kontrawize

Versioning data models; it's not just the model that counts!

At the risk of repeating things I've covered in previous posts, one things that I'm often involved in is the question of versioning policy for UML data models and XML schemas. Now, the policy "question" is usually about how you can make changes (usually additions) to a model/schema with minimum impact on applications which use the model/schema, and ideally with zero impact on applications which can legitimately ignore the changes.

This all sounds reasonable, but the problem is that when these policy decisions are made, they are often done is a way that is almost sure to fail. Why? Well, the problem is that people often treat this as a model/schema issue only, as though some particular versioning approach in the model/schema alone is going to be the silver bullet that smooths out all of the potential downstream issues related to model/schema changes. However, it just doesn't work that way.

You can't have a workable policy on managing data change unless your applications are part of that same policy. There is no silver bullet that can work regardless of how your applications have been written. To go back to the relational database example that I sometimes use, in the bad old days when code generators wrote code that was tightly bound to each table and column in a database, database changes almost always broke the applications that used the database. The situation with databases is very different now, because most people follow the policy of interacting with their relational database via SQL (often using ODBC/JDBC), and that SQL layer (when properly used) provides sufficient decoupling to help avoid a lot of potential downstream issues, especially when all that has happened is an addition to your data model.

By contrast, with XML, most applications do not routinely access the XML via an XQuery or an XSLT transform, or via some other layer which would provide the same level of indirection and decoupling that SQL does for relational databases. Instead, a lot of use is made of XML schema compilers, which do the same tight binding that the old relational database schema compilers used to do, with all of the same downstream data change problems.

This is why versioning policy has to apply right along the data processing chain. You need to have good versioning policies for your models, but that alone isn't enough. You also need to have application design policies that match your model/schema policies so that the sum of the two is a robust approach to data change management.

After all, there's no point in building a bridge that only goes half-way across a river, is there?

Hibernating a Linux laptop with VMware Server

I'm written in the past about how it isn't possible to hibernate a Linux laptop (in my case, openSUSE 10.2) when VMware Server is running. I decided it was time to try again and see what can be done, since laptops are a lot more convenient if you can hibernate them rather than doing a full power-down and power-up every time.

One thing that has changed is that VMware Server now seems to do a better job of properly shutting down your virtual machine before it shuts down, rather than just performing a hard power-off. Either that, or it's simply that I have it better set up now. Anyway, each virtual machine can be set to shut down or (hard) power-off when VMware Server is shut down, and you should definitely choose the "shut down" option, for the sake of your file system. That said, I prefer to manually shut down or hibernate my virtual machines, and that's probably the best option (although you could do some automation using the "vmrun" command if you want).

The last time I tried to find a way to automate the shutting down of VMware Server when I hibernate my laptop, I couldn't find how to do it. Back then, I was looking for somewhere in the GNOME GUI to add a hook to a pre-hibernation script. It turns out that in openSUSE 10.2, the place to look is the "pm-utils" package (other varieties of Linux may use similar but different packages). With "pm-utils", you can add a hook hibernate/thaw script in the "/etc/pm/hooks" directory. What follows is the script that I added; the scripts are called in alphabetical order; there are already scripts starting with "00" and "01", so my script is called "02vmware.local":

#!/bin/bash
# [ABC] local power management script for VMware Server.
# VMware can't be hibernated, it needs to be stopped and restarted.

RETVAL=0
case "$1" in
	hibernate|suspend)
		/etc/init.d/vmware stop
		;;
	thaw|resume)
		/etc/init.d/vmware start
		;;
	*)
		;;
esac

exit $RETVAL

You can see that it just calls the standard VMware Server start/stop command from "/etc/init.d". The events that the script can receive are

• hibernate - suspend to disk;
• suspend - suspend to RAM;
• resume - resume from RAM;
• thaw - restore from disk.

This works, with a couple of caveats. My touchpad is disabled when my HP Pavilion dv2000 (dv2058ea) laptop resumes, but fortunately there is an enable/disable button just above the touchpad, so I can just press that. Also, the wireless network doesn't always resume correctly, which means having to run

sudo /etc/init.d/network restart

which isn't such a problem. This is what works for me, anyway.

Get those proposals in for XML 2007, Boston, 3-5 December

The deadline for proposals for the XML 2007 conference is the 31st August, so you have just over a week to get your proposals in. As noted by David Megginson, the conference chair, there are no "late breaking proposals" this year, so this is the one and only, final deadline for submissions.

Skype alternatives: Gizmo, JAJAH, part 2

During Skype's recent outage, I mentioned Gizmo and JAJAH as alternatives. It seems that JAJAH did particularly well out of this snafu, as its registrations are reported to have gone up 50% since the event.

I think you know that I'm not suggesting that the majority of that 50% were readers of this weblog.

Skype alternatives: Gizmo, JAJAH

Today's (apparently worldwide) problems with the Skype system meant that I couldn't use it for my conference calls, and I usually do. What do you do when that happens? Well, one thing I do is to always have active subscriptions to alternative VOIP services, so that if one isn't working, I can switch to another.

For one of my telecons today, I used Gizmo. That worked well. My only niggle with Gizmo is that, with the Linux version, it won't die when you try to close it, you have to kill the process. Otherwise, it was fine.

For a later telecon, I used JAJAH. The cool thing about JAJAH is that it phones you, you don't talk into a computer headset, just into the nearest landline or mobile phone. It's very useful if you are going to be near a landline where you can receive calls but can't make outgoing calls (especially long distance calls). JAJAH calls you first, then sets up a VOIP connection to the phone on the other end. On a 90 minute call today, it was perfect, you couldn't tell it was a VOIP call rather than a traditional long-distance call (UK to USA).

Skype is good most of the time, but it's definitely necessarily to have your alternatives planned out ahead of time.

Extreme Thumb Makeover for iPhone User, part 2

So, it was a good story, but it was a hoax.

Oxford and the CSW XML Summer School 2007

These are some photos I took while in Oxford for the CSW XML Summer School 2007.

The Summer School is held at Wadham College:

Here are some photos from near the Bodleian Library, just a block away:

Finally, some of the presenters finishing the day with a quiet drink in the college bar:

The full set of my photos is on Fotki. There are also photos from other participants on flickr.

Extreme Thumb Makeover for iPhone User

Find that your fat fingers are too large for your iPhone keyboard? For every stupid answer (like surgery), there's a willing volunteer.

Lenovo to sell laptops with Linux pre-installed

See the article at Pocket-lint. The next time I buy a laptop, I really want to get it with Linux pre-installed. With both Lenovo and Dell planning to offer Linux, I might even have a choice of laptops!

I'm afraid that putting openSUSE on my HP laptop has thrown up too many hardware driver issues, and so it hasn't been a good experience. For laptops, and for the foreseeable future, I think there will be a lot to be said for taking whatever Linux distribution the manufacturer offers, and just being grateful for it.

Does tight data binding always cost you more, in the long run? (part 3)

In the original post on this topic, I mentioned that I was inspired by the comments of one of my colleagues at Miley Watts LLP. Well, that colleague, Adrian Miley, has posted a comment, and it's a good one, so I recommend that you read it.