Is there a more efficient way to decoding JSON from QWebSocket?

martin_ky

Many (if not most) WebSocket APIs out there use JSON format, which is sent as text frames, which use UTF-8 encoding on the wire. In QWebSocket, text frames are handled by following APIs, which expose the message as an already decoded QString:

QWebSocket::sendTextMessage(const QString &message)
QWebSocket::textMessageReceived(const QString &message)

On the other hand, the native JSON support in Qt which is implemented by QJsonDocument provide APIs for serializing and deserializing: toJson() and fromJson(), which take an UTF-8 encoded QByteArray.

That means, if I want to consume a JSON WebSocket API in Qt, I have to:

receive a message as QString that is already decoded from UTF-8 internally by QWebSocket
convert it to UTF-8 QByteArray so I can pass it to QJsonDocument::fromJson()

Analogous but reverse process goes for sending JSON messages.

This seems very inefficient, as there are apparently two needless conversions: UTF-8 -> QString -> UTF-8, before the JSON message can actually by read by QJsonDocument.

Surely, there must be a more efficient way. Am I missing something? Is there a way to send/receive an UTF-8 encoded string directly using QWebSocket? Can these conversions be avoided, or is the Qt API lacking here? Thanks for any clues.

SGaist

Hi,

From a very quick look at the class, what about using sendBinaryMessage ?

JKSH

@martin_ky said in Is there a more efficient way to decoding JSON from QWebSocket?:

Surely, there must be a more efficient way.

You have a good point: https://bugreports.qt.io/browse/QTBUG-133100

Kent-Dorfman

Looks like there are QByteArray versions of send and receive for QWebSocket, at least in 6.9

martin_ky

Folks suggesting to use sendBinaryMessage() and binaryMessageReceived() - I don't think that's a choice that I can make as a consumer of a WebSocket API. As far as my understanding of WebSockets protocol go, they work in one of two modes - either transport text frames (encoded as UTF-8 bytes) or binary mode (raw bytes). If the server chooses to send a JSON as a text message, I cannot receive it via the binaryMessageReceived() signal - QWebSocket does not emit this signal when a text message is received. To my knowledge, as of Qt 6.9.1, there is no way to access the UTF-8 bytes of a received text message via the QWebSocket API, only the already and often unnecessarily decoded QString.

@JKSH: Thanks for the QTBUG - that is exactly my point.

JonB

@martin_ky You might like to post something to this effect into the bug report.

Kent-Dorfman

The only way for the API to know if it is receiving data that is to be binary or text is through the mime-type header. I'd suggest verifying if websocket protocol uses mime-types. If what @martin_ky is true, then he should see appropriate mime-type headers in the transactions.

The other option is to throw out QWebSocket and do TCP level session transport where everything is consider a stream of octets...or of course live with the QString/byte-array translations...I mean, is the amount of data being processed really a bottleneck in translating back and forth?

martin_ky

@JonB The bugreport QTBUG-133100 is accurate. I upvoted it. You can too, if you want.

@Kent-Dorfman There is no such thing as mime-type in the WebSocket protocol. Basically, there is just 1 bit in the wire format that differentiates between a text message and a binary messages. When QWebSocket receives a text messages it decodes the UTF-8 payload and emits the QString signal. I see no way around it in Qt 6.9.