Sunday, August 30, 2015

Programming with the metric system - Draft ideas

In a paper called 'Software Development for Infrastructure' Bjarne Stroustrup presented the new features of C++11 with some interesting examples. The most fascinating one was derived from the NASA accident in September of 1999. The root of the accident was a mismanagement of the metric system units due to a poorly designed API that basically was relying on comments.

I'm currently working on a project that requires managing metric units correctly. The language used is Java. Also, I have developed an external, open source library, that provides the essential API for managing metric units. It's a very young project so it supports a very small range of metric units (only those needed by the bigger project) but it is sufficient to demonstrate the basic design principles for managing metric units.

You can find the library here.

Let's examine it's usage in a few examples. Let's say we want to keep track of velocity.

Actually, this is already provided in the library.

public final class VelocityUnit {
    public final DistanceUnit DISTANCE_UNIT;
    public final TimeUnit TIME_UNIT;

    public final double DISTANCE_VALUE;

    public VelocityUnit(DistanceUnit distance_unit, TimeUnit time_unit, double distance_value) {
        DISTANCE_UNIT = distance_unit;
        TIME_UNIT = time_unit;
        DISTANCE_VALUE = distance_value;

    public VelocityUnit convert(DistanceUnit distance_unit, TimeUnit time_unit) {

        if (distance_unit == DISTANCE_UNIT && TIME_UNIT == time_unit)
            return this;

        double newTimeUnitWorthOfCurrentTimeUnit = 1/time_unit.convert(1, TIME_UNIT);

        double newTotalDistance = DISTANCE_VALUE*newTimeUnitWorthOfCurrentTimeUnit;

        double newDistanceValue = distance_unit.convert(DISTANCE_UNIT, newTotalDistance);

        return new VelocityUnit(distance_unit, time_unit, newDistanceValue);


    public String getMetricSignature() {
        return DISTANCE_UNIT.signature + "/" + TIME_UNIT.signature;

    public String toString() {
        return String.format("%.2f%s",DISTANCE_VALUE, getMetricSignature());

In the constructor, the programmer passes the metric unit of the distance and the time. This implementation is quite simple, so it gets the difference in distance as a third parameter and it assumes the value of time is a single unit of whatever metric is passed. You can then convert it to use different distance units or time units accordingly. 

But that doesn't tell much about differences between two people, what about hiding the information from the third party developer? Let's see another example now of how the developers will get what they expect by interacting with a black box class. In the example below, the developer can put values in whatever metric unit they like and get it back in whatever format they like. The magic is by telling the class to explicitly work with only one metric unit.

package com.vaslabs.units.examples;

import com.vaslabs.units.DistanceUnit;

public class ExampleDistanceCalculation {

    private final DistanceUnit PREF_DISTANCE_UNIT = DistanceUnit.METERS;

    private double pointA;
    private double pointB;

    public ExampleDistanceCalculation() {


    public void setPointA(double value, DistanceUnit distanceUnit) {
        pointA = DistanceUnit.PREF_DISTANCE_UNIT.convert(distanceUnit, value);

    public void setPointB(double value, DistanceUnit distanceUnit) {
        pointB = DistanceUnit.PREF_DISTANCE_UNIT.convert(distanceUnit, value);

    public double getDistance(DistanceUnit distanceUnit) {
        return distanceUnit.convert(DistanceUnit.PREF_DISTANCE_UNIT, (pointB - pointA));
The ExampleDistanceCalculation will work with meters while the third party developers can choose their own metric system. For instance, you can have a sensor and some software that give you values in centimeters. You can have a class like the above as a middleware (with CM instead of METERS) and allow all the other developers to work on the metric unit of their preference. It is also useful when delivering to the userland, as users may have different preferences on metric units.

Saturday, April 26, 2014

Pi-web-agent Quokka

The pi-web-agent version 0.2 codenamed Quokka has been released since the 24th of April. It provides a better user interface which is faster and more interactive and some extra cool features such as:

  • Pi camera controller (take snapshots or watch a live stream).
  • File manager - browse and download files. 
  • Radio - stream from internet radio or other audio by providing the URL.
The firewall management was also improved which allows now to control access from various protocols and IP addresses.

A video that demonstrates the application:

How to get it:

Download the application from pi store:

Give a like to the developers and their project:

Monday, April 7, 2014

The next day for your business: Windows XP?

I should have written this article about a year ago to give to someone that cares the time to plan ahead. Frankly, I don't care much. If you have a business and you are technologically impaired it's your fault.

Computers are not an unnecessary "shit that I have to buy, just don't spend much". They are your records, data center, analysis tools and your professional image all in one box. When you've installed or bought computers with Windows XP you did the right thing. They were the best you could find, a value for money deal like no any other. The main reason for that was the Microsoft monopoly. Linux distributions were good but you couldn't find the tools you needed easily, and Apple oh Apple. . .

 But in 2014 things are different. You can have a free office solution that may lack the User Interface eye candy of Microsoft's but it does the job and guess what: it's free. Also you have a large range of free Linux based Operating Systems you can use. People tend to agree that Ubuntu or Linux Mint are the most user friendly ones.

But if you feel that open source and free software is "insecure and vulnerable and Jesus everyone can see the code, is that even safe?" you can buy from Red Hat and have the support you used to have with Microsoft. Which in fact you didn't have, but this time it will be a real thing.

 So before upgrading to Windows 7 or 8 (Jesus are you thinking "what about Vista?" now?) or buying new machines, think about spending a fifth of that money to install a free operating system (which is also safer) and train your employees to use them. If they can't learn it, fire them and get new ones.

 So what are the benefits of Linux based Operating Systems?

 Remember when you needed to update manually Firefox, Google chrome and a bunch of other applications that weren't Microsoft's? Well, no more. Every application (assuming you've installed them correctly which requires an IQ roughly above the 20's) gets updated automatically along with the system updates. And guess what: If you screw up or something breaks, you can roll back (again, if you have the IQ index mentioned before).

"But why do I need to keep updating?". Well that's the reason you are switching from XP to something else right? Anyway, most of your employees play solitaire or they are on Facebook. So get them something that's free and actually works.

Friday, December 27, 2013

The pi-web-agent

Remember the Hackmanchester winning project pi-web-agent? Well, we released the first version of the pi-web-agent in the pistore. Here is the wiki page of the project as generated and exported from our github repository


The pi-web-agent is a web application that aims to provide a more user friendly way of interacting with the Raspberry Pi and performing basic tasks by eliminating the need of using the command line directly.

How to use

After starting the pi-web-agent service by executing or sudo /etc/init.d/pi-web-agent start , you can access the application with your browser via either https://raspberrypi:8003 or https://ip_address_of_your_pi:8003 if your internet router does not resolve hostnames to IPs. To access the application inside your Pi just access the local host without https:

Provided functionalities

The web application currently provides the following functionalities:

  • Firewall management by controlling the iptables.
  • A package management system for installing useful applications easily.
  • Service management for starting or stopping services
  • Update management for updating the underlying Linux distribution with a simple click
  • GPIO management for controlling the pins on the Raspberry Pi (special thanks to the author of wiringPi for his excellent open source program)
  • General purpose information of the system (memory usage, disk capacity, ip, cronjobs, swap usage)
  • Tightvnc is provided, by setting up a vncboot service and enabling users to use tightvnc java applet to access the system by the tightvnc viewer (special thanks to tightvnc for their open source tightvnc client)
  • Power management for rebooting or powering off the system with a simple click

Firewall management

Currently the Firewall management section displays the current state of the iptables. Enabling input for altering the iptables state is under development

Package management

The package management provides a list with useful packages and a short description. You can request an uninstall or install of the application by simply clicking on the switch button.

Service management

Service management allows you to stop or start services. Only services with known state are shown.

Update management

The update management aims to arrange or the hassle about updates for you. It takes care of checking for updates and notifies you on the live information feed. The update section also provides information of weather there is an update or not and if yes, it provides a list of packages with there description that need update. The update can be initiated with a simple click of a button at the end of that list.

GPIO management

The GPIO management provides access to the General Purpose Input Output pins on the Raspberry Pi. You can convert a pin to input or output and activate outputs. Currently only GPIO0-GPIO7 pins are available. The solution is under development to provide more functionality on the second release.


VNC is very important because most users want to access their pi from their laptop and have an image of the desktop in their screen. That's why the application has the tightvnc server as a dependency and provides the tightvnc client java applet. The whole vnc solution is pre-setup and only clicking at the vnc section should work. The tightvnc service on the RPi should be started manually because you need to setup a password.


Currently the web application agent supports the Raspberry Pi with Raspbian installed. Any debian based Linux distribution should also work but is not thoroughly tested yet.


Vasilis Nicolaou, Angelos Georgiadis, Georgios Chairepetis, Kyriacos Georgiou and Maria Charalambous


GPLv2. Imported projects have their own license.

Developer information

Please consult the README file in order to setup an environment for testing purposes of the application. Note that architecture specific code won't work (just the GPIO for the moment). The application is based on the micro-CernVM web appliance agent developed at CERN by Vasilis Nicolaou and documentation section contains documents for that web application but are highly relevant to the forked version (the pi-web-agent)


Report (only relevant information of the web application, ignore update management section)

Presentation (first 9 slides)

Follow usr/share/pi-web-agent/doc for documentation on key python modules.