Friday, April 19, 2013

Running a script at startup with Raspberry Pi and Raspbian

First of all, I'm sorry for the lack of updates. I've been very busy with teaching and grading. Plus, my research is not leading me to discover new libraries or programming models, so I have nothing to write here.
Lately, I've been working with deploying my Raspberry Pi integrated with some sensors. I wanted to a robust deployment, but how to do that? Should I bring my monitor wherever I want to deploy the Pi to set it up?
Luckily there is a very easy way to do that. In this way, whatever happens (like power supply unplugged), whenever the Pi is turned on again, it will start again what it was doing, effectively removing the need of connecting it to a screen and a mouse/keyboard.
The procedure is very simple: the idea is to add a script to the /etc/init.d/ path as many of the scripts running at booting runs from there. Here's an example:

#! /bin/sh
# /etc/init.d/myscript

# do something like running your node.js server or client!
cd mystuff/mynodeserver/
node my_server.js

exit 0

Once you save this file in the location specified before, you should make it executable. I usually run  chmod 777 but some people prefer to give just the root the power to execute that, so they call chmod with 755 instead.
Once this is done, you just have to update the symbolic link to make the script execute at startup and that's it! To update them just run update-rc.d myscript default.
To remove it, run update-rc.d -f myscript remove.
And it is as simple as that. I hope I helped somebody with this little trick!

Thursday, April 4, 2013

ZeroMQ on Node.JS and Socket inspection

First of all, I'm sorry for the lack of updates. Lately I've been writing papers and not really coding. Moreover in the last few days I was on vacation, so no computer either.
Anyways. In my project I'm using ZeroMQ which is a very good socket library. I use that to make my workers communicate with each other.

Lately my main concern was message loss. Since I increase and decrease the number of workers, it may happen that some worker gets shut down when it is receiving, processing or sending a message.
The very first approach I had to solve the issue was to save the timestamp when a message was received and then wait some time. If a message was not received within that time (last_received_message - time_now > some_variable) then no messages will ever arrive anymore and I would shut down the worker. Moreover a flag would help me if a message is being processed (that is, when receiving a message a flag is set to true, when the message leaves the worker, the flag is set to false).

The problem is that I cannot possibly access the socket's queue to check what is inside and if I have to wait some more time before shutting the worker down. Eventually I found out about the getsockopt() function and its return values.
Before showing the code, I have to tell that this is not a final solution, nor the very right way to do it. For what concerns my sockets, I use PULL and PUSH. This means that I can only have two valid options for both. For the PULL socket which is read-only:

0 = nothing to read
1 = have something to read

For the PUSH socket which is write-only they are:
0 = can't write
2 = can write

BUT. The getsockopt(ZMQ_EVENTS) & ZMQ_POLLOUT > 0 does not mean there are no messages in the queue. It just means that the queue is not full and the socket is ready to accept some more for sending. On the other hand getsockopt(ZMQ_EVENTS) & ZMQ_POLLIN == 0 guarantees that the incoming queue is empty.

if(msg.command == 'kill'){
       var time_now = new Date().getTime();
       //if 10 seconds passed without receiving any message or no message received at all (producer or useless worker)
       if(time_now - last_message_received > 10000 && !execution_flag || !last_message_received || receiver.getsockopt(zmq.ZMQ_EVENTS) | zmq.ZMQ_POLLIN == 0 && !execution_flag){
    }, 1000);

So basically I set up a timeout each second that checks if something has been received, if the worker is working on something or if is not working at something AND the POLLIN value is 0.
I still have to check this approach, but the given values for the bitmasks are correct.
If you have a better idea I'm open to suggestions. For now I think I will keep it this way.