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I'm currently studying for a project involving a web-server and some raspberrys. The challenge would basically be to watch raspberry "status" on a web interface.

Raspberrys are connected to the Internet through a GSM connection (mostly 3G). I'm developping using Node.js on both the clients and the server, and I'd like to use websockets through socket.io in order to watch the raspberry connection status (actually, this is more like watching the raspberry capability to upload data through my application), dealing with "connected" and "disconnected" events.

Would an always-alive websocket connection be reliable for such a use-case? Are websocket designed-to (or reliable-for) staying opened? Since this is a hard-testable situation, does anyone know a data-consumption estimate for an always-alive websocket? If I'm going in a wrong way, does anyone ever worked on such a use-case via another reliable way?

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Cyril CHAPON
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  • what do you mean by "data-consumption estimate for an always-alive websocket"? – micnic Feb 18 '15 at 10:00
  • WebSockets are designed to stay open. I already did that over GSM and it worked quite well. TCP handles the reliability for you. Data consumption is depending on the data you are transmitting, just for the connection it is not very much. – deyhle Feb 18 '15 at 10:15
  • @micnic, I mean data in bits/bytes. In here data can become expensive for a wide fleet, so we try to scale GSM data subscriptions as low as possible. – Cyril CHAPON Feb 18 '15 at 10:40
  • @deyhle, thanks about that piece of information. Actualy, this would be "just to keep it opened", but I read somewhere that the "hearbeat" could be consuming. Just trying to know a consumption "range", like 1kB/day, 1mB/day. Would you say that's insignificant ? – Cyril CHAPON Feb 18 '15 at 10:43

1 Answers1

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Would an always-alive WebSocket connection be reliable for such a use-case ? Are WebSocket designed-to (or reliable-for) staying opened ?

Yes, WebSocket was designed to stay open and yes it's reliable for your use-case, a WebSocket connection is just a TCP connection which transmits data in frames.

Since this is a hard-testable situation, does anyone know an data-consumption estimate for an always-alive websocket ?

As I wrote, data in WebSocket connections is transmitted using frames, each frame has a header and the payload. The data sent from the client to the server is always masked and like this adds 4 bytes (the masking key) to each frame. The length of the header depends on the payload length:

  1. 2 bytes header for <=125 bytes payload
  2. 4 bytes header for <=65535 bytes payload
  3. 10 bytes header for <=2^64-1 bytes payload (rarely used)

Base Framing Protocol: https://www.rfc-editor.org/rfc/rfc6455#section-5.2

To keep the connection open, the server sends at a specific timeout (depends on the implementation, usually ~30 seconds) ping frames which are 2-127 bytes long, usually 2 bytes (only the header, without payload) and the client responds with pong frames which are also 2-127 bytes long.

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