Chris Read

How often has someone come up to you and asked you what build of your software is currently deployed in a specific environment?

How many times have you come across a .jar or .dll file and wondered what version it is? Especially when using Open Source Software?

The most frightening one for me is when I’ve looked at a cluster of production servers and noticed that the .war file for the application deployed on it is a different size on one of the nodes. Which one was the correct one to deploy? Luckily this happened to me a long time ago, but I know that people out there are still having this problem today.

The solution is what I call “Self Identifying Software”. Every build of your software needs to have something that tells you what version it is and how to get back to the source code that created it. Having a build label or release number visible in your application is a good start, but it does not make your software Self Identifying. Product companies have been doing this for ever. The problem is that for that number to be useful (particularly when you’re trying to access the source code to reproduce and fix a bug) you then need to refer to a build system or release notes to find out where the source code came from (if you’re lucky). It often also does not apply to development builds. To be truly Self Identifying you need to make sure that every build (including builds developers create on their workstations) also includes enough information from the SCM system so that anyone who has access to the source code can go right back to the exact source code that produced that binary. For example, if you use Subversion as your SCM then this will be a URL and a revision number.

This is not exactly a new concept, it’s something I (and others) have been doing for a number of years now. The reason I’ve decided to write about it now though is that recently I was showing a new guy around one of the projects I’m working on at the moment, and when I showed him how to determine which version of the app was deployed he was delighted.

How do I get my software to identify itself?

There are generally two scenarios where I would want to get the identity of some software. The first is where I have a service or system running somewhere that I would like to know the version of, and the second is where I have a binary artefact and I’m trying to determine it’s provenance.

When querying a service, the easiest is to be able to hit a well known URL (or make a well known service call) that tells me what I need to know. What the exact URL or service call is is not important, as long as those who need it know how to invoke it. This is most useful then you don’t actually have access to the binaries that power the service.

However, if you do have access to the binaries then it’s usually easiest just to examine the meta data of the file to see where it came from. Most modern binary formats allow for this in some way. If you’re working with Java then all .jar, .war and .ear files allow you to put this into the META-INF/MANIFEST.MF file. Creative use of versioninfo resources will allow you to achieve similar results if you’re dealing with Windows. Note that we’re not talking about putting this information into the file name itself – file names change.

How do I use it?

Having Self Identifying Software goes beyond just being able to quickly track down the source code that built the binaries you’re dealing with. Here are a few other uses I’ve found:

• When used as part of a Continuous Deployment system you can verify that you’ve got the correct version of the binaries before you start deploying things, and you can then use the service call to verify that the correct version of the app is up and running at the end of the deployment processes.
• When used as part of a dynamic system dashboard you can quickly and easily see what’s installed where instead of relying on spreadsheets or documents that often get forgotten.

If you find any more, please let me know…