i of 150");
    }
    $stmt->execute([':status' => "done"]);        
}
// and...
public function progressAction()
{
    $path = \realpath('./data/progress.sqlite');
    $db = new \PDO("sqlite:$path");
    $query = 'SELECT status FROM progress';
    $stmt = $db->query($query);
    // and this is working as well..
    $text = file_get_contents('./data/progress.txt');
    return new JsonModel(['status'=>$stmt->fetchColumn(),'text'=>$text]);
}

and then some Javascript (jQuery)

    var check_progress = function() {
        $.get("/my/job/progress").then(r=>{
        $("#progress").text(r.status);
        if (r.status === "done") { return; }
        window.setTimeout(check_progress,300);
    });

    $.post("/long/running/process",data).then(check_progress);

Voilà!