waelchatila.com - Web category

Turn your roomba into a walking google bot

Tue, 02 Mar 2010 07:26:00 GMT

When my latest son was born I got a little worried since he didn't look at all like me. I decided I needed a discrete device to spy on my wife while I was at work. I started hacking my roomba and made a GåågleBot (pronounced /google-bot/). GåågleBot is a "home crawler" consisting of a vacuum roomba with an on board webserver and camera. While the vacuum goes about its business, it extracts text from the images it takes. The text is later put in a database on the roomba and searchable through a web interface. This was a good pretext for my wife. I, of course, also added the ability to remote control the roomba using AJAX...for spying purposes...hehe.

I created a dedicated site for the little fellow for fun with showcase app. http://www.GaagleBot.com

This is the next version of My AJAX Lego Robot. I'm working on yet another version of my AJAX Remote Controlled thingies...stay tuned.

Running Jetty as Non Root

Wed, 21 Jan 2009 07:16:00 GMT

I'm kicking the tires of Jetty and am in the process of setting it up on a production machine and found that the latest Jetty realeas (6.1.14) has a really easy way to start jetty out running as root and later switch to a uid of your choice which enables you to open up server sockets on the privileged ports 80 & 443. See the README file under {jetty.root}/extras/setuid. The process is just a one-liner...well, there are some more perks as you'll see at the end.

The README seems little bit out of date as my

mvn install

did all of the compiling and copying steps. However, when I try to launch jetty with the new jetty-setuid.xml config file I get the following error.

2009-01-20 23:10:33.887::WARN: Config error at <Set name="uid">jetty</Set> 2009-01-20 23:10:33.888::WARN: EXCEPTION java.lang.reflect.InvocationTargetException at java.lang.reflect.Constructor.newInstance(libgcj.so.81) at org.mortbay.xml.XmlConfiguration.set(XmlConfiguration.java:405) at org.mortbay.xml.XmlConfiguration.configure(XmlConfiguration.java:248) at org.mortbay.xml.XmlConfiguration.configure(XmlConfiguration.java:214) at org.mortbay.xml.XmlConfiguration.main(XmlConfiguration.java:974) at java.lang.reflect.Method.invoke(libgcj.so.81) at org.mortbay.start.Main.invokeMain(Main.java:194) at org.mortbay.start.Main.start(Main.java:523) at org.mortbay.start.Main.main(Main.java:119) Caused by: java.lang.NumberFormatException: invalid character at position 1 in jetty at java.lang.Integer.parseInt(libgcj.so.81) at java.lang.Integer.<init>(libgcj.so.81) at java.lang.reflect.Constructor.newInstance(libgcj.so.81) ...8 more

My jetty-setuid.xml file looks like:

<Configure id="Server" class="org.mortbay.setuid.SetUIDServer"> <Set name="startServerAsPrivileged">false</Set> <Set name="umask">2</Set> <Set name="uid">jetty</Set> <Set name="gid">jetty</Set> </Configure>

For some reason its expecting an integer instead of "jetty"...well well

> su jetty > id uid=1002(jetty) gid=1003(jetty) groups=1003(jetty)

My jetty-setuid.xml now looks like:

<Configure id="Server" class="org.mortbay.setuid.SetUIDServer"> <Set name="startServerAsPrivileged">false</Set> <Set name="umask">2</Set> <Set name="uid">1002</Set> <Set name="gid">1003</Set> </Configure>

...works like a charm.

Single Table Polymorphic Inheritance with ActiveRecords

Fri, 23 May 2008 05:27:00 GMT

Implementing a single table polymorphic inheritance in rails is dead easy. Simply do

class Fruit < ActiveRecord::Migration def self.up create_table :persons do |t| t.column :type :string t.column :shape, :string t.timestamps end end def self.down drop_table :fruits end end

or if you a migrating...

class Fruit < ActiveRecord::Migration def self.up add_column :fruits, :type, :string end def self.down remove_column :fruits, :type end end

Now, all you have to do is to create your sub-classes

class Fruit < ActiveRecord::Base def calc_volume ... end end class Apple < Fruit def calc_volume ... end end

Don't forget to put Apple in a separate file by itself apple.rb or else ruby will spit out an unconditional warning if you reference Apple in your code with Fruit::Apple

warning: toplevel constant Fruit referenced by Fruit::Apple

Internet Explorer 6 & 7 document.domain bug

Wed, 31 Oct 2007 17:25:00 GMT

Disclaimer: Im not sure this a bug discovery...couldn't find anything that resembled it after a couple of minutes of googling, or a super silly security restriction.

I've been coding some javascript ajax code that inserts an iframe, and into the created iframe, inserts a form and submits the form to a (cross domain) server. This is an old cross-domain ajax technique by now, but while implementing it I came across this ridiculously annoying bug in IE. The line in red below will result in an "Access Denied" exception being thrown...again, only in IE. Tried this with IE6 under WinXP Media Center Edition SP2, IE7 under WinXP Home Edition SP2, and IE7 Windows Vista Home. Needless to say...it works just fine in FireFox.

Try it by clicking the button below.

This page should be served off of http://waelchatila.com. As you can see, the document.domain is explicitly set to the domain the page was served from. The iframe src is also from the same server. I've tried it with a whole variety of variations, none of the demonstrated here but tested for.

iframe's document.domain = 'waelchatila.com';
iframe src is empty
iframe src is "javascript:false"

There should be no problem accessing the iframe's content in either case.

Now, if the document.domain = 'waelchatila.com'; line is removed, it works. In order for this to work, you'll need to reload this page if you clicked the previous button since document.domain has now already been assigned.

The same results present itself if the iframe is created with document.createElement('iframe') and document.appendChild(...)

I've yet to find a workaround for this issue. I'll be positing another entry if I found out how. Please let me know if you know of one!

DisplayTag Paging

Tue, 07 Nov 2006 23:37:41 GMT

DisplayTag is a very versatile jsp tag library. It provides a whole lot for free. Paging, sorting, even / odd highlighting, grouping, exporting to PDF, Excel, CSV, XML, just to mention a few things it can do.
I use it in pretty much every project. However, I've yet to see a built in way to keep track of which page you were at last. In this blog entry I explain one general way to get around this limitation.