Sending multiple buffers through websockify about websockify

[kanaka]

Websockify is made up of three parts:

• websocket.py: a general websocket server library/module to incorporate into other python programs to turn them into python servers
• websockify: a runnable program that is a WebSocket to TCP proxy/bridge that uses websocket.py to communicate with the browser and makes regular TCP socket connections on the back-end to the server being bridged.
• include/websock.js: a Javascript library that makes it easy to interface with websocket.py. It provides an interface that is different from normal WebSocket. The API is documented here: https://github.com/kanaka/websockify/wiki/websock.js

The WebSocket protocol is message based. When you send 1 message you will receive 1 message (i.e. onmessage will fire once). websock.js+websocket.py is designed to handle binary data that is either streaming or message based data. For that reason all data received is pushed onto a receive queue. If you want to process a message at a time then you will need to rQshiftBytes() or rQshiftStr() all the data off of the receive queue every time you receive a message (every time the message handler fires).

However, TCP is streaming and not message based. When you send TCP data, it is all guaranteed to arrive at the destination (and in order) but it may get fragmented into multiple chunks or multiple chunks may be combined into fewer chunks.

So when you use websockify as a proxy/bridge program to a TCP server, you will lose any notion of messages. In other words, you will need to implement your own protocol/message/framing handling in your client such that it understands the data coming from the target TCP server and is able to re-assemble the chunks into the proper pieces. For example, if the only thing that your server is sending is a series of PNG images, your client can look into the inital bytes of the PNG header and pull out the length. Then you can shift off that many bytes from the receive queue (if there are enough, otherwise you have to wait for more frames to arrive). Or if you have varying types of data you are sending, you may need to add your own header to each message that contains the length of the message so you know how long it is.

Alternately, since you have a python server, you could incorporate websocket.py into your python server. When you call send_frames() it takes a list of frames/messages to send. Each of those items from the list will arrive as a single message in the browser.

[ntenisOT]

Thanks a lot for the comprehensive respond. Is it possible to provide a sample code on how to get the length of the PNG image and wait for more frames to arrive? I am only sending PNG images.

PS. What if I incorporate websocket.py into my python server and then use websockify to proxy the data through a second server. Will I receive all frames in one message?

[kanaka]

Actually, I forgot, PNG doesn't have an overall length of the whole file. It only has a length for each chunk inside the file. That's a lot more work. I suggest adding a 4 byte length of the whole file before each image. More information about the PNG format:

• http://en.wikipedia.org/wiki/Portable_Network_Graphics
• http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html

If you use websockify, then part of your transport will be TCP. If you have TCP in the mix anywhere, and you want to handle messages instead of streams of bytes, then you need to handle this yourself. So unless you have WebSocket all the way to your server, you will need to handle messaging/framing yourself. You could use a WebSocket to WebSocket proxy if you are wanting to have a server in the middle, but this is not the purpose of websockify.

[ntenisOT]

Thanks kanaka I will follow your suggestion. Thanks a lot once again for your help!

[kanaka]

I'm closing this. Feel free to post here if you have any related questions (I'll see it in my feed).