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If you use a Windows based workstation or notebook computer virtualization offers a way for you to run a linux server environment using the same linux distribution and configuration you'll use in production. Virtualization allows you to explore clustering and network setups that can't be tested on your workstation alone and simplifies your development environment by keeping the LAMP environment contained inside a VM.

While VMWare offers these capabilities with their VMWare workstation product, Sun has created a free alternative called VirtualBox, with many of the same capabilities in VMWare workstation. VirtualBox runs on a variety of intel chip based operating systems including OS/X, Windows XP & Vista, Linux and Solaris, and supports the installation of many different "Guest" operating systems. In this article, I'll detail the installation and configuration of Centos. Centos is a great choice for a Linux server operating system, as it is widely used by hosting companies due to its Redhat Enterprise Linux (RHEL) core.

Our goals in this setup will be:

• Centos server running the LAMP stack
• XP can be used to develop code using the IDE of your choice.
• The XP Workstation can communicate with the linux server using standard tools: putty, winscp, firefox
• The setup works even when no other networking is available. When a network is available, no network reconfiguration is required.
• Use XP to setup private domain resolution so apache vhost configurations can be tested.

Let's get started.

Continue reading "Run a Centos Lamp development server on XP, Vista or Win 7 using VirtualBox" »

The MySQL timestamp is an oddity, being both a mySQL "Data Type" as well as a type of specialty column that provides a built in default. It doesn't help matters, that the timestamp was changed significantly around mysql version 4.1.

The Old TIMESTAMP

In older mysql versions, the TIMESTAMP was not in the same format as a DateTime column, and you could also set up truncation by defining the TIMESTAMP to have a fixed size. For example, you could define a TIMESTAMP column to be a TIMESTAMP(4) which would then only store the 4 digit Year portion of a DateTime value. I won't go into much detail on the pre version 4.1 TIMESTAMP, however, if you're stuck with an older version of MySQL I recommend you read the manual carefully before you attempt to use any of the information here. I'm going to concentrate on the current TIMESTAMP.

TIMESTAMP Properties

At its most fundamental, the TIMESTAMP is really nothing more than a Unix TimeStamp, which is to say, that internally it is stored as an integer value of seconds. Where a MySQL DATETIME column can be used to store any date and time from Jan 1, 1000 to 12/31/9999, the TIMESTAMP is limited in the same ways that the Unix timestamp is currently limited -- it can only store values from Jan 1, 1970 to Jan 9, 2038.

Those familiar with Unix design, will recognize the Jan 9, 2038 date as being the next big "Y2K" computing panic, and if you're young enough, you may realize a large payday in your future, selling remediation services to companies in roughly another 28 years. The folks at http://www.y2038.com/ are already estimating this to be as much as a 10 trillion dollar jackpot, although no doubt by that time most of the coding will be done by the Morlocks from their underground cave cities. Outsourcing of IT to Morlocks will be a major industry trend by the year 2020, mark my words.

Continue reading "Too much information about the MySQL TIMESTAMP" »

You can't add to Null

Here's something about mysql create table definitions that can easily catch you if you aren't careful. Consider this table definition:





What the user wanted was a simple table to keep track of user submitted screen shots. Of course the real table had a submission date, and name column but I've omitted those in order to focus on what can happen when you allow a numeric column to have NULL values.

In the application in question, when a user votes for the screen shot they like, the system should "count" the vote, by increasing the values in the "votes" column by one. Initially the developer working on this application was trying to read the value from the database, and in a PHP web script, they would increment this value and take the result and set "votes" to be equal to it in an UPDATE statement. I explained that this could cause lost votes, because if two or more users were voting at nearly the same time, each would overwrite the value of the "vote" column. In fact there are scenarios far worse than that --- a user with a cached page could vote and set the vote count back days or weeks. I didn't bother to mention the possibility that someone might recognize what was going on in the web form, and start tampering with it, since it was plainly evident that the form was passing the current number of votes.

One of the many benefits of using a relational database is built in concurrency. In an UPDATE statement, you can add to the value of the column without having to know what its original value is, just as computer languages allow assignment to a variable that references the variable's current value (ie. $a = $a + 1, $a++).



All that's needed is to have the serverside language provide a value for a particular "id" and the votes will be tallied and updated correctly. Even more importantly, mysql will serialize the updates, insuring that no votes are lost.

However, given the original Mysql CREATE TABLE statement , what will happen if our code embeds the UPDATE statement provided?

Continue reading "Mysql Update: Null + 1 is Null! " »

For some time now I've felt like an honorary citizen of Ireland, having married a girl from Dublin and been blessed with two children upon which my wife and I have bestowed the most Irish of names. I've visited the country numerous times over the last 15 years and find myself becoming more intrigued with its history, culture and music with each visit. I've witnessed first hand Ireland's rapid evolution under the influence of unprecedented economic growth and the tidal wave of change brought by the European union. Any such metamorphosis brings with it changes that are both good and bad. The skies of Ireland's major cities have in recent years become thick with giant construction cranes, and its citizens have seen real estate prices hit astronomical highs. Ireland's youth no longer are confronted with the economic necessity of emigration, and expatriates have begun to return home in increasing numbers. The country now faces ironic and unforeseen challenges in the wake of its stunning reversal of fortune.

Continue reading "Heathers" »

Here are the slides for the presentation on subversion I gave at the September 2008 LampSIG meeting. I hope some may find them useful, however they were meant only to provide a skeleton for the talk, and aren't a complete tutorial by any means. The following links to other sites were mentioned in the talk:

The Red bean book, aka Subversion manual
The svnmerge python script
Subversion tips Article
Subversion Cheat Card

This error pops up for a couple of reasons. At the top of the script there will probably be a line that looks like this:



This is telling Linux that this script should be interpreted using the /bin/sh program. So your first step is to verify that program exists. I tend to use:



This will typically come back with a response like this:



This is telling us that the path to the sh program is in fact /bin/sh, matching the path specified at the top of the script. Ok, so what gives? Well, it's possible that this script was made on an operating system that has line ending characters different than linux. This could have been on on a Mac or PC, or the file could have been converted when it was packaged. In this case, you get the relatively misleading bad interpreter: No such file or directory message, which is really trying to look for sh, although you don't get any indication of the fact.

So, how to fix? Read on.

Continue reading "Linux shell scripting: bad interpreter: No such file or directory" »