I’m addressing the v2 question first. It is hard to know what to do in v1 if you don’t know where you want to end up when you go to build and deploy v2 (and later versions) of your product.

First, I'm going back to the three different product categories that I outlined earlier. I'm going to collapse the two different types of applications into just one category. Today's post is about applications. The next one will be about components.

Applications

For v2, you will want one of two situations:

• my v2 application completely replaces my v1 version entirely, or
• my v2 application installs side-by-side with v1 and the two versions do not affect or interact with one another

In most cases, I suspect that folks want the former. As an example, almost no one expects or wants to continue to use Office 2003 after installing Office 2007 on their machine. Something like TurboTax is the example that breaks that rule.

If you opt to install your application side-by-side with the older version of your application, then you might need to consider the safety of side-by-side versions of the various libraries that you depend on. The specific scenario that I’m imagining is that v2 of your application might be perfectly side-by-side with the v1 version, but the newer version of the MathLib library that you depend on might not be properly designed for a side-by-side install. Let's assume that you purchased MathLib from MathCorp, and that the versioning/servicing scheme of MathLib is different (naturally) than what you employ for the libraries that you build for your own application.

There are (at least) two obvious gotchas that I can imagine:

• MathLib doesn’t install side-by-side, but always overwrites older library versions with newer versions, and the MathLib developers made a breaking change this release (oops!), or
• MathLib reads mathematic constants from a file at a particular location, and the format of the file changed, but the location did not (oops!), which will break the old version

And to the other option, if you opt to replace the earlier version of your application, you don't risk breaking the earlier version of your app (since it is now gone), but you still have the risk that you might affect another app on the machine that relies on MathLib in the same ways described above.

What I’ve just described is essentially the well-known “dll hell” problem. I’d hope that most developers are familiar with this problem and have enough information to avoid it. However, it is sometimes non-obvious that you are about to walk off that cliff. This post is really intended to get folks to think a little more about that cliff.

This whole scenario assumes that MathLib is installed in some global fashion, either to the GAC, or to a global directory, such as "C:\Program Files\MathCorp\MathLib". If MathLib is installed with the application, in the application directory, then this whole problem goes away. At that point, you really are back to the monolithic application that I discussed earlier where the entirety of the application is installed and serviced uniformly, in time and location.

To be clear, I'm not trying to push developers toward private installs. There are downsides to approach that too. The meta-message is that you need to understand the potential impact of deploying your v2, and align your v2 goals around avoiding those impacts.

What's with the picture in the post? I recently purchased a digital SLR and am getting into photography as a hobby of sorts. The pic is just to add something extra to the post. I like this particular picture since there is a lot to look at visually, and because the focus is pretty narrow and well off center. It is also fun to see a child’s toy as an artifact to study.