QHttpServer and large file

mrdebug

My QHttpServer server has to transfer a large file.
With a little file I can use the write() function of QHttpServerResponder object like this

responder.write(QBABuffer, "application/octet-stream");

where QBABuffer is the intere file.
But with a large file, how can I send back to the client the file in little chunks?

mrdebug

This code seems to work but there is a problem

        if (File.open(QIODevice::ReadOnly)) {
            responder.writeBeginChunked("application/octet-stream");
            while (!File.atEnd()) {
                QByteArray QBABuffer= File.read(1024);
                if (QBABuffer.length()) {
                    if (File.atEnd()) responder.writeEndChunked(QBABuffer);
                    else responder.writeChunk(QBABuffer);
                }
            }
            File.close();
        }

The responder.writeChunk(QBABuffer); function is not blocking so the file will be load totally in ram before to send to the client.
Is there a way to flush the socket?

Kevin Hoang

If you want to keep your current logic, you’ll need to add a blocking call like responder.socket()->waitForBytesWritten(-1); right after writeChunk.
However, I think that's not a very good idea.

A better way would be to open the file and use the QTcpSocket::bytesWritten event to send data chunk by chunk to the client. It would look something like this:

QTcpSocket *socket = responder.socket();
QObject::connect(socket, &QTcpSocket::bytesWritten, socket, [socket, file, responder = std::move(responder)]() mutable {
    if (file->atEnd()) {
        responder.done();
        file->deleteLater();
        return;
    }

    QByteArray chunk = file->read(1024);
    socket->write(chunk);
});

// Send the first chunk right away
QByteArray chunk = file->read(1024);
socket->write(chunk);

This way, you avoid blocking the event loop and can handle multiple clients much more efficiently.

mrdebug

Hi, thank you for the suggestion but I haven't got socket() function in QHttpServerResponder object.
Which Qt version are you using?

Kevin Hoang

What I shared was just an idea based on building a custom HTTP server.

From what I see, Qt currently only offers a very basic HTTP server. If you want better performance, it’s probably best to look at Qt’s source code and create your own server based on it. I haven’t had a chance to test if writeChunk in Qt 6.8 really works as expected, but I have a feeling it might not.

mrdebug

Solved using QtWebApp
https://github.com/fffaraz/QtWebApp
instead of QtHTTPServer

Kevin Hoang

Congratulations! Glad to hear you got it resolved. I noticed that QtHttp uses socket->waitForBytesWritten(-1); when writing chunks — it’s a simple and straightforward approach. I’m still curious why QHttp doesn’t apply the same method.