(Originally published on Mike Clark’s pragmaticautomation.com)
In 1999 I was consulting for a company that builds telecom equipment. They were developing a new variety of … well, let’s just call it a “local area telecom network.” It included multiple nodes for traffic switching, call control and setup, and administration. It was to be a full-fledged product, but (as so often happens) there was an artificially imposed deadline: they wanted to demonstrate the product at a trade show in just a few months. Those kinds of things can be death to a project, but this company was attacking it rationally—for the demo, they were quickly pulling together “version 1” using existing internal simulation, testing, and prototyping tools, while a different team was working on building “version 1.5” from scratch, and there was a reasonable amount of communication and feedback between the two teams.
I was working on the Java-based operations and management system for the version 1 team, and was asked to design and build a system that could start and configure the various separate processes on a node, hooking them together so that they would be ready to join the network and start doing work. The testing group had an existing tool that did a similar job, and although it wasn’t suitable for use even in the demo product, the team asked that I keep the command structure of that testing tool. I don’t remember the details, but that tool could respond to three or four very simple commands: “connect”, “send”, “wait”, for example.
At that time I was a fan of the Tcl language, which was designed to be a neutral, extensible, embedded command language for applications. Tcl’s inventor, John Ousterhout, designed the language after noticing a pattern in application development: nearly every application needs a command language of some sort, but the developers are focused on the application itself, so they cut corners with the command language, thinking they can get by with less. The command languages end up having “insufficient power and clumsy syntax,” in Ousterhout’s words … and then, contrary to the developers’ expectations, users find that they need to use that command language to do complex things.
I saw this pattern developing in our telecom system. The command language I was asked to develop was almost hopelessly simple, but the people asking for it were adamant that it was all they needed. And yet I noticed that the command syntax was completely compatible with Tcl syntax. Plus, an implementation of Tcl atop Java, called Jacl, was available for me to use. Because Jacl, like Tcl, was designed to be extended with application-specific commands (and had a very nice API for doing so) I made the case that it would be just as easy — probably easier, in fact — for me to implement the desired commands as Jacl extensions than to write a custom parser. Then they would have a real programming language at their disposal “in the unlikely event that they ever needed it.” My clients reluctantly agreed to this plan, but only because it would not involve extra cost. They were sure they knew their needs.
Implementing the commands as Jacl extensions was easy, and the payoff was huge. During preparations for the first deployments (for the trade show demo and a couple of early customers) the team wrote hundreds of lines of Tcl scripts to automate the setup and configuration of the different pieces of software that made up the network. If that power hadn’t been available to them, they would’ve had to make requests to the development team for added features in the system, which would’ve been a huge bottleneck. It was generally acknowledged that the use of Tcl for the configuration language was one of several decisions that helped the deployment teams meet the project deadlines. It also made the system more robust: the configuration scripts were originally intended just for starting the system, but they turned out to be very useful for dynamic error detection and recovery as well. Tcl has fallen out of favor (for reasons both good and bad) and today I’d probably use Ruby or Python. But that’s an implementation detail.
I’m a big believer in the YAGNI principle: if you don’t have a real, demonstrated need for some architectural or design feature today, assume that “you ain’t gonna need it” and keep your design simple and focused on the needs of the day. But YAGNI is a principle, not a law, and there are some places where I’ve learned that it doesn’t apply. This is one: if you know you need a scripting, configuration, or extension language — something that allows your system to be automated — you can safely assume that you need a complete programming language in that role. YIGNI: you is gonna need it.