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I would want to send a message from the server actively, such as using UDP/TCPIP to a client using an arduino. It is known that this is possible if the user has port forward the specific port to the device on local network. However I wouldn't want to have the user to port forward manually, perhaps using another protocol, will this be possible?

2 Answers2

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1 Arduino Side

I think the closest you can get to this is opening a connection to the server from the arduino, then use available to wait for the server to stream some data to the arduino. Your code will be polling the open connection, but you are avoiding all the back and forth communications to open and close the connection, passing headers back and forth etc.

2 Server Side

This means the bulk of the work will be on the server side, where you will need to manage open connections so you can instantly write to them when a user triggers some event which requires a message to be pushed to the arduino. How to do this varies a bit depending on what type of server application you are running.

2.1 Node.js "walk-through" of main issues

In Node.js for example, you can res.write() on a connection, without closing it - this should give a similar effect as having an open serial connection to the arduino. That leaves you with the issue of managing the connection - should the server periodically check a database for messages for the arduino? That simply removes one link from the arduino -> server -> database polling link, so we should be able to do better.

We can attach a function triggered by the event of a message being added to the database. Node-orm2 is a database Object Relational Model driver for node.js, and it offers hooks such as afterSave and afterCreate which you can utilize for this type of thing. Depending on your application, you may be better off not using a database at all and simply using javascript objects.

The only remaining issue then, is: once the hook is activated, how do we get the correct connection into scope so we can write to it? Well you can save all the relevant data you have on the request to some global data structure, maybe a dictionary with an arduino ID as index, and in the triggered function you fetch all the data, i.e. the request context and you write to it!

See this blog post for a great example, including node.js code which manages open connections, closing them properly and clearing from memory on timeout etc.

3 Conclusion

I haven't tested this myself - but I plan to since I already have an existing application using arduino and node.js which is currently implemented using normal polling. Hopefully I will get around to it soon and return here with results.

Typically in long-polling (from what I've read) the connection is closed once data is sent back to the client (arduino), although I don't see why this would be necessary. I plan to try keeping the same connection open for multiple messages, only closing after a fixed time interval to re-establish the connection - and I hope to set this interval fairly high, 5-15 minutes maybe.

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Madsn
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    Thank you so much for replying in such detailed, I've learnt a lot from it. However I'm worrying will too many connections to the server an issue? –  Apr 04 '13 at 12:28
  • You're welcome, you might consider accepting it as an answer if you found it helpful. Regarding your concern for the number of connections, the software should be able to easily handle quite a large number of connections. But your server environment will likely be a limiting factor - see http://stackoverflow.com/questions/2350071/maximum-number-of-concurrent-connections-on-a-single-port-socket-of-server – Madsn Apr 05 '13 at 22:48
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We use Pubnub to send notifications to a client web browser so a user can know immediately when they have received a "message" and stuff like that. It works great.

This seems to have the same constraints that you are looking at: No static IP, no port forwarding. User can theoretically just plug the thing in...

It looks like Pubnub has an Arduino library: https://github.com/pubnub/arduino

Chris Dutrow
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